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JULY 2020

The world’s best-selling astronomy magazine www.Astronomy.com Vol. 48 • Issue 7

ALL ABOUT BONUS
ONLINE
STARS CONTENT
How stars are born CODE p. 4
and die p. 16
Meet the most
extreme stars p. 24
The nearest stars
up close p. 30
The Sun’s lost
siblings p. 44
How pulsating stars
unlock the cosmos p. 56

AND MORE!

Bob Berman’s tribute to carbon p. 14
Astronomy’s editors answer your questions p. 70

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Online Content Code: ASY2007 JULY 2020
Enter this code at: www.astronomy.com/code
VOL. 48, NO. 7
to gain access to web-exclusive content

CFHT/COELUM – J.-C. CUILLANDRE & G. ANSELMI

CONTENTS 16 ON THE COVER

FEATURES 36 50 Giant spheres of stars, globular
clusters like M15 in Pegasus
16 Sky This Month Is Eta Corvi a window contain aged suns that winked
to our past? on during the early days of the
How stars are Feast on a full planetary universe.
born and die lineup. MARTIN RATCLIFFE By studying this strange
star, astronomers hope to COLUMNS
Stellar evolution is a circle AND ALISTER LING better understand what
of life — dying stars spew happened early in the life Strange Universe 14
their contents into the galaxy, 38 of our own solar system.
paving the way for the next BOB BERMAN
generation. JIM KALER Star Dome and NOLA TAYLOR REDD
Paths of the Planets Secret Sky 62
24 56
RICHARD TALCOTT; STEPHEN JAMES O’MEARA
Meet the most ILLUSTRATIONS BY ROEN KELLY How pulsating stars
extreme stars unlock our universe For Your Consideration 64
44
Some huge, some small. Some RR Lyrae variables allow JEFF HESTER
zip, some crawl. The cosmos The Sun’s lost siblings precise distance measurements,
is full of objects that defy reveal the history of the Observing Basics 66
expectations. JAKE PARKS Astronomers think it’s possible regions they populate,
to identify the stars that and trace how galaxies are GLENN CHAPLE
30 formed from the same nebula structured. ATA SARAJEDINI
as the Sun. YVETTE CENDES Binocular Universe 68
Stellar neighbors 70
close-up PHIL HARRINGTON
Ask Astro
Astronomers are learning 7
a lot from the stars in our The Sun’s light.
part of the Milky Way. QUANTUM GRAVITY

BRUCE DORMINEY Everything you need to
know about the universe
ONLINE Dave’s Trips and Sky This News this month: growing
FAVORITES Universe Tours Week The latest space salads, 139 minor
The inside updates from planets found, and more.
Go to www.Astronomy.com scoop from Travel the world A daily digest the science
for info on the biggest news and the editor. with the staff of of celestial and the hobby. IN EVERY ISSUE
observing events, stunning photos, events.
informative videos, and more. Astronomy. From the Editor 5
Astro Letters 6
4 ASTRONOMY • JULY 2020 Advertiser Index 65
New Products 69
Reader Gallery 72
Breakthrough 74

Astronomy (ISSN 0091-6358, USPS 531-350)
is published monthly by Kalmbach Media
Co., 21027 Crossroads Circle, P. O. Box 1612,
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FROM THE EDITOR Editor David J. Eicher
Design Director LuAnn Williams Belter
What we owe stars
EDITORIAL
The Sun, our star, I hope that every day, at least for a Senior Editor Richard Talcott
appears like a giant moment, you think of the Sun — Production Editor Elisa R. Neckar
glowing ball of not necessarily because you’re a Senior Associate Editor Alison Klesman
radiation in this solar observer, but out of gratitude. If not for Associate Editor Jake Parks
image from the the Sun, of course, none of us would be here. Copy Editor McLean Bennett
Solar Dynamics Circumstances in our solar system’s forma- Editorial Assistant Hailey McLaughlin
Observatory. As the tion and evolution led to Earth being in the
universe’s natural right place to receive enough solar radiation ART
nuclear reactors, to permit water, and, thus, life. Our local Contributing Design Director Elizabeth Weber
stars tell us an nuclear reactor in the sky gives us our exis- Illustrator Roen Kelly
enormous amount tence, and we owe our lives to it. Production Specialist Jodi Jeranek
about how the The subject of stars is a pretty fundamen-
universe works. tal one in astronomy. After all, stars are CONTRIBUTING EDITORS
practically everywhere when we look up in the sky; galaxies are filled Michael E. Bakich, Bob Berman, Adam Block,
NASA/SDO with them, and the universe is filled with galaxies. It’s a little funny Glenn F. Chaple Jr., Martin George, Tony Hallas,
thinking back over conversations with astronomers and appreciating Phil Harrington, Korey Haynes, Jeff Hester, Alister Ling,
Follow the how relatively few are actively researching fundamental properties Stephen James O’Meara, Martin Ratcliffe, Raymond Shubinski
Dave’s Universe blog: of stars, given how important stars are to the story of the cosmos. But
www.Astronomy. enough are studying how stars work that our collective store of knowl- SCIENCE GROUP
com/davesuniverse edge about them is growing. Executive Editor Becky Lang
Follow Dave Eicher This special issue presents a pretty impressive tale of what we know.
on Twitter: We’re fortunate to have the dean of writers about stars for a popular EDITORIAL ADVISORY BOARD
@deicherstar audience, Jim Kaler, now an emeritus professor at the University of Buzz Aldrin, Marcia Bartusiak, Jim Bell, Timothy Ferris,
Illinois, kicking off our package with a story about how stars are born Alex Filippenko, Adam Frank, John S. Gallagher lll,
and die. Their life cycles tell us a great deal about how the universe Daniel W. E. Green, William K. Hartmann, Paul Hodge,
works and about the past and future of our own solar system. Edward Kolb, Stephen P. Maran, Brian May, S. Alan Stern,
Associate Editor Jake Parks takes us on a journey through stellar James Trefil
extremes — the largest, smallest, brightest, and so on — an always-
fascinating and ever-changing tapestry of understanding of these Kalmbach Media
cosmic engines. Science writer Bruce Dorminey helps us visit the
nearest stars, from the Alpha Centauri system and beyond, in a fas- Chief Executive Officer Dan Hickey
cinating look at our galactic neighborhood. Senior Vice President, Finance Christine Metcalf
Some 4.6 billion years ago, the Sun was born in an open cluster. Senior Vice President, Consumer Marketing Nicole McGuire
Now it is a single star. What happened to its sisters? Radio astronomer Vice President, Content Stephen C. George
Yvette Cendes describes the ongoing research on the Sun’s lost siblings, Vice President, Operations Brian J. Schmidt
a detective story spanning years and looking back on a nearly infinite Vice President, Human Resources Sarah A. Horner
time span. Science writer Nola Taylor Redd examines the weird star Senior Director, Advertising Sales and Events David T. Sherman
Eta Corvi, which hides some strange behavioral secrets that could shed Advertising Sales Director Scott Redmond
light on many other stars. And physics professor Ata Sarajedini Circulation Director Liz Runyon
describes a class known as RR Lyrae stars, one of the tools in an Director of Design & Production Michael Soliday
astronomer’s box that allows us to gauge distances in the cosmos. Managing Design Director Lisa A. Bergman
I hope you’ll enjoy this special package — and that you’ll continue Retention Manager Kathy Steele
to think of the Sun, and give it an occasional wink of appreciation. Single Copy Specialist Kim Redmond

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ASTRO LETTERS

The Moon at First A keen eye and many people have strong views about if Pluto is a
Quarter, captured planet or not. But nature isn’t so obliging. I think we
from the International I thoroughly enjoyed seeing should accept categorization is often necessary for us to
Space Station. NASA the Moon get some love in the understand generalities, but we can’t apply those rules
March issue with Michael E. to everything or have everything fit into tidy boxes.
We welcome Bakich’s “Explore the Moon at
your comments First Quarter.” I did notice one — James Fradgley, Wimborne, U.K.
at Astronomy Letters, little erratum — he swapped the
P.O. Box 1612, terms for scotopic vision (low The mystery of ‘Oumuamua
Waukesha, WI 53187; light) and photopic vision (well
or email to letters@ lit). In a past job, I worked in the I enjoyed your article, “Our first interstellar visitor,” in
astronomy.com. refractive surgery market writing software that imaged the February 2020 issue immensely, and read it from
Please include your and measured a patient’s cornea with infrared light. end to end with great pleasure. It was the most exhaus-
name, city, state, and We used infrared so the eye was fully scotopic, or dark tive article that I have seen on ‘Oumuamua in the popu-
country. Letters may adapted. The terms just jumped out at me when I saw lar press. I learned a lot about it and had my own suspi-
be edited for space them there, and of course I knew it was backwards. cions confirmed. It could be a flat disk, which is alluded
and clarity. to in your article, but less likely, it could have one bright
— Richard S. Wright Jr., Lake Mary, FL side and one dark side.

From the editors: Good eye, Richard — you’re right! Great job, Alison Klesman! I will look for your work
in future issues of Astronomy. I would also like to con-
Pluto’s status gratulate the management of Astronomy on 46 years of
publication. I am a retired electrical engineer from the
The article “Lowell Observatory turns 125” in the Jet Propulsion Laboratory and have had an amateur
February 2020 issue mentions Pluto’s planetary status. interest in astronomy all my life. I even taught
It’s a pressing human need to put everything into boxes, astronomy! — Chuck Lahmeyer, Jefferson City, MO

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Astronomy? Blogs
And More!
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0DFKHVQH\ 3DUN ,/ 86$ Go to Astronomy.com
3K

6 ASTRONOMY • JULY 2020

QG QUANTUM GRAVITYEVERYTHING YOU NEED TO KNOW ABOUT THE UNIVERSE THIS MONTH

NASA, ESA, AND STSCI. BOTTOM FROM LEFT: INDIAN SPACE RESEARCH ORGANISATION, DEPARTMENT OF SPACE, GOVERNMENT OF INDIA; STEFAN PAYNE-WARDENAAR; NASA/SVS SNAPSHOT HOT LIFTING OFF SLOW CRASH LAVA-LESS
BYTES The Indian Space New data from the Gaia A recent analysis of
HUBBLE’S Research Organisation satellite suggest the martian meteorites
COSMIC REEF announced that its Milky Way’s warped indicates that the
first crewed mission, disk is the result of Red Planet was never
On April 24, NASA celebrated named Gaganyaan, an ongoing collision covered in a global
the Hubble Space Telescope’s could launch three between our galaxy magma ocean in the
30th anniversary with a star- astronauts into low and another, smaller past, unlike Earth
studded image of NGC 2014 Earth orbit as early one — possibly the and the Moon.
(right) and NGC 2020. Located as December 2021. Sagittarius Dwarf.
in the Large Magellanic Cloud,
the region is called the Cosmic
Reef, in part for NGC 2014’s
coral-like appearance. While
NGC 2014 contains several
stars 10 times the Sun’s mass
each, its blue-hued neighbor is
shaped by a single 15-solar-mass
Wolf-Rayet star blasting material
into space. — ALISON KLESMAN

WWW.ASTRONOMY.COM 7

QUANTUM GRAVITY

GROWING SPACE SALAD

By next year, NASA astronauts could be eating
space-grown lettuce, tomatoes, and peppers.

Aboard the International Space for analysis. But the munchies marked CHOW TIME. Astronauts Kjell Lindgren and
Station (ISS) in August 2015, a milestone in human spaceflight. It
Japanese astronaut Kimiya Yui trimmed was the first time an orbiting crop had Scott Kelly nibble on red romaine lettuce
a few leaves of red romaine lettuce and been grown with NASA hardware and grown on the International Space Station. NASA
passed them to NASA astronauts Scott then eaten (though scientists suspect
Kelly and Kjell Lindgren. Each drizzled astronauts might have stolen a few bites LETTUCE REJOICE
a few drops of dressing onto the pre- from a previous sample).
cious produce, then popped it into their From 2014 to 2016, astronauts grew
mouths. “That’s awesome, tastes good,” We now know that space lettuce “Outredgeous” red romaine lettuce
Lindgren said. doesn’t just taste good. It’s also safe to inside the ISS Vegetable Production
The 2015 harvest was too scant for eat and as nutritious as lettuce grown System chambers, or Veggie. Meanwhile,
a proper space salad, especially since back on Earth, according to a new scientists at NASA’s Kennedy Space
half the crop was sent back to Earth study published March 6 in the journal Center ran a control experiment to
Frontiers in Plant Science. precisely replicate Veggie’s conditions
using temperature, humidity, and
carbon dioxide measurements beamed
from the ISS back to Earth.

When NASA researchers tested both
versions of the lettuce, they found the

8 ASTRONOMY • JULY 2020

NASA/JPL-CALTECH/MSSS QUICK
TAKES
Curiosity views the martian horizon
AURORAL SIGNAL
Following the death of NASA’s Opportunity rover in 2018 due to a massive martian dust
storm, Curiosity became the sole rover left exploring Mars’ surface. The car-sized scout Astronomers think they’ve found
has been churning out great science and images consistently for almost eight years, a terrestrial-mass exoplanet some
and it’s not done yet. Between November 24 and December 1, 2019, Curiosity used its
Mastcam to capture more than 1,000 images that were later stitched together to create 30 light-years away by spotting
this highest-resolution panorama (almost 1.8 billion pixels) of Mars to date. Next year, aurora-like radio signals in the star’s
Curiosity will get some new company when the Perseverance rover (and its more
advanced Mastcam-Z camera) touches down in Jezero Crater. — J.P. magnetic field, caused by the
potential planet’s presence.
space-grown variety was strikingly interesting. The pepper’s complex
similar to the ground-grown controls. genome could undergo interesting FAREWELL, AL
Each had equivalent levels of nutrients changes when grown in the high-
and antioxidants, and even the same radiation environment of space. Al Worden, Command Module Pilot
diverse microbial communities. Plus, of Apollo 15, died March 18 at the
neither crop showed signs of potentially On Earth, these peppers are typically age of 88. In 1971, Worden set the
problematic bacteria like E. coli. milder than jalapeños. But “plants record for “most isolated human
often produce the chemical responsible being” while orbiting the Moon
But researchers say there’s still a long for spiciness, capsaicin, in response to some 2,235 miles (3,597 km) away
way to go before astronauts can help stress,” says Matt Romeyn, a Kennedy from his crewmates on the lunar
themselves to a cosmic salad bar. The Space Center scientist overseeing the surface — greater than the separation
lettuce was a gateway plant — it’s easier pepper experiment. “We currently have
and faster to grow than most fruiting no data on how the stress of micrograv- between other Apollo crews.
crops, yet it thrives in similar condi- ity could affect capsaicin levels. At the
tions. Now scientists need to figure out same time, we have grown peppers in EX-THEORY?
how to nurture slower-growing plants. the lab that were not stressed at all and
the fruit was bland and missing a bit Astronomers searched the Milky
“Tomatoes and peppers, which we of heat that we were after, so it will be Way for an X-ray signal seen in
hope to grow this year and next, will interesting the first time an astronaut other galaxies and thought to come
need similar growing conditions. But bites into a pepper grown on ISS.” from dark matter. However, they saw
because they take a lot longer to grow” no such signal in our own galaxy.
— 28 days for mature lettuce versus By next year, astronauts could be The team says their result, which is
80 days for the first fruit from dwarf eating fresh tomatoes in orbit, too — still highly controversial, rules out
tomatoes — “they are much more of once NASA sorts out how to precisely the theory that these X-rays come
an investment of time and resources,” deliver water and nutrients for the
Christina Khodadad, a Kennedy Space 100 days it takes the fruit to reach from dark matter.
Center researcher and lead author of maturity. “Fresh-picked ripe tomatoes
the new study, tells Astronomy. are a rare treat for many, so we thought DELAYED LAUNCH
these would also be a treat for the
STRESS TEST astronauts,” says study co-author and The launch of ExoMars — a joint
Veggie researcher Gioia Massa, also at lander and rover mission led by the
This August, space farming will see Kennedy Space Center.
its most challenging crop yet: the chili European Space Agency and
pepper. Peppers are particularly tough If everything goes according to plan, Russia’s Roscosmos — has been
to grow because their seeds need two ISS astronauts soon will finally have postponed until 2022, due to a
weeks of perfect conditions before they the makings of a true space salad. combination of technical issues
germinate. But they’re also scientifically and the coronavirus pandemic.
— ERIC BETZ, JAKE PARKS
NO LAND

Ancient Earth was a water world
with little or no dry land some

3.24 billion years ago, new research
suggests. An early lack of sufficient

dry land would have major
implications on the origin and
evolution of life, both for Earth

and other planets.

SEND MY HEART

Researchers shipped samples
of heart muscle tissue to the
International Space Station as part
of the Engineered Heart Tissues
experiment, which aims to explore
why weightlessness reduces heart
function in astronauts. — J.P.

WWW.ASTRONOMY.COM 9

QUANTUM GRAVITY

Astronomers find 139 DISTANT WORLD. Trans-Neptunian objects are
new minor planets
frozen, far-off worlds orbiting our Sun beyond the
Researchers searching through Gary Bernstein of the University of ice giant Neptune. This artist’s concept shows
data from the Dark Energy Pennsylvania tells Astronomy he’s been Sedna, one of the largest known TNOs, which has
Survey, or DES, have discovered 139 fascinated by TNOs since “before Planet a reddish surface — and possibly a moon (shown
new minor planets orbiting the Sun. Nine was a thing.” That interest led at the dwarf planet’s upper right). The Sun appears
These worlds, called trans-Neptunian the team to design a novel algorithm only as a small, bright point of light. NASA/JPL-CALTECH
objects (TNOs), orbit beyond Neptune, that could identify distant solar system
which sits about 30 astronomical units worlds by looking for objects in the up in unexpected ways, as if an unseen
from the Sun. (One astronomical DES images that moved against the object is herding them into specific
unit, or AU, is the average Earth-Sun background stars from night to night. orbits. Some astronomers think these
distance.) The team then confirmed they could strangely orbiting TNOs point to a mas-
The method that allowed the team filter out fake objects and validated their sive, distant world called Planet Nine.
to spot so many small worlds is now movement-spotting algorithm with
expected to reveal thousands of distant known TNOs. Their final results were Hypothesized to be five to 15 times
objects in coming years — making published March 10 in The Astrophysical the mass of Earth and orbiting some
these first hundred or so just the start. Journal Supplement Series. 400 AU (or more) from the Sun, Planet
Currently, we know of nearly 3,000 Nine would have enough gravitational
TNOs, but astronomers estimate the So far, the team has only analyzed a pull to cause some TNOs to cluster as
total number is closer to 100,000. Taken small subset of the DES data. Ultimately, they make their closest approaches to
together, the newfound distant objects, they expect their algorithm can uncover the Sun. But the evidence for Planet
as well as those to come, could possibly 500 or more TNOs in the full dataset. Nine is so far indirect and sparse. That’s
resolve one of the most fascinating And if the same method is applied to why discovering more TNOs, particu-
questions in modern astronomy: Is even more sensitive upcoming surveys — larly beyond the Kuiper Belt, will give
there a massive and mysterious world such as those planned with the Vera C. astronomers more clues that could
called Planet Nine lurking in the Rubin Observatory — the group expects point to the location of Planet Nine —
outskirts of our solar system? to find thousands of new TNOs. or eliminate its existence altogether.
Researchers uncovered the new TNOs
in data taken by the Blanco 4-meter tele- A MAP TO PLANET NINE None of the 139 newfound minor
scope in Chile as part of the DES, which planets in this research bolster the case
was not originally designed to look for As more TNOs have been discovered for Planet Nine, though. “If this were
minor planets. But study co-author over the years, astronomers have noticed the first dataset that came out, then no
that a small subset of these objects have one would have come up with the Planet
peculiar orbits. They seem to bunch Nine hypothesis because there appears
to be no clustering [in the orbits of any
new TNOs],” says study co-author Masao
Sako of the University of Pennsylvania.
However, he adds that this doesn’t
disprove the existence of Planet Nine
either. Their method could uncover other
TNOs that do support the planet, or
maybe even spot the world itself.

Regardless of whether Planet Nine
exists or not, understanding the orbits
and properties of TNOs will provide
insights into the history of the giant
planets. The distribution of TNOs could
also reveal the fates of past giants that
were kicked to the outskirts of the solar
system, or even out of the solar system
entirely, during its early years.

“It’s a fantastic example of how a sur-
vey designed for one area of astronomy
— to study the expansion history of
the universe — can also produce great
science in a completely unrelated area,”
says Alexander Mustill, an astrophysicist
at Lund University in Sweden who was
not involved in the study. — ERICA NAONE, J.P.

10 ASTRONOMY • JULY 2020

DECODING THE HERTZSPRUNG-RUSSELL DIAGRAM

–10 100 solar radii 1,000 solar 1,000,000

S U P E R G I A NTS radii

–5 10 solar radii Betelgeuse

Rigel 10,000

Spica

0 1 solar radius MAIN SEQUENCE Aldebaran GIANTS

Sirius 100 Luminosity (solar units)
Absolute magnitude5 0.1 solar radius 1
The Sun 0.01
ASTRONOMY: ROEN KELLY0.01 solar radiusTau Ceti
10
WHITE DWARFS Barnard’s Star
Sirius B Proxima Centauri

0.001 solar radius 0.001
15

Spectral class

20 O B AF GK M 0.0001
2,500
40,000 20,000 14,000 10,000 7,000 5,000 3,000

Surface temperature (kelvins)

CRYSTAL BALL. Since it was developed in the early 1900s, astronomers have

used the Hertzsprung-Russell (HR) diagram to categorize stars by size, FAST FACT

temperature, and brightness. The HR diagram is a simple but extremely Astronomers Ejnar

powerful tool for understanding a population of stars. A given star’s Hertzsprung and Henry Norris

location on this diagram is related to its age and mass; these properties Russell independently
allow astronomers to predict how that star will evolve by calculating the developed a diagram in 1911
track it will follow through this diagram over time. Main sequence stars, and 1913, respectively, to plot a
such as the Sun, are in the relatively stable hydrogen-burning phase of star’s color, or spectral class,
their lives. As these stars age and run out of hydrogen fuel, they will move against its absolute magnitude.

off the main sequence and onto the giant branch as they fuse helium or

heavier elements in their cores. The most massive stars ultimately reach the supergiant branch. But

when stars like the Sun can no longer generate fusion in their cores, they leave behind white dwarfs,

which only shine from residual heat and slowly cool over trillions of years. —A.K.

11

QUANTUM GRAVITY ALMA (ESO/NAOJ/NRAO), TAFOYA ET AL.

Researchers map dark
matter via slime mold

MOLDY MAP. Researchers seeded an algorithm — inspired by the food-seeking behavior of slime OLD STAR
REVEALS
mold — with the positions of about 37,000 galaxies. The galaxies served as the “food,” and the YOUNG JETS
resulting 3D map shows where the algorithm predicted invisible cosmic web filaments (purple) of
gas and dark matter connecting the galaxies (yellow) would be. NASA/ESA/J. BURCHETT AND O. ELEK (UC SANTA CRUZ) Planetary nebulae occur when
Sun-like stars puff off their
A brainless, single-celled organism with a The algorithm mimics the simple outer layers late in life. These
knack for finding food is helping astrono- organism’s food-seeking behavior, in which layers glow for a short cosmic
mers study the largest, most mysterious it deploys tendrils of reconnaissance mold time as radiation from the
structure in the universe: the cosmic web. to hunt for nearby food. If a specific mold star strikes them, pushing
And things could get a bit … slimy. thread stumbles on a meal, it thrives, creat- them outward. Although
ing a strong connection between the food some planetary nebulae are
The cosmic web is a vast network and the rest of the colony. So, by substitut- round, others are bipolar,
of interconnected filaments made of ing individual galaxies for the mold-based shaped like a butterfly or an
dark matter and cold gas that forms algorithm’s “food,” the researchers were hourglass. Bipolar nebulae
the scaffolding upon which the entire able to generate 3D maps that predicted are particularly interesting
universe is built. These filaments can where the galaxy-connecting filaments to astronomers, but they are
stretch hundreds of millions of light-years, of the cosmic web should be based on the also hard to study because
connecting groups and clusters of galaxies locations of existing galaxies. the gas and dust from the star
together. However, because the cosmic hides the innermost regions
web is incredibly faint and the dark matter This told researchers where to look for from optical telescopes. To
within it doesn’t interact with light, it’s the faint threads in archived observations. see past this veil, astronomers
extremely difficult to map. “Wherever we saw a filament in our recently used the Atacama
model,” lead author Joseph Burchett said Large Millimeter/submil-
To tackle this challenge, researchers at in a press release, “the Hubble spectra limeter Array (ALMA) to
the University of California, Santa Cruz, showed a gas signal.” This means the study W43A, a planetary
sifted through archived data for more than researchers were able to use the algorithm nebula 7,000 light-years away,
37,000 galaxies to chart their positions in both to effectively pinpoint where threads at radio wavelengths. ALMA
the sky. They then used a sophisticated of the cosmic web should be, and to actu- revealed the star’s fast-moving
algorithm to map out the underlying, ally find them. bipolar jets (blue), which are
invisible filaments of gas and dark matter blasting away from the star at
between those galaxies. They published “These results not only confirm the about 109 miles (175 kilome-
their results March 10 in The Astrophysical structure of the cosmic web predicted by ters) per second. Based on
Journal Letters. cosmological models,” Burchett said, “they this speed and the distance
also give us a way to improve our under- the jets have covered, the
The algorithm the researchers used standing of galaxy evolution by connecting researchers calculate the jets
to make their map isn’t your average it with the gas reservoirs out of which could be as young as 60 years
algorithm. It’s inspired by a slime mold galaxies form.” — JOHN WENZ, J.P. old. They also saw slower-
species called Physarum polycephalum. moving outflows (green) and
clouds of dust (orange) swept
up by the jets as they flow
outward, punching through
the material previously shed
by the star. —A.K.

12 ASTRONOMY • JULY 2020

Iron rains HEAVY METAL RAIN. On the nighttime side of the gas giant planet WASP-76 b, liquid iron falls as rain. The out-of-
from the
sky on this-world weather is pictured in this artist’s rendering. ESO/M. KORNMESSER
this world
Sun. Daytime temperatures SPectrograph for Rocky the light from the star passes,”
On the exoplanet WASP-76 b, can reach up to 4,300 degrees Exoplanets and Stable said study co-author Núria
the nighttime forecast often Fahrenheit (2,400 degrees Spectroscopic Observations Casasayas-Barris, a researcher
calls for rain. But the droplets Celsius) — hot enough to (ESPRESSO) on the European at the Instituto de Astrofísica
aren’t made of water. Instead, vaporize metal. Southern Observatory’s Very de Canarias and Ph.D. student
the downpour is made of iron. Large Telescope (VLT) in Chile. of the University of La Laguna
Winds driven by extreme ESPRESSO was originally in Spain, in a press release.
WASP-76 b is slightly temperature differences designed to study Earth-like
smaller than Jupiter and sits between the day and night planets around stars like our The technique worked: The
some 640 light-years from sides then push the vaporized Sun. However, the team sus- team’s discovery of iron rain
Earth in the constellation metal around the planet to the pected the VLT’s size would on WASP-76 b occurred during
Pisces. Its bizarre weather is nighttime hemisphere. There, also allow ESPRESSO to study ESPRESSO’s first-ever science
caused by its truly extreme drastically cooler temperatures the atmospheres of other exo- observations. And that means
orbit. Gas giant worlds like let the iron condense into planets in general. in the future, ESPRESSO may
WASP-76 b are called hot drops and fall as rain. Its dis- reveal even more unusual
Jupiters because they orbit covery is the first time astrono- “Ultrahot giant planets are weather patterns on planets
uncomfortably close to their mers have detected this kind the best laboratories we have circling other stars. “What
home stars — in this case, of day-to-night chemical dif- for studying extreme climates we have now is a whole new
nearly 10 times closer than ference on a hot Jupiter like on exoplanets. If we observe way to trace the climate of the
Mercury is to our Sun. WASP-76 b. The find was pub- an exoplanet during its tran- most extreme exoplanets,” said
lished March 11 in Nature. sit across the disk of its star, lead author and University
That proximity leaves we can study the part of its of Geneva astronomer David
WASP-76 b tidally locked to Researchers found the atmosphere through which Ehrenreich. — E.B., A.K.
its star, with one side perma- planet using the Echelle
nently baking in light and the
other stuck in eternal dark-
ness. As a result, WASP-76 b’s
dayside gets hit with thou-
sands of times more radiation
than Earth receives from the

23.5 Pop-up robot scouts practice their skills NASA/JPL CALTECH
The number
of hours in a NASA’s Jet Propulsion Laboratory (JPL)
day on Earth has a new team of small scouts ready
about 70 million to explore the Moon. The Autonomous
years ago, based Pop-Up Flat Folding Explorer Robot,
or A-PUFFER, is a shoebox-sized robot
on a study designed to go where astronauts cannot,
measuring how including lunar craters or caves that
humans might find inaccessible. Here, one
fast ancient of three A-PUFFERs tackles the Moon-like
clams grew. terrain of JPL’s Southern California Mars
Yard during a test in February. These
robots are designed to work together in
teams, collaboratively and autonomously
reconnoitering an area quickly to generate maps of previously unexplored regions. NASA hopes
a team of A-PUFFERs could be on its way to the Moon within a few years. — A.K.

WWW.ASTRONOMY.COM 13

STRANGE UNIVERSE

A tribute to carbon Consider:Earth’satmosphereis78percentnitrogen
and 21 percent oxygen, which leaves a mere 1 percent
for everything else. Almost all that residual stuff is the
The fuel for our cells is born in the stars. single element argon. Carbon dioxide is 20 times less

prevalent than even argon. Amazingly, CO2 only com-
prises 1/25 of 1 percent of the air we breathe. It’s barely

there at all.

With each exhale, we release a gaseous mixture that

is 78 percent nitrogen, 17 percent oxygen, 1 percent argon,

and 4 percent carbon dioxide. Comparing inhalations

with exhalations, the nitrogen and argon concentrations

have not budged because our bodies have no use for

them. The big change is that oxygen has gone down by

4 percent and carbon dioxide has gone up by 4 percent.

These numbers can also solve an issue that may have

troubled you: How can delivering the “breath of life” to

an accident victim be helpful if we’re only breathing out

useless CO2? Now you know: We’re not! The breath you
give to an unconscious person has nearly as much oxy-

gen as fresh air.

But back to our big baffling brouhaha. If we breathe

in air that’s only 1/25 of 1 percent carbon dioxide and

then breathe out 4 percent carbon dioxide, it means

The ruby-hued star We astronomers love carbon. Though nowa- we’re introducing a steady stream of enhanced carbon
VX Andromedae has days it gets unrelenting bad press thanks to into our planet’s air. Where does this carbon come
an atmosphere rich in climate change, carbon is so essential to life from? How can our bodies possibly cough up such an
carbon, which lets red that it grabs attention wherever we detect it. Who doesn’t unrelenting, nonstop stream of carbon when we breathe
light through while perk up when reading about organic compounds found in almost none of it?
scattering light that
is bluer and greener. Ever wonder about this? Want to stop and guess?

DAVID RITTER

in comets or in martian soil, or causing the redness on The answer is … sugar.

Titan? Indeed, an entire field — organic chemistry — is Most of what we eat gets changed into glucose, which

devoted to the study of material containing carbon. is then consumed by our cells. It’s also utilized by the

Carbon is the universe’s fourth-most mitochondria in each cell to supply us with

abundant element. During stellar life cycles, Every energy. These tiny organelles have their own
nuclear fusion turns hydrogen into helium, romantic life cycles and independent genetic material,
and helium into carbon and oxygen. So, and live inside us in a symbiotic way. It has

when reaching old age, Sun-like stars col- sigh that even been suggested by a panspermia advo-

lapse into tiny balls of oxygen and carbon. If escapes our cate that perhaps mitochondria came to
blown outward, carbon becomes lots of fun lips has a Earth via meteorites and comets, invaded
even when not creating life-forms. sweet early plant and animal bodies, and estab-
origin. lished a mutually beneficial relationship.
We enjoy observing stars surrounded by Regardless of their origin, we supply them
carbon dust. Because that element scatters

blue and some green light but lets red emis- with a warm, safe home and they give us

sion pass, such “carbon stars” are the reddest energy. And the raw material they use is glu-

objects anywhere. If you ever find yourself in the deep cose, whose chemical formula is C6H12O6. Our exhaled
south — not the tobacco-growing Carolinas but llama- carbon comes from that C.

loving Patagonia — point your scope at DY Crucis, The waste products of glucose metabolism are water

BY BOB BERMAN which hugs the Southern Cross’ leftmost star. It may be (H2O) and carbon dioxide (CO2). So the conclusion,
the reddest thing your telescope can visually display. little known by the public, is that every romantic sigh

Join me and Pulse As long as we’re pondering carbon, here’s a question that escapes our lips has a sweet origin.
of the Planet’s that should drive you crazy. And that sweetness, in turn, has its foundation in the
Jim Metzner
in my podcast, Back in school, they taught us that we breathe in stars.
Astounding Universe, oxygen and breathe out carbon dioxide. But how can
at www.astounding your body be constantly supplying that carbon for you BROWSE THE “STRANGE UNIVERSE” ARCHIVE
universe.com to exhale? AT www.Astronomy.com/Berman

14 ASTRONOMY • JULY 2020

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Stellar evolution is a circle of life —
dying stars spew their contents into
the galaxy, paving the way for the next
generation. BY JIM KALER

e live in a relatively quiet district of a galaxy 100,000 light-
years across that contains around 200 billion stars arranged
in a disk beset with spiral arms. As galaxies go, it’s pretty
big, though the supermassive black hole at the center is
relatively small, just 4 million solar masses. There is ample evidence
from the Milky Way’s rotation that our galaxy, like all others, con-
tains considerable dark matter, whose role in star formation eludes us.
But many of the processes of stellar evolution have become apparent,
most notably that through death comes life. Stellar evolution is cyclic,
with new stars replacing those that pass away.

Stellar building blocks

To forge a star you need gas, dust,
gravity, and violent stirring. From a
dark location in northern summer and
fall, an observer can see the Milky Way
cascading in its turbulent passage out
of Cygnus through Aquila, Sagittarius,
and south toward the Southern Cross.
Its glow is the combined light of the
billions of stars in our galaxy’s disk.

About 11,000 years ago, a star exploded in the
constellation Vela the Sails. This photo shows the
northern portion of the remnant, as well as the
pulsar that the stellar progenitor left behind.
Although the pulsar itself is invisible in this image,
it is located on the right-hand side. HAREL BOREN

Optical and radio observations show Thousands of stars are igniting within the vast 30 Doradus Nebula, located in the Milky Way’s largest
that gas is plentiful, and myriad opaque satellite galaxy, the Large Magellanic Cloud. The visible concentration of blue stars to the left of center is
patches without apparent stars reveal R136, which contains many of the most massive stars astronomers have ever spotted, some 100 times the
that dust is pervasive. mass of the Sun each. These huge, hot stars are blasting their birth nebula with winds of particles and
energetic radiation, while smaller stars that are still forming remain hidden within the cloud’s dusty depths.
The dust consists of microscopic min-
eral grains made of silicon, magnesium, NASA, N. WALBORN AND J. MA`IZ-APELL`ANIZ (SPACE TELESCOPE SCIENCE INSTITUTE), R. BARB`A (LA PLATA OBSERVATORY, LA PLATA, ARGENTINA)
iron, and many other metals, as well as
carbon in its varied forms. On average, these clouds are so thick that the Incas hydrogen and 10 percent helium —
our galaxy’s disk contains just one grain of South America made them into con- roughly similar to the Sun — and at these
per cubic meter. But there are a lot of stellations. Among the closest are the low temperatures, we would expect little
cubic meters between stars, so, overall, Taurus-Auriga clouds, which are only a chemical activity.
dust constitutes roughly 1 percent of the thousand light-years away, allowing us
total mass of interstellar matter. to study them in great detail. To the contrary, we find through radio
emissions that the clouds are filled with
While interstellar dust may be thinly Opaque clouds of interstellar dust molecules. More than 200 molecular spe-
spread, it also tends to clump together, keep out heat radiated by nearby stars, cies are present, dominated by molecular
even forming dense clouds. Some of and the gas within the dark clouds falls hydrogen (H2), but we also observe car-
nearly to absolute zero. The gas has a bon monoxide (CO, which is used as a
chemical composition of 90 percent tracer for the hard-to-observe hydrogen),

WWW.ASTRONOMY.COM 17

carbon dioxide (CO2), methyl alcohol events force the interstellar clouds into the supermassive black holes residing
(CH3OH), ethyl alcohol (CH3CH2OH), turbulent clumps, within which new stars in galactic cores.
and possibly even complex molecules are made. Given the low temperatures,
such as urea (CH4N2O) and others ragged blobs within the clouds condense, While the clouds are filled with T
important to life. Some molecules that causing their central cores to slowly heat Tauri stars, none of these stars is visible
do not exist on Earth abound in space, up. The cores eventually become hot to the naked eye. Moving outward per-
while many molecules responsible for the enough that they visibly glow, first with pendicular to the disk, the jets hammer
emissions we see remain unidentified. infrared radiation and then with visible the surrounding interstellar gas into
light, as heat is released by gravitational bright shock waves, which are common
The real showpieces are the gaseous, contraction. These developing protostars phenomena both on Earth and in the
dusty diffuse nebulae. These occur dot the dust clouds of Taurus, Auriga, universe in general. A shock wave is
where the interstellar clouds lie in close Orion, and many other such regions. formed in a fluid when a body moves
proximity to hot stars with temperatures faster than the natural speed of the wave
more than 26,000 kelvins or so. The Named after their faint prototype, within it, as with the bow-wave off the
ultraviolet radiation given off by these slightly older T Tauri stars appear highly prow of a speedboat. Here, this violent
stars can destroy molecules, ionizing variable as they sporadically gain mass, meeting results in glowing nebulae called
(removing electrons from) the interstel- accreting it from a disk of material Herbig-Haro (HH) objects, which occur
lar gas, which causes it to glow. With just swirling around their equator. At the where the jets are brought to a halt by the
binoculars, you can see the vast Orion same time, these stars lose mass via interstellar gasses. New stars appear as a
Nebula (M42) in the Hunter’s sword, powerful jets emerging from their poles. pair of HH objects connected by jets
as well as many other such nebulae. Amazingly, this disk/jet structure shows from the star in the middle. Four and a
Telescopes reveal jaw-dropping beauty. up not just in growing stars, but also in half billion years ago, the Sun would have
stars that are ejecting their outer enve- looked like this. In many cases, we see
Making new stars lopes as they prepare for death, in star only a single jet with or without its star,
systems where mass is being transferred as various portions of the structure can
Blast waves from nearby exploding stars, from one to the other, and even around be hidden by local dust clouds.
cloud-cloud collisions, and other violent

18 ASTRONOMY • JULY 2020

LEFT: The disk of the Milky Way contains vast
swaths of star-building material: gas and dust. The
latter is revealed by the apparent absence of stars
whose light the dust blocks out. This image
captures a portion of the dust-laden region
sometimes called the Great Rift, the entirety of
which runs from Centaurus to Cygnus. HYPATIA ALEXANDRIA

As a new protostar contracts under Deep within the Crab Nebula (M1) supernova remnant is its beating heart: the pulsar left behind when its
the force of gravity, the core heats. massive progenitor star finally ceased nuclear fusion and collapsed. The pulsar itself is the rightmost of the
Eventually the temperature becomes two brighter stars at the center of this Hubble Space Telescope image. Eventually, the nebula itself will drift
high enough to initiate nuclear reactions away into space, seeding future generations of stars. NASA AND ESA, ACKNOWLEDGMENT: J. HESTER (ASU) AND M. WEISSKOPF (NASA/MSFC)
(around 5 million kelvins), in which four
hydrogen atoms are turned into the next Main sequence dwarfs detectors allow us to look at the Sun’s core
heavier atom, helium, with a slight loss and show that our theories are correct.
(0.7 percent) of mass (m). Consequently, Once formed, the star remains stable as Trillions of them pass through you every
energy (E) is created according to it consumes its hydrogen fuel. Seventy second and you don’t feel a thing.
Einstein’s famous relationship E=mc2 percent of the Sun’s nuclear energy is
(c is the speed of light). The new source supplied by the proton-proton (pp) chain, The range of masses of hydrogen-
of energy brings the contraction to a halt whereby four protons join in a three-step fusing stars — called main sequence stars
as the star stabilizes at a central tempera- process to make helium, with the ejec- to differentiate them from stars that are
ture that depends on the new star’s mass tion of protons, gamma rays, and neu- dying — runs from 0.075 to over 120 solar
(the Sun, for example, stabilized at about trinos (near-massless particles that carry masses. For historical reasons, all of these
16 million kelvins). energy at nearly the speed of light). The ordinary stars are called dwarfs, but don’t
other 30 percent comes from the carbon let the term fool you. The comparatively
Multitudes of stars are often created cycle, in which carbon and hydrogen modest Sun — a yellow dwarf — is about
at roughly the same time, and their combine to create a chain of six reactions 864,000 miles (almost 1.4 million kilome-
mutual gravity binds them into an open that generate nitrogen, oxygen, and ulti- ters) across, while the most massive dwarfs
cluster with a large range of masses, like mately ends with carbon and helium, the are many times that. On the other hand,
the Pleiades (M45), the Hyades, or the former of which allows the cycle to begin the coolest red dwarfs are not much bigger
Beehive (M44). These clusters slowly again. This also produces gamma rays than Jupiter.
evaporate, their constituents dispersing and neutrinos, as well as positrons (posi-
with time. We believe our Sun may have tively charged electrons). There may be only a few monster stars
been born into one such cluster. in a galaxy, while dim red dwarfs consti-
Because our star is so dense, the heat tute up to 70 percent of the local stellar
Additionally, much of this action takes from the gamma radiation takes hundreds population. Below 0.075 solar mass,
place within the larger dark clouds and is of thousands of years to work its way out stellar cores are so cool that the pp chain
invisible until stellar radiation and winds of the Sun. By contrast, the neutrinos — won’t work, resulting in a brown dwarf
dissipate the parent dust clouds. When the unhindered by frequent interactions with that is still capable of fusing its natural
Sun was born, only a few other stars might other atoms — leave directly. Neutrino deuterium (hydrogen atoms with both a
have been visible from its location because
of the dust in the local birth cloud.

The prototype T Tauri is a young, variable star
whose brightness changes over time as mass from
its circumstellar disk falls onto the star. A nearby
nebula, Hind’s Variable Nebula (NGC 1555), shines
with reflected light from T Tauri. ATLAS IMAGE COURTESY OF

2MASS/UMASS/IPAC-CALTECH/NASA/NSF

WWW.ASTRONOMY.COM 19

STELLAR FEEDBACK

Mass

ABOVE: The young stars of the Beehive Cluster (M44) 200 AU
will slowly move apart over time. This 600-million-
year-old open cluster may be what our own Sun’s Galactic Interstellar
birth cluster looked like before it dispersed. STEPHEN RAHN spiral arms medium

proton and a neutron in the nucleus) Time
down to a mass of 1.2 percent the Sun’s
mass, or 13 Jupiters. However, we’ve From Earth, we see a side-on view of T Tauri star A 3-trillion-mile (4.8 trillion km) jet from a young star FROM LEFT: NASA, ALAN WATSON (UNIVERSIDAD NACIONAL AUTONOMA DE MEXICO, MEXICO), KARL STAPELFELDT (JPL), JOHN KRIST (STSCI) AND CHRIS
found planets around other stars heavier HH-30, which is only about half a million years old. (invisible inside dust at the lower left of the image) BURROWS (ESA/STSCI); J. MORSE/STSCI, AND NASA/ESA
than that, blurring the line between stars This image highlights the forming star’s jet and disk, forms Herbig-Haro object HH-47. The sinuous glow
and planets and leaving open key ques- the latter of which is split by a dark lane of dust. is caused when the powerful jet meets the
tions about how the two are formed. interstellar gas and dust.
Twice a giant
The luminosities of dwarf stars are fire will go out. No longer supported by
critically dependent on mass. At the low While details differ, the end products of the energy of fusion, the helium core
end, stars run entirely on the pp chain, stars in the midrange of stellar masses will shrink, as a thin shell of fusing
their cool reddish surfaces radiating at are similar. In 5 billion years, the Sun hydrogen surrounds it. Squeezing down
rates less than 1/1,000 that of the Sun. At will have converted its internal hydro- under gravity’s relentless fist, the core
the high end, they employ the carbon gen to helium and the central nuclear will also heat, causing the star’s outer
cycle and shine with the light of more
than a million Suns, allowing them to be
visible in other galaxies. Their brilliance
and winds are so powerful that they
shred the local interstellar gas and dust,
creating blobs that can contract and
form new stars, continuing a steady cycle
of birth and death that created our own
Sun and its planets.

Fusion rates climb so rapidly with
increasing mass and core temperature
that the lifetimes of stars actually decrease
as mass increases. They run from the age
of the galaxy — some 13 billion years —
for the least massive stars to just a few
million years for the most massive. In the
middle, the Sun has a hydrogen-burning
lifetime of about 10 billion years, of which
5 billion are history.

20 ASTRONOMY • JULY 2020

IN THE GALAXY Compression Cosmic rays
(shock waves) Enriched gas
Stars are the fundamental building and dust
blocks of the universe. A star’s Gas
evolution progresses at a rate that and dust
depends on its initial mass at birth.
(Sun-like stars advance down the Black holes
middle of this diagram.) As stars
age and ultimately die, they pave High mass
the way for new stars through
feedback such as shock waves, Supergiants Supernovae
gas and dust, and cosmic rays.
Gas Gas Neutron
ASTRONOMY: ROEN KELLY, AFTER JIM KALER

Gas and dust and dust stars

T Tauri Main Red Giant star Planetary White
stars sequence giants evolution nebulae dwarfs

stars

Low mass

Present

envelope to expand and cool as the star NGC 6543 (also called the Cat’s Eye Nebula; left) and NGC 6326 (right) are planetary nebulae that develop
brightens to become a giant. as Sun-like stars slough off their outer layers in the later stages of their lives. When light from the dying
star at the center of the debris field hits this gas and dust, the material glows, creating ethereal shapes.
When the core hits 100 million kel- Planetary nebulae ultimately fade over tens of thousands of years, as the central star becomes a white
vins, the helium nuclei that had been dwarf and slowly starts to cool. NASA, ESA, AND THE HUBBLE HERITAGE TEAM (STSCI/AURA); ESA/HUBBLE AND NASA
made earlier fuse into carbon, which
requires that three helium atoms hit with the light of thousands of Suns. As the second phase of brightening
each other simultaneously. The new Atoms heavier than the iron given to proceeds, winds blow ever stronger from
helium burning, plus the old hydrogen the star at birth begin to capture neu- the stellar surface. The Sun will lose half
fusion in the surrounding shell, once trons that decay into protons, making yet its mass this way, bigger stars losing much
again stabilize the star against collapse. heavier elements as the star begins to fill more, as they expose their hot inner cores.
In the core, when the newly made car- in much of the chemist’s periodic table. No longer supported by nuclear burning,
bon is hit with yet another helium
nucleus, it makes oxygen.

Now the process repeats itself. The
star is stuck with a core made of carbon
and oxygen with no means of support,
so it contracts and heats. Around it are
shells of fusing helium and hydrogen,
which alternately turn on and off. In the
right mass range, fresh carbon can be
swept to the surface by convection to
make a red carbon star.

Externally, the giant grows even big-
ger and brighter, perhaps becoming as
big as the inner solar system, radiating

WWW.ASTRONOMY.COM 21

RECIPE FOR A
TYPE IA SUPERNOVA

A type Ia supernova occurs when a white dwarf
gains mass, often from a binary companion. If the
white dwarf accretes enough material to push it
over the Chandrasekhar limit of 1.4 solar masses,
it will explode. Alternatively, two white dwarfs
circling each other can merge, also generating
such an explosion. G299, a 4,500-year-old
supernova remnant within the Milky Way (pictured,
far right), was created through one of these two
scenarios. ASTRONOMY: ROEN KELLY; NASA/CXC/U. TEXAS

ACCRETION SCENARIO

Companion star

Mass White
transfer dwarf

Companion star Supernova

ABOVE: The Cygnus Loop Nebula is all that remains into a space the size of a sugar cube). The with the extra mass, the chain can go
of a massive star that ended its life in a devastating star’s old outer envelope — rich in heavy further. Carbon and oxygen fuse to a
type II supernova explosion several thousand years chemical elements as well as carbon, mix that includes neon and magnesium,
ago. Now, the material that once made up that star nitrogen, and oxygen — flees into space, which then goes on to fuse to silicon and
will drift into space, ultimately providing the material leaving the still-glowing white dwarf sulfur before reaching iron. Each time
to form new stars. NASA/JPL-CALTECH behind. The rate at which white dwarfs the core initiates a new kind of fusion,
cool is so slow that every white dwarf it is surrounded by shells running the
RIGHT: Betelgeuse, which marks Orion the Hunter’s ever made since the beginning of the previous reactions. Fusion reactions that
right shoulder, is a red supergiant star. In 1996, it universe is still hot enough to be visible. create nuclei on the periodic table up
became the first star other than the Sun to be to iron generate energy. But above that
directly imaged. This Hubble Space Telescope Go out with a bang limit, creation of new and heavier ele-
image shows the star’s atmosphere; the large, bright ments requires energy. Iron is the most
spot visible in the center is roughly twice as wide as In a star of greater mass, hydrogen and tightfisted of all elements — it’s hard to
Earth’s orbit around the Sun and 2,000 kelvins helium fusion proceed as before. But break apart into its constituent protons
hotter than the surface of Betelgeuse. ANDREA DUPREE and neutrons, which is why it is so com-
mon. Externally, the star grows enor-
(HARVARD-SMITHSONIAN CFA), RONALD GILLILAND (STSCI), NASA AND ESA mously, becoming a supergiant. Such
stars could enclose the orbit of Jupiter,
the cores are held up by free electrons even nearly that of Saturn.
through a quantum process called degen-
eracy, which makes them incompressible. Around 1930, Subrahmanyan
Chandrasekhar discovered that when a
For a few tens of thousands of years, star’s core mass reaches about 1.4 solar
the exposed core remains hot enough to masses, Einstein’s theory of relativity tells
light up the shells of matter that it had us that electron degeneracy can no longer
previously ejected. The system becomes a support the star’s core. The whole mess
strikingly beautiful expanding planetary
nebula, while the inner core becomes a
white dwarf made of carbon and oxygen
with a density of a million grams per
cubic centimeter (the equivalent of com-
pressing 2,204 pounds [1,000 kilograms]

22 ASTRONOMY • JULY 2020

MERGER SCENARIO

White White
dwarf dwarf

Supernova

comes crashing down, as everything appears to flash at every rotation. Or, if them to measure the universe’s expansion
(including the iron in the core that took the star’s initial mass is high enough, a rate by comparing how far an object is
so long for the star to make and much of black hole will form with a gravitational expected to be with its actual distance.
the material in the enclosing shells) turns pull so great that nothing, not even light, The last two supernovae seen in our gal-
back into neutrons. We expect this to can escape. axy were Kepler’s Star in 1604 and Tycho’s
happen when the star’s initial mass Star in 1572. Both were type Ia. Before
exceeds about eight Suns. Double stars have their own tales to that was the type II Chinese “guest star”
tell. A star in a binary system can pass of 1054, whose violently expanding rem-
The resulting neutron star has a diam- some — even much — of its mass to a nant, the Crab Nebula (M1), can be
eter of about 12.4 miles (20 km, or about white dwarf companion. Alternatively, viewed with a small telescope. Hidden
the size of Manhattan) and a density a two mutually orbiting white dwarfs can inside this remnant is the pulsar left
million times that of a white dwarf. Upon merge. If the result in either case exceeds behind by the massive progenitor.
its birth, the neutron star first overcom- the Chandrasekhar limit of 1.4 solar
presses and then violently bounces back, masses, it will explode as a type Ia super- But this is not the end of the story. The
sending a monstrous shock wave through nova — which is even brighter than the expanding supernova remnant, rich with
what’s left of the star. This event blasts type II version and yields even more iron heavy elements, including mass injected
the material outward in a mighty type II — as the stars annihilate themselves, by the now-dead star’s giant and supergi-
supernova that sends the temperatures leaving nothing behind. ant winds, finds its way back to the inter-
into the billions of kelvins and can be stellar clouds. Its detritus becomes the
seen billions of light-years away. Because they all occur at the material that will ultimately make new
Chandrasekhar limit, type Ia supernovae stars, thus completing the cycle.
Nuclear reactions run amok, but as all have about the same maximum bright-
the ruined star expands, it also cools. ness. So, by measuring how bright they Jim Kaler is a professor emeritus of
This freezes in a specific distribution of appear, astronomers can easily determine astronomy at the University of Illinois at
elements, including one-tenth of a solar the distance to these objects. They are so Urbana-Champaign. His work on stellar
mass of iron. Left behind might be a bright that astronomers can see them evolution has received worldwide recognition.
spinning, highly magnetic pulsar that across the universe, and subsequently use

WWW.ASTRONOMY.COM 23

NASA/GSFC/SDO Some huge, some small. Some
zip, some crawl. The cosmos
is full of objects that defy
expectations. BY JAKE PARKS

THE SUN IS a pretty boring star. Still burning through the
hydrogen in its core, our middle-aged Sun is comfortable at its
current, relatively petite size. And though it will stay this way
for about 5 billion years more, our star will eventually run low
on hydrogen and switch to fusing helium deep within. This will
inflate the Sun into a red giant over the span of just a couple of
hundred million years. After engulfing the innermost planets,
possibly including Earth, the Sun will continually shed its outer
layers, eventually leaving behind a smoldering white dwarf
surrounded by a beautiful planetary nebula of glowing gas.

That’s the picturesque life that most stars live. But just like
people, some stars have wildly different experiences. So, let’s
do a quick review of some of the universe’s most extreme stars.

The familiar: Sun

Name: Sun
Type: Main-sequence (G2V)
Distance: 93 million miles

(150 million kilometers)
Radius: 432,000 miles

(696,000 km; 109 R )
ʇ

Mass: 1.99 x 1030 kilograms
(333,000 M )

ʇ

Luminosity: 3.83x1026 watts
Temperature: 5,780 kelvins

24 ASTRONOMY • JULY 2020

Lor
e
m

The biggest: UY Scuti

JUST LIKE IN the DC Name: UY Scuti UY Scuti Saturn Uranus
Universe, sometimes the Type: Red supergiant
clearest way for astronomers Neptune
to express something is truly (M2-M4Ia-Iab)
extraordinary is to add the Distance: 9,500 light-years*
prefix super. It’s the case with Radius: 1,700 R
Superman, as well as with
supergiant stars — a fitting ʋ
category for the largest known
star in the universe, UY Scuti. Mass: 7–10 M
ʋ
One day, the Sun will
become a red giant. But if Luminosity: 340,000 L
it had started its life with a ʋ
dozen or so times its current
mass, it could have eventually Temperature: 3,400 K
evolved into a red supergiant.
(UY Scuti has already shed a In 1860, astronomers
lot of mass.) The biggest of
these stars, sometimes called at Bonn Observatory in
hypergiants, can swell to
more than 1,000 times the Germany first cataloged UY
size of the Sun. But UY Scuti,
located near the center of the Scuti as part of a star survey. If the Sun were replaced by UY Scuti, the surface of the red supergiant would
Milky Way in the constella- But later, researchers noticed fall between the orbits of Jupiter and Saturn. ASTRONOMY: ROEN KELLY
tion Scutum, is around 1,700
times the Sun’s width. UY Scuti’s brightness

Although it’s difficult to pin down changes over a period of But like any red supergiant star no longer generates
the exact traits of any given star,
based on what we know, the largest about 740 days, leading them — including Betelgeuse — enough outward pressure
star is UY Scuti, which is some
1,700 times as wide as the Sun. to reclassify it as a variable UY Scuti is destined to end to keep it from imploding

ASTRONOMY: ROEN KELLY, AFTER SCTESTER star. Some of these stars vary its life with a bang. After under its own gravity.

in brightness for external rea- exhausting the helium fuel in The end result? A power-

sons, such as being eclipsed its core, it will ferociously ful core-collapse (type II)

by another star or clouds of forge increasingly heavy ele- supernova that will finally

gas and dust from our van- ments. And as long as UY — albeit briefly — make UY

tage point. However, intrinsic Scuti doesn’t expel too much Scuti visible to the naked eye

variables like UY Scuti expe- mass over the course of its from Earth.

rience physical changes remaining life, it will eventu-

within, such as pulsations. ally start producing iron. *DISCLAIMER: Astronomers often

In the case of UY Scuti, it Making iron is a death calculate the physical properties
varies in brightness because sentence for stars. Unlike of stars by comparing available
it’s constantly yo-yoing in when it combines lighter ele- data to best-fit models — meaning
terms of size — making exact ments, when a star forces two estimates can vary wildly depending
on whom you ask. And this is

measurements of its girth a iron nuclei together, it doesn’t definitely true for UY Scuti. Based

challenge. release any energy; it instead on parallax data obtained by the
takes energy away from the Gaia satellite and released in 2018,
environment. This causes a some think UY Scuti might be only
runaway collapse where the about 5,100 light-years away, which
would make it much smaller and less

luminous. In that case, with a reliable

size of about 1,400 times as wide as

the Sun, VY Canis Majoris would be

UY SCUTI | Red supergiant a prime contender for biggest star.

Diameter: 1.47 billion miles
2.37 billion km

SYMBOL KEY SUN | Yellow dwarf

R = 1 Earth radius Diameter: 864,000 miles
ʇ 1.39 million km

M = 1 Earth mass
ʇ

R = 1 solar radius
ʋ

M = 1 solar mass
ʋ

L = 1 solar luminosity
ʋ

The most massive: RMC 136a1

Name: RMC 136a1 LOOKS CAN BE deceiving. stars that’s ionizing the gas a rare Wolf-Rayet star, it’s
Type: Wolf-Rayet Just because a star is a certain within NGC 2070. This huge incredibly hot, chock-full of
size doesn’t mean it has a cer- open star cluster lies in heavy elements, and sports
(WN5h) tain mass. That’s absolutely the heart of the Tarantula extremely powerful stellar
Distance: 163,000 the case with the most massive Nebula, which is the brightest winds that are blowing off
known star in the universe — star-forming region in our its outer layers.
light-years RMC 136a1 — which packs a galactic neighborhood.
Radius: 30 R lot of heft into a surprisingly Thanks to Hubble Space These stellar winds are so
slim frame. Although thought Telescope observations, powerful — reaching a veloc-
ʋ to be more than 300 times the astronomers know RMC ity of around 5.8 million mph
mass of our Sun, RMC 136a1 136a1 is just one of more
Mass: 315 M is only about 30 times as wide than 200 bright, massive FAST FACT
ʋ as our home star. stars in the immediate area,
all found within a cluster Although RMC 136a1 is expected
Luminosity: Located in the Milky Way’s called RMC 136. However, to lose nearly 85 percent of its
8,700,000 L largest satellite galaxy, the RMC 136a1 is the bright- mass over the course of its life, it
Large Magellanic Cloud, est of these beacons. In
ʋ addition to holding the will still be heavy enough to
RMC 136a1 is just one title for the most massive collapse into a black hole
Temperature: of many blazing star, RMC 136a1 also takes after going supernova.
53,000 K the crown for the most
luminous star. (9.4 million km/h) — that by
the end of its life, the star is
Although the exact age of expected to expel enough gas
this stellar heavyweight is to end up weighing just over
still uncertain, according to 50 solar masses. However,
that’s still plenty big enough
a 2016 study, RMC 136a1 to produce an astounding
could be as young as a supernova. After all, the pro-
few hundred thousand genitor of Supernova 1987A,
to a million years old, also located in the Large
so it’s thought to still Magellanic Cloud, was
be burning hydro- only about 20 solar masses.
gen in its core.
And because
RMC 136a1 is

Massive and luminous stars like RMC 136a1 sport extremely
powerful stellar winds, which are streams of charged
particles flowing from the star’s surface. They also emit
intense ultraviolet radiation that would be strong enough
to sterilize the surface of Earth. ASTRONOMY: ROEN KELLY

26 ASTRONOMY • JULY 2020

The smallest: EBLM J0555-57Ab

JUPITER EBLM J0555-57AB
Gas giant Low-mass star

SATURN TRAPPIST-1
Gas giant Low-mass star

Astronomers found tiny star EBLM J0555-57Ab only when it passed in front of its larger Name: EBLM
binary companion, which blocked some of the bigger star’s light. Detecting such a transit J0555-57Ab
is also the way researchers find many exoplanets. AMANDA SMITH, UNIVERSITY OF CAMBRIDGE
Type: Unknown
WHEN IT COMES mass of Jupiter and understood, brown Distance: 630
dwarfs are not-quite-
to stars, size matters. a sliver wider than planets, not-quite-stars light-years
whose cores can only Radius: 0.084 R
If a star is exceptionally Saturn, EBLM J0555- fuse a heavy form of
hydrogen called deute- ʋ
massive, it gobbles up 57Ab skirts the lower rium, as well as possibly
lithium. “Understanding Mass: 0.081 M
its fuel, causing it to boundary of what it the boundary that sepa- ʋ
rates stars from brown
live fast and die hard. takes to be a star. dwarfs will improve our Luminosity:
understanding of how 0.0002 L
However, if a star is “Our discovery both form and evolve,”
Serge Dieterich of the ʋ
small and light, it has reveals how small stars Carnegie Institution for
Science, an astronomer Temperature:
a slow metabolism, can be,” lead author who studies the smallest 2,300 K
stars, said in a statement.
allowing it to live an Alexander Boetticher — just a little more
EBLM J0555-57Ab massive than EBLM
extremely long life. But of the University of may be tiny, but there J0555-57Ab. And
are other stars out there because stars with less
just how small can a star Cambridge said in a that compare with its than 25 percent the
puny mass. For instance, Sun’s mass are the most
be? Well, EBLM J0555- press release after find- the star TRAPPIST-1, common type of stars
which hosts at least and excellent candidates
57Ab is right at the limit. ing the diminutive star seven rocky planets, tips for hosting Earth-sized
the scales at 0.089 M planets, learning more
At just 85 times the in 2017. “Had the star about the lives of the
ʋ smallest stars may help
formed with only a researchers uncover
potentially habitable,
slightly lower mass, Earth-like planets
around them.
FAST FACT the fusion reaction
of hydrogen in its

The most massive stars may core could not
only live for a few million years, be sustained,
but the smallest stars can slowly and the star

burn through their hydrogen would instead

over the course of hundreds have trans-

of billions or even trillions formed into a
of years. brown dwarf.”

Although not well

WWW.ASTRONOMY.COM 27

The hottest: WR 102

Name: WR 102 “ The flame that burns FAST FACT result of rapidly
Type: Wolf–Rayet twice as bright burns converting
”half as long. — Laozi Researchers expect hydrogen to
(WO2) WR 102 to erupt as a Type Ic helium in
Distance: 9,500 supernova, meaning it will its fiery
core via the
light-years expel its outer layers of C-N-O cycle.
Radius: 0.5 R hydrogen and helium

ʋ before exploding.

Mass: 17 M WHILE STARS MAY not only burn incredibly However, the
ʋ
follow the exact ratio set by hot and bright, but hottest star, WR 102,
Luminosity:
280,000–380,000 L this quote from the 6th- their stellar winds also is an especially rare

ʋ century-b.c. Chinese work Tao blast much of their potential WO-type Wolf-Rayet, which

Temperature: Te Ching, the gist holds true. fuel into space. The hottest is a late-stage star that has a
210,000 K

The faster a star burns through known star, WR 102, is one surface heavily enriched with

its fuel, the shorter its life. And such Wolf-Rayet, sporting a ionized oxygen. All said,

this is surely the case for Wolf- surface temperature more than astronomers only know of

Rayet stars. These stars not 35 times hotter than the Sun. about 10 WO-type Wolf-Rayet

Like Baskin-Robbins, Wolf- stars in the entire universe.

Rayet stars come in a variety of Even for a Wolf-Rayet star,

flavors. The most massive star, WR 102 has intense stellar

RMC 136a1, has a spectral winds. Currently, they are

type of WN, meaning blowing about a Sun’s worth of

it’s rich in ionized mass from the star’s surface

nitrogen as a every 100,000 years. That

means WR 102 is losing sev-

eral hundred million times

more mass each year than the

Sun. Although that may not

seem like much for a massive

star, keep in mind that at this

rate, WR 102 would be com-

pletely gone in less than 2 mil-

lion years. But who can wait

that long?

Astronomers are interested

in WR 102 not just because of

its exceptionally hellish sur-

face temperature and rapid

mass loss, but also because

the star is a prime candi-

date to go supernova in the

relatively near future. In a

2015 paper that explored

WR 102 how much time a variety
of WO-type Wolf-Rayets

have left before exploding

as supernovae, WR 102

was found to have the

worst prognosis.

According to the

WR 102 hides near the center authors: “WR 102 is
of the nebulosity captured a post-core helium burn-
in this infrared image. The ing star and has a remain-
star’s extreme radiation is ing lifetime of less than
ionizing the surrounding gas, 2,000 years.”
causing it to glow. JUDY SCHMIDT

28 ASTRONOMY • JULY 2020

The fastest: S5-HVS1

THE SUN ZIPS through to a process FAST FACT
space at a brisk 490,000 mph called the Hills
(790,000 km/h) relative to the mechanism, Astronomers have seen stars
Milky Way. That’s fast, but it’s which was out- travel faster than S5-HSV1, but not
nothing to crow about. Instead, lined some three consistently. In 2018, the star S2, which
the bragging rights for the fastest decades ago by passes very near Sagittarius A* every
known star (that’s not a white astronomer Jack 16 years, reached a top speed of some
dwarf) belong to a speed demon Hills. The idea is 17 million mph (27 million km/h) during
known as S5-HVS1. This that S5-HVS1 was its closest approach to the black
middle-aged, hypervelocity star once part of a binary
is fleeing our galaxy at more system that tangoed hole — that’s almost 3 percent
than 3.9 million mph (6.3 million with Sagittarius A*. the speed of light.
km/h). For reference, that’s about When the stellar pair
0.6 percent the speed of light. ventured too close, the Astronomers think S5-HVS1
black hole captured the achieved such a breakneck
Astronomers first found the companion star, releasing speed following its ejection
star streaking through the south- S5-HSV1 from its binary dance from a binary system that
ern constellation Grus in 2019. and flinging it through space. passed too close to the Milky
After tracing its orbit back in Way’s central black hole, as
time, they quickly realized it is “My favorite part of this dis- seen in this artist’s concept.
coming from the center of the covery is thinking about where
Milky Way, near our roughly this star came from and where JAMES JOSEPHIDES (SWINBURNE ASTRONOMY
4-million-solar-mass supermas- it’s going,” co-author Alex Ji said. PRODUCTIONS)
sive black hole, Sagittarius A*. “It was born in one of the craziest
places in the universe, near a WWW.ASTRONOMY.COM 29
“This is super exciting, as we supermassive black hole with lots
have long suspected that black of other nearby star friends; but
holes can eject stars with very it’s going to leave our galaxy and
high velocities. However, we die all alone, out in the middle of
never had an unambiguous asso- nowhere. Quite a fall from grace.”
ciation of such a fast star with
the galactic center,” Sergey That may not be an ideal life
Koposov of Carnegie Mellon for a star, but at least it didn’t
University, lead author of the suffer the fate of its companion.
study, said in a press release. “We
think the black hole ejected the Jake Parks is associate editor of Astronomy.
star with a speed of thousands
of kilometers per second about
5 million years ago. This ejection
happened at the time when
humanity’s ancestors were just
learning to walk on two feet.”

Though single now, research-
ers suspect S5-HVS1 wasn’t
always alone. The evidence sug-
gests the star was ejected thanks

Name: S5-HVS1
Type: Main-sequence (A)
Distance: 29,000 light-years
Radius: 2.5 R

ʋ

Mass: 2.35 M
ʋ

Luminosity: 10 L
ʋ

Temperature: 10,000 K

Gliese 380

Distance: 15.9 light-years

Gliese 687

Distance: 14.8 light-years

Gliese 725
A and B

Distance:
11.5 light-years

Gliese 1245
A, B, and C

Distance:
14.8 light-years

Astronomers are learning a lot from the 90° 61 Cygni
stars in our part of the Milky Way. Kruger 60 A and B
A and B
BY BRUCE DORMINEY Distance: Distance:
13.1 light-years 11.4 light-years

hat do we really know Runaway star Plane of the Milky Way Ross 248 180°
about our Sun’s nearest
stellar neighbors? At just 6 light-years away, Barnard’s Distance:
We get so consumed with what’s 10.3 light-
beyond our galaxy that we forget that
our own stellar neighborhood is pretty years
fascinating. Sure, we’re surrounded by
mostly faint red dwarf stars, but they Star has the fastest proper motion GX Andromedae and
are our nearest stellar neighbors and (motion across our line of sight) of any GQ Andromedae
we interact with them on timescales known star and is steadily making its
of tens of thousands of years. Distance: 11.7 light-years
With the exceptions of Sirius, the
brightest star in the night sky, and way north through eastern Ophiuchus.
Alpha Centauri, none of these stars
shows up easily to the naked eye. But Its discovery serendipitously stemmed
they have influenced our solar system’s
evolution in the past and will continue from a number of galactic surveys
to do so in the future.
We often hear about our neighbors started by American astronomer
in the Alpha Centauri system, which
includes Proxima Centauri, the closest Edward Emerson Barnard in the early
known star to Earth at only 4.2 light-
years away. But let’s take a look at some 1890s, when he began using Yerkes Teegarden’s Star
other fascinating stars, all of which lie Observatory’s 40-inch refractor.
within 15 light-years of Earth. Distance: 12.6 light-years

In 1916, while examining a pho-

tograph of a region in Ophiuchus,

Barnard noticed what he thought TZ Arietis
was an undiscovered star. By compar-
Distance: 14.5 light-years

ing an August 1894 plate of the same Van Maanen’s Star
region, he found the magnitude 9.5
Distance: 14.0 light-years

star — but some 4' from its 1916 posi-

tion. Examination of a 1904 plate of the

same part of the sky showed that this

new star was traveling at a rapid pace.

It was later christened Barnard’s Star.

30 ASTRONOMY • JULY 2020

Gliese 412 A and B Wolf 424 A and B KEY

Distance: 15.9 light-years Distance: 14.3 light-years A-class dwarf
F-class dwarf
Gliese 388 G-class dwarf
K-class dwarf
Distance: M-class red dwarf
15.9 light-years T-class brown dwarf
White dwarf
Lalande 21185 Ross 128 LHS 292

Distance: Distance: 10.9 light-years Distance: 14.8 light-years
8.3 light-years
Wolf 1061
DX Cancri
Distance: 13.9 light-years
Distance:
11.8 light-years Wolf 359

Distance:
7.8 light-years

Barnard’s Star 0° DENIS 1048–3956

Distance: Alpha Centauri A Distance: 13.1 light-years
6.0 light-years
Ross 154 Distance: 4.4 light-years
Procyon A and B
Distance: 9.7 Alpha Centauri B
Distance: light-years
11.4 light-years Distance: 4.4 light-years

Sun Proxima Centauri

Distance: 4.2 light-years Gliese 674 Gliese 440

Distance: Distance:
14.8 light-years 15.1 light-years

Luyten’s Star Sirius 270°
A and B
Distance:
12.3 light-years Distance: 8.6 light-years

LHS 288

Distance:
15.6 light-years

SCR 1845–6357 A and B

Distance: 12.6 light-years

EZ Aquarii Ross 614 A and B AX Microscopii Epsilon Indi A, This diagram shows the stars
A, B, and C B, and C out to 16.2 light-years from
Distance: Distance: the Sun. Two-thirds are
Distance: 13.3 light-years 12.9 light-years Distance: 11.8 light-years M-class dwarf stars. The
11.3 numbers are the distances in
BL Ceti and Gliese 1061 Kapteyn’s Star light-years, as measured by
light-years UV Ceti the European Space Agency’s
Distance: Distance: Hipparcos satellite. The sizes
Ross 780 Distance: 12.0 12.8 light-years shown are scaled relative
8.7 light-years light-years to each other, not to the
Distance: Gliese 832 distances between them.
15.2 Lacaille 9352 All stellar data are from the
Distance: Research Consortium on
light-years Distance: 16.1 light-years Nearby Stars. ASTRONOMY: RICHARD
10.7 light-years
YZ Ceti TALCOTT AND ROEN KELLY
Epsilon Eridani
Distance:
12.1 light-years Distance: 10.5 light-years

Tau Ceti

Distance: 11.9 light-years

Gliese 1002 Gliese 1
Distance:
Distance: 14.2 light-years
15.3 light-years Omicron 2

Eridani A, B, and C DENIS 0255–4700
Distance: 16.2 light-years
LP 944-20 Distance: 16.2 light-years
Distance: 16.2 light-years

WWW.ASTRONOMY.COM 31

Barnard’s Star in Ophiuchus lies 6 light-years away, making it the nearest single star to us; it also has The term “dog days of summer”
the largest proper motion of any known star. An exoplanet more than three times as massive as Earth comes from the ancient Greeks, who
orbits Barnard’s Star. This artist’s impression shows the planet’s surface. ESO/M. KORNMESSER believed Sirius’ annual appearance
brought the worst of the summer’s hot
At some 10 billion years old, roughly refraction variations, as well as color and dry period. They feared this bright
twice the age of our Sun, Barnard’s Star effects, says Philip Ianna, an astrono- star would literally cause people to go
is among the oldest within Earth’s vicin- mer at the Research Consortium on mad, and called anyone thought to be
ity. This red dwarf is the closest known Nearby Stars Institute in Chambersburg, affected by Sirius’ brightness “star-
single star to our Sun and appears rela- Pennsylvania. struck.” That’s hardly the current use of
tively inactive. the term, but it does explain its origins.
“The Sproul people deserve credit for Thus, the days coinciding with Sirius’
Aside from its high proper motion, working diligently to sort out and under- annual reappearance from July 3 through
Barnard’s Star is best known as the star stand their flaws,” says Ianna. “Other August 11 were dubbed “dog days.”
that became an obsession for Dutch- long-focus refractors are not known to
born American astronomer Peter van de have ‘flaws’ because no one has studied Sirius itself is a young A-spectral type
Kamp. By the early 1960s, van de Kamp them adequately.” star thought to be only 250 million years
was absolutely certain that the star har- old. Although it shows no signs of plan-
bored at least one or two planets. In 2018, Barnard’s Star finally got a ets, it doesn’t travel through space alone.
bona fide planetary detection. But its
Van de Kamp arrived in 1937 3.2-Earth-mass planet lies close to the In 1844, German astronomer Friedrich
at Swarthmore College’s Sproul system’s so-called “snow line,” where Bessel deduced from changes in Sirius’
Observatory, where he began surveying water condenses into ice. Thus, even proper motion that it has an unseen com-
nearby stars with the 24-inch refrac- though the planet is close to its parent panion. In 1862, American telescope
tor. Over the following decades, he took star, 0.4 astronomical unit away (1 AU maker Alvan Graham Clark was testing a
thousands of plates of Barnard’s Star (as equals the average Earth-Sun distance), lens in Cambridgeport, Massachusetts,
I note in my book Distant Wanderers: it may still be inhospitable. when he first observed Sirius B.
The Search for Planets Beyond the Solar
System [Copernicus, 2002]). By 1963, van According to Eric Mamajek, an Then, in 1915, the 60-inch reflector at
de Kamp had accumulated enough data astronomer at the University of Mount Wilson observatory in California
to announce that it had a perturbation Rochester in New York, future study of characterized Sirius B as a whitish star in
in its proper motion, which he claimed Barnard’s Star will likely tell astronomers roughly a six-year orbit around Sirius A.
indicated a planet 1.6 times the mass of how planet formation turns out for a star With this new data in hand, astronomers
Jupiter with an orbital period of 25 years. about one-sixth the mass of the Sun, but then concluded that Sirius B was a white
that has only about a third, or less, of its dwarf, a stellar remnant left behind by a
Van de Kamp’s claims were largely metal content. low-mass star.
discounted after it became known that
the telescope had a history of structural Winter’s gem Sirius B used to be bigger and more
problems that contributed to the star’s luminous than Sirius A, probably about
perceived perturbations. His images Sirius (Alpha [α] Canis Majoris) cap- five times the mass of the Sun and nearly
were also generally underexposed, which tured the attention of early skygazers. a thousand times as luminous during the
enhanced small displacements of one star Its heliacal rising (its first visibility in time it was a main sequence star, says
in comparison to another. the east before sunrise) each year made Mamajek. But he says the star exhausted
it important to the Egyptians, who wor- its fuel, became a red giant, and blew off
All refractors have similar issues: tube shipped it as the goddess Sopdet and
and lens flexure, thermal lens aberra- saw its appearance as heralding the Nile Sirius (Alpha [α] Canis Majoris) is the brightest
tion changes, atmospheric seeing, and River’s beneficial annual flooding. star in the night sky. This image shows Sirius B,
its white dwarf companion, to the lower left.

NASA, ESA, H. BOND (STSCI), AND M. BARSTOW (UNIVERSITY OF LEICESTER)

32 ASTRONOMY • JULY 2020

e

cr d _
CETUS Deneb
j
NGC 7000 i
CYGNUS
a IC 1318

Tau (o) Ceti m

5° o 61 Cygni
h
ABOVE: Tau (τ) Ceti lies 12 light-years
from the Sun. At magnitude 3.5, it’s one of RIGHT: At magnitude 5.2, p ¡
the few nearby stars visible to the naked 61 Cygni is one of the few 5°
eye. Two of its four planets may lie within nearby stars visible to the Veil
the star’s habitable zone, a region where unaided eye. It was the first Nebula
liquid water could exist if the planet has a star to have its parallax
high enough atmospheric pressure. measured. ASTRONOMY: RICHARD

ASTRONOMY: RICHARD TALCOTT AND ROEN KELLY TALCOTT AND ROEN KELLY

its outer layers about 120 million years to detect interstellar transmissions on his observations of 61 Cygni, a binary
ago, leaving behind a dimming, cooling from the vicinity of the star using the star system 11.4 light-years away in
white dwarf. 26-meter telescope at the National Radio Cygnus. Unfortunately, Strand was using
Astronomy Observatory’s Green Bank the Sproul Observatory refractor when
So, the total mass of the system in the facilities in West Virginia. Of course, he he claimed that 61 Cygni had an unseen
two stars was probably more like seven came up empty, but it marked an impor- companion 16 times the mass of Jupiter.
times that of the Sun, says Mamajek. tant milestone in the advancement of the His claims were never substantiated. In
But now, only about 3 solar masses search for extraterrestrial intelligence. fact, 61 Cygni is a binary system which
remain in the system: 2 solar masses in consists of two K dwarf stars.
Sirius A and one in Sirius B. Tau Ceti potentially harbors four
super-Earth-mass planets, two of which However, 61 Cygni remains histori-
Researchers don’t think the Sirius could be habitable. If the planets are con- cally important because it was the first
system harbors planets. That’s in part firmed, it would be the nearest solar-type star other than our Sun to have its paral-
because the separation between the stars star to host super-Earth habitable-zone lax (shift in position due to Earth’s orbit)
changed when Sirius B expelled most of planets, says Edward Guinan, an astron- measured. This accurate measurement
its mass, which may have caused plan- omer at Villanova University. for a star was a critical discovery, says
etary orbits to destabilize — and perhaps Mamajek. It was the first time the actual
even cross. Slightly cooler and less luminous, distance to another star was measured;
but with a longer lifetime than our Sun, as a result, he says, the floodgates opened
This means that any putative planets Tau Ceti has radiation and evolutionary on measuring distances to other stars.
would have been in a constantly chang- characteristics that make it and other
ing dynamical environment, says Adam cooler G and K dwarf stars near-perfect Engage!
Kraus, an astronomer at the University hosts for habitable planets, says Guinan.
of Texas at Austin. He says the odds are For these reasons, we refer to them as As the fifth-nearest star system and an
high that, at some point, any planet Goldilocks stars, he says. These less active red dwarf, Wolf 359 has earned a
would have been either kicked out of the luminous stars have main sequence, special place in the hearts of Star Trek
system or kicked into one of the stars. hydrogen-burning lifetimes that are typi- fans. It’s where the United Federation of
cally two to three times longer than our Planets suffered a devastating defeat at
“It would have been astounding to Sun, and Guinan says planets orbiting the hands of the Borg Collective in the
witness the changes that have taken place them would, in theory, have stable long- year 2367, in the TV series Star Trek: The
in the Sirius binary system over its astro- term climates. Next Generation.
physically short life span,” says Mamajek.
No answer “I have to admit that I started observ-
Anyone home? ing Wolf 359 over 20 years ago in part
As I also note in my book, “the late Kaj because of its Star Trek fame,” says
Tau (τ) Ceti, a Sun-like spectral type G Strand, the first astronomer to claim the Guinan. Thought to be less than a billion
star only 12 light-years away in Cetus, is ‘detection’ of an extrasolar planet, did years old, it’s an M dwarf that lies only
the closest known solitary G-type star to so when the rest of the world was more 8 light-years away in Leo. Unfortunately,
the Sun. It’s best known as the first star concerned about events here on Earth.” Guinan reports that there have been no
to be searched for signs of intelligent life. I write that Strand based his 1943 claim planets observed circling Wolf 359.
That’s because in April 1960, American
astronomer Frank Drake attempted

WWW.ASTRONOMY.COM 33

e d This M dwarf, located some 12 light-
years away in Aries, was found in 2003
Denebola LEO after now-retired NASA Goddard Space
` f Flight Center astrophysicist Bonnard
Teegarden began looking for high-
Wolf 359 _ i proper-motion stars using Near-Earth
r Regulus Asteroid Tracking data.
m
l “Initially, astronomers thought the
star was among the nearest, but more
5° accurate parallax measurements show
that it is at a distance of 12 light-years,
Wolf 359 is a red dwarf in the constellation Leo the Lion. It lies 7.9 light-years from Earth and glows making it the 27th-closest star,”
faintly at magnitude 13.5. Its luminosity is only 1/50,000 that of the Sun. ASTRONOMY: RICHARD TALCOTT AND ROEN KELLY Teegarden says.

Heading our way silicon needed to form planets are defi- The nearest now
cient in the Kapteyn system.
The red dwarf star Ross 128, which The four closest systems are currently
American astronomer Frank Elmore “It’s good to see that super-Earth believed to be Alpha Centauri (A, B, and
Ross placed in a catalog of high-proper- planets can form in such low-metal envi- Proxima), Barnard’s Star, WISE 1049–
motion stars, is the 12th-closest stellar ronments,” says Guinan. He says this 5319, and WISE 0855–0714.
system to Earth. It lies only 11 light- gives hope that the hundreds of millions
years away in the constellation Virgo of metal-poor stars located in the oldest WISE 1049–5319 is a young binary
the Maiden and is now known to have and most metal-poor parts of our Milky system consisting of two brown dwarfs,
an Earth-sized exoplanet that orbits this Way Galaxy could also host planets. discovered in 2015 and located some
faint star roughly every 10 days. 6.5 light-years away in Vela. WISE 0855–
“When I think about the planet can- 0714 lies at a distance of just over 7 light-
Although that’s incredibly close to its didate Kapteyn b, I wonder about the years in Hydra. The two have bumped
parent star, the planet receives only 1.38 possibility of life some 6 billion to Wolf 359 from the Sun’s third-nearest
times more radiation than Earth. As a 7 billion years older and possibly more stellar neighbor.
result, its temperature is estimated to advanced than us,” says Guinan.
range up to as high as 68 degrees Guinan wonders whether WISE
Fahrenheit (20 degrees Celsius). But it’s “Future study of the Kapteyn system,” 0855–0714 could even be a rogue planet.
still uncertain whether the planet, desig- says Mamajek, “will help astronomers “WISE 0855–0714 is very faint and cool,
nated Ross 128 b, lies inside this system’s learn how planetary systems around low- and is more like a large Jupiter planet
habitable zone, much less whether its mass stars evolve when the parent star than a star,” says Guinan. “If it is a super-
surface harbors liquid water. only starts out with a metal content that Jupiter, it may have been ejected from its
is a tenth that of our Sun.” birthplace star system and is more like a
Ross 128 is moving in our direction. rogue planet. If any of these rogue plan-
In less than 80,000 years, it’s expected to Successful search ets got too close to our solar system, it
replace Proxima Centauri as our nearest would be disastrous for us.”
stellar neighbor. Finally, there’s Teegarden’s Star, which is
the only star named after a living person. One passage of a body like WISE
0855–0714, even at a distance of 50 AU,
could disrupt the solar system, he says.

Not so heavy COLUMBA `
CAELUM
Lying some 13 light-years away in the far-
southern constellation Pictor, Kapteyn’s d _ _ Kapteyn’s Star,
Star likely takes the cake as one of the b which glows at
most metal-poor stars out there. (Metals, Kapteyn’s Star magnitude 8.9,
to astronomers at least, are any elements PICTOR lies in the
heavier than helium.) Yet it is thought southern
to be orbited by two super-Earth-mass constellation
planets. Given that Kapteyn’s Star likely Pictor the Painter,
originated in the halo of our galaxy, this 12.8 light-years
would make its planets some of the old- away. In 2014,
est ever detected in the habitable zone of astronomers
another star. discovered two
planets orbiting it.
Thought to be some 11.5 billion years
old, Kapteyn’s Star has only 13 percent as ASTRONOMY: RICHARD
much metal as the Sun, says Guinan.
This would imply that the iron and TALCOTT AND ROEN KELLY

`c h
e 5°

DORADO

34 ASTRONOMY • JULY 2020

Stellar census Alpha Centauri AB

Just how many nearby M dwarfs and Proxima Centauri
smaller stars might be nearby?
Proxima Centauri (Alpha [α] Centauri C) is the nearest star to us, not counting the Sun. It glows dimly at
There may be some stars within a few magnitude 11.1 and lies 4.2 light-years away. DIGITIZED SKY SURVEY 2/DAVIDE DE MARTIN/MAHDI ZAMANI
parsecs (1 parsec equals about 3.26 light-
years) of the Sun that still haven’t been the impact of stellar activity on the exo- In about 40,000 years, Voyager 1 will
recognized, but most of the discovery planets that orbit them. pass the nearest star, Proxima Centauri.
space likely lies in very low-mass L, T, Voyager 2 and Pioneer 10 will pass Ross
and Y dwarfs, says J. Davy Kirkpatrick, We need to know what a given star’s 248, some 10 light-years away in the con-
an astronomer at Caltech. These include ultraviolet and X-ray output actually is, stellation Andromeda. In some 928,000
stars that range down to masses of small, says Winters, while noting that it would years, Pioneer 11 will pass within a light-
stellar-type objects known as brown also “be awesome if we could send year of the red dwarf star TYC 992-192-1.
dwarfs. Finding those — particularly robotic probes out to individual stars
the cold Y dwarfs, says Kirkpatrick — for in-depth studies.” A comforting thought
will require going to far-infrared wave-
lengths, where they emit most of their While four NASA probes launched in Perhaps the recent film Ad Astra, set in
radiation. the 1970s continue to make their way our near future, sums this up best. The
into the interstellar depths, the space main character marvels at the fact that
“It is likely that the nearest dozens of agency appears to have largely aban- his father was the first person to reach
stars, especially Sun-like ones, will be the doned the idea of such a mission. Of the outer solar system; that’s no small
best targets for directly imaging light course, NASA’s New Horizons Pluto feat, given that Uranus and Neptune are
from planets similar to Earth,” says flyby performed stunningly well. And 19 and 30 AU away, respectively.
Mamajek. However, he says, recent given its current velocity, it will eventu-
observations seem to suggest that small ally reach interstellar space — perhaps As the film reminds us, beyond the
stars have lots and lots of small planets. with enough fuel to continue providing hazards of cosmic radiation and our own
telemetry data to its ground-based psychological frailties, the biggest chal-
But Kraus says astronomers are start- controllers. lenge in space is the void. Nevertheless,
ing to find that planets are substantially it’s comforting to contemplate our local
less common in binary systems, espe- The four spacecraft will eventually neighborhood. Despite the peril and
cially those on the scale of the solar pass by several of our stellar neighbors seeming impossibility of actually getting
system. He says planets do form and within the next few million years. there, it’s heartening that we are some-
survive, though, even in very dynami- Although they will all have long ceased how connected to these nearby stars.
cally active environments. operation by then, astronomer Coryn
Bailer-Jones at the Max Planck Institute Bruce Dorminey is a science journalist
Space missions like NASA’s Transiting in Heidelberg, Germany, has used data and author of Distant Wanderers: The
Exoplanet Survey Satellite (or TESS) are from the European Space Agency’s Gaia Search for Planets Beyond the Solar
surveying the brightest M dwarfs for spacecraft to calculate which stars they System (Copernicus, 2002).
transiting planets, says Jennifer Winters, are most likely to pass.
an astronomer at the Harvard-
Smithsonian Center for Astrophysics.
But, she adds, even if all M dwarfs have
planets, only a fraction of them will have
the optimal alignment to our line of sight
that would enable them to be observed.
Thus, only an estimated one of 30 M
dwarfs with a radius about a tenth to a
third that of our Sun will be detectable.

“We not only want to detect the plan-
ets; we want to characterize them by
studying their atmospheres to see if they
could harbor life,” says Winters.

It’s critical to understand how a given
star’s flares impact exoplanet atmo-
spheres at different wavelengths, says
Knicole Colon, an astronomer at NASA’s
Goddard Space Flight Center in
Maryland. NASA’s upcoming James
Webb Space Telescope has the potential
to precisely characterize infrared activity
on the closest stars, she says, and there-
fore contribute to our understanding of

WWW.ASTRONOMY.COM 35

SKY THIS MONTH Visible to the naked eye
Visible with binoculars
THE SOLAR SYSTEM’S CHANGING LANDSCAPE AS IT APPEARS IN EARTH’S SKY. Visible with a telescope
BY MARTIN RATCLIFFE AND ALISTER LING

JULY 2020

Feast Jupiter (center) and Saturn (lower
on a full left) shine against the backdrop of
planetary our galaxy’s dusty disk last year. This
lineup month, the two giant planets appear
even closer together in the sky as
All seven major part of Sagittarius, away from As the solar system’s largest they reach opposition within a week
planets are on dis- the brightest regions of the planet reaches its closest point of each other. TIMOTHY CORBIN/FLICKR
play these short summer nights. Milky Way and roughly mid- to Earth this month, Jupiter is
Take in Saturn’s rings, Jupiter’s way between 2nd-magnitude well placed for telescope obser- Jupiter’s disk is 48" across,
dynamic atmosphere, and the Nunki (Sigma [σ] Sagittarii) vations. It reaches its peak ele- offering a wealth of atmospheric
red deserts of Mars late at night. and 3rd-magnitude Dabih (Beta vation near 30° (for those near detail. Look occasionally for the
Stay up for dramatic views of [β] Capricorni). Jupiter moves 40 degrees north latitude) in the Great Red Spot. This swirling
Venus and its phases near a 4° westward relative to the stars south around local midnight storm has shrunk over the past
waning crescent Moon and two in Sagittarius during July. (1 A.M. local daylight saving time). decade, and observers are keen
bright star clusters in Taurus. to track its progress this year.
Binocular views of Uranus and Giant planets shine all night The twin dark equatorial belts
Neptune, plus Mercury’s late generate a lot of activity along
July predawn show, provide a A Q UA R I U S their borders, while the temper-
full course of planetary delights. ate zones carry subtler features.
On July 1, a gibbous Moon Saturn Jupiter Viewing through an eyepiece
is already in the southeastern for many minutes allows the eye
sky as night falls. As the sky CAPRICORNUS Nunki to adjust to Jupiter’s brilliance
darkens between 9:30 and and capture fleeting moments
10 P.M. local time, you’ll spot SAGIT TARIUS Kaus Australis of good seeing.
Antares, the brightest star in
Scorpius, about 10° southeast 10° The Galilean satellites —
of the Moon. At first, no planets Io, Europa, Ganymede, and
seem visible, but both Jupiter July 14, midnight Callisto — now shine at their
and Saturn rise in the southeast Looking south-southeast brightest. The angle between a
within an hour of sunset. satellite and its corresponding
Jupiter stands 15° high on Jupiter reaches opposition July 14, shining at magnitude –2.8. On July 20, shadow decreases through
July 1 by 11 P.M. local time, Saturn follows, reaching magnitude 0.1. By midmonth, the two giants are opposition and switches after-
shining at a brilliant magnitude visible all night. ALL ILLUSTRATIONS: ASTRONOMY: ROEN KELLY ward, when the leading shadow
–2.7. It reaches opposition on during a transit prior to opposi-
July 14, remaining visible all tion becomes a trailing shadow.
night at a slightly brighter mag- Eclipses and occultations of
nitude –2.8. It’s in the eastern

36 ASTRONOMY • JULY 2020

RISING MOON I Punched, flooded, sprayed, and strafed

MUCH OF THE MOON’S history is encapsu- Schickard
lated in one feature: the crater Schickard, way

down in the lunar southwest. Protective mea-

sures are necessary to carry out this observa-

OBSERVING tion. Screw in a dark filter, mask off part of your N
HIGHLIGHT scope’s aperture, or pump up the magnification E
to reduce the glare of an almost-full lunar disk.
VENUS reaches greatest
Start on the evening of the 2nd, when the
brilliancy at magnitude –4.7 on rays of sunlight strike the flooded crater at a
July 10. Two days later, it passes glancing angle, highlighting the differences in
height among the smaller features. This is an old
1° north of Aldebaran.

crater, its rounded rim pounded down across

the ages. The splatter of smaller craters Schickard

across the smooth floor sport the classic

sharp edges of relative youth. The west-

these moons occur frequently, ern wall, steep at the instant of forma- This month, watch as the crater
so check Jupiter regularly each tion, slumped abruptly down into Schickard sprouts a stripe of lighter
evening. At opposition, eclipses terraces that remain visible to this day. color across its broad floor. MOON:
as the moons pass into Jupiter’s Disappearing shadows under a climbing
CONSOLIDATED LUNAR ATLAS/UA/LPL. INSET: NASA/GSFC/ASU

shadow occur at the same time Sun in the next two evenings almost erase

as occultations. these features. west sprayed the whole region with lighter-hued

Saturn lies 6° east of Jupiter During the following nights, Schickard trans- material. A final surge of lava managed to cover
on July 1 and nearly 8° east by forms into a two-faced, striped crater, with an the northern and southern portions but did not
July 14, thanks to Jupiter’s unusual swath of lighter gray painted across the rise enough to erase this lighter stripe.
brisker retrograde motion west middle of this timeworn feature. The 140-mile-
compared with the more dis- wide depression was carved out some 4 billion When the Sun returns on August 1, the
tant Saturn. The ringed planet years ago as bombardment was ending. Lava rough lunar landscape casts plenty of shadows
reaches opposition on July 20 soon welled up and covered its central peaks. to camouflage the different shades of material.
at magnitude 0.1. Then the giant Orientale impact event to the However, in a couple of days the stripe will
return to prominence once more.

It’s challenging to figure out

what to observe with both giant METEOR WATCH I Summer meteors shine
planets beside each other. One
option is to focus on Jupiter

first, making note of its clouds, Southern Delta Aquariid meteors JULY IS A GOOD MONTH
then swing to Saturn to check

out its moons and atmosphere. Enif for meteor observing, with
Some minutes later, swing back warm weather and increas-
to Jupiter and you’ll notice the PISCES ing hours of darkness as the

cloud patterns have changed. DELPHINUS month progresses. Some
(Yes, they move that quickly.) minor showers are active,
The moons’ relative configura- CETUS all contributing to an overall
tion may have also changed. increase in summertime
A Q UA R I U S meteor rates. A Full Moon
Saturn’s disk spans 19" and occurs on July 5, so the
the rings’ major axis stretches Fomalhaut Radiant best viewing is later in the
nearly 42". The planet’s polar month, when the Southern
axis tilts 21° toward us, reveal- GRUS CAPRICORNUS Delta Aquariids are active
ing the northern side of the (July 12 to August 23).
ring system and offering views SOUTHERN DELTA Saturn
of the three major rings: the AQUARIID METEORS New Moon occurs on
outer main A ring, the brighter Jupiter July 20 and the predawn
central B ring, and the dusky Active dates: July 12–August 23 hours are the best time to
inner C ring. The A and B Peak: July 29 10° look for shower members,
rings are separated by the Moon at peak: Waxing gibbous although sporadic meteors
2,980-mile-wide Cassini divi- Maximum rate at peak: July 29, 4 A.M. can occur any time of night.
sion, visible in a small scope. Looking south-southwest In late July, the Perseids
begin, quietly at first, but
At opposition on July 20, Although the Moon will brighten the sky in late the waxing Moon interferes
look for relative brightening July, our satellite sets with several hours of most of the night in the
darkness to spare for catching the Southern latter days of the month.
— Continued on page 42 Delta Aquariids’ peak.

20 meteors/hour WWW.ASTRONOMY.COM 37

STAR DOME N

HOW TO USE THIS MAP M31 NGC 869 C A M E L OPA R DA L I S
NGC 884
This map portrays the sky as seen NE A N D R O M E DA
near 35° north latitude. Located CEPHEUS URSA ar
inside the border are the cardinal CASSIOPEIA MINOR M81 M82
directions and their intermediate
points. To find stars, hold the map NCP
overhead and orient it so one of Polaris
the labels matches the direction
you’re facing. The stars abovePEGASUS Deneb DRACO
the map’s horizon now match L A C E RTA
what’s in the sky.
CYGNUS Vega LY R A M13
The all-sky map shows CORONA
E how the sky looks at: M27 S A G I T TA HERCULES BOREALIS
DELPHINUS
midnight July 1 M15 M57
11 P.M. July 15 Enif VULPECULA
10 P.M. July 31
Planets are shown EQUULEUS C ASPEURTP E N S
at midmonth
A Q UA R I U S Altair C SAEURDPAE N S
MAP SYMBOLS
AQUILA OPHIUCHUS
Open cluster
Globular cluster S M11
Diffuse nebula CUT
Planetary nebula U M M16
Galaxy CAPRICORNUS
Saturn Jupiter M17
STAR M22 M20
MAGNITUDES Antares M4
M8
Sirius SE S A G I T TA R I U S
0.0 3.0 M6
1.0 4.0
2.0 5.0 M7

STAR COLORS ACUOSRTOR NA I SCORPIUS NGC 6231
AL
A star’s color depends S
on its surface temperature.
TELESCOPIUM
The hottest stars shine blue
S
•• Slightly cooler stars appear white
• Intermediate stars (like the Sun) glow yellow
• Lower-temperature stars appear orange
• The coolest stars glow red
• Fainter stars can’t excite our eyes’ color

receptors, so they appear white unless you
use optical aid to gather more light

BEGINNERS: WATCH A VIDEO ABOUT HOW TO READ A STAR CHART AT
www.Astronomy.com/starchart.

JULY 2020 SAT.

SUN. MON. TUES. WED. THURS. FRI.

Mi MINORL E O NW 1 234 ILLUSTRATIONS BY ASTRONOMY: ROEN KELLY
URSA MAJOR
CANES VENATICI 5 6 7 8 9 10 11
B O Ö T E S M51 Denebola
Arcturus S NGP LEO 12 13 14 15 16 17 18

E 19 20 21 22 23 24 25

C 26 27 28 29 30 31

I Note: Moon phases in the calendar vary in size due to the distance
from Earth and are shown at 0h Universal Time.
MA M64
REN CALENDAR OF EVENTS

CO 2 Asteroid Herculina is at opposition, 10 A.M. EDT
BE 4 Earth is at aphelion (94.5 million miles from the Sun), 8 A.M. EDT
5 Full Moon occurs at 12:44 A.M. EDT; penumbral lunar eclipse
W
Asteroid Vesta is in conjunction with the Sun, 2 A.M. EDT
VIRGO he Sun (ecliptic) The Moon passes 1.9° south of Jupiter, 6 P.M. EDT
Path of t 6 The Moon passes 2° south of Saturn, 5 A.M. EDT
M104 10 The Moon passes 4° south of Neptune, 3 A.M. EDT
Venus is at greatest brilliancy (magnitude –4.7), 4 A.M. EDT
M5 11 The Moon passes 2° south of Mars, 4 P.M. EDT
12 Venus passes 1.0° north of Aldebaran, 3 A.M. EDT
Spica Mercury is stationary, 3 A.M. EDT
The Moon is at apogee (251,158 miles from Earth), 3:27 P.M. EDT
LIBRA HYDRA
LUPUS Last Quarter Moon occurs at 7:29 P.M. EDT
SW Dwarf planet Ceres is stationary, 10 P.M. EDT
Asteroid Pallas is at opposition, 10 P.M. EDT
14 Jupiter is at opposition, 4 A.M. EDT
The Moon passes 4° south of Uranus, 8 A.M. EDT
15 Pluto is at opposition, 3 P.M. EDT
17 The Moon passes 3° north of Venus, 3 A.M. EDT
18 The Moon passes 4° north of Mercury, midnight EDT
20 New Moon occurs at 1:33 P.M. EDT
Saturn is at opposition, 6 P.M. EDT
22 Mercury is at greatest western elongation (20°) 11 A.M. EDT
25 The Moon is at perigee (228,889 miles from Earth), 1:02 A.M. EDT
27 First Quarter Moon occurs at 8:33 A.M. EDT
29 Southern Delta Aquariid meteor shower peaks

WWW.ASTRONOMY.COM 39

PATHS OF THE PLANETS

AND LAC

AUR LYR HER

GEM Venus shines brightest TRI Asteroid Pallas reaches
before dawn in early July ARI opposition July 12
CNC
Path of the Moon
Sun PEG
Uranus DEL
Neptune SGE
Mercury appears ORI PSC Ceres
bright in the morning Mars EQU
TAU OPH
sky in late July AQL
Celestial equator Asteroid Herculina
MON reaches opposition
Pluto reaches July 2
opposition July 15
Iris
CET

CMa LEP ERI
PYX
FOR PsA Saturn appears at Jupiter appears at
PUP SCL its best in late July its best in mid-July
COL
SCO
CAE

Moon phases Dawn Midnight

21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 43
31 30
To locate the Moon in the sky, draw a line from the phase shown for the day straight up to the curved blue line.

THE PLANETS Uranus
IN THEIR ORBITS
Venus
Arrows show the inner
planets’ monthly motions Jupiter Neptune
and dots depict the
outer planets’ positions Saturn Mercury Mars
at midmonth from high Opposition is July 20
above their orbits.

Venus Mercury Pluto MERCURY VENUS
Greatest western elongation Opposition is July 15
is July 22 July 31 July 15
PLANETS –0.8 –4.7
Mars 6.3" 34.5"
Date 66% 31%
Ceres Magnitude 1.072 0.484
Angular size 0.319 0.728
Earth Illumination 7h29.2m 4h42.2m
Aphelion Distance (AU) from Earth 21°32' 17°40'
is July 4 Distance (AU) from Sun
Right ascension (2000.0)
Jupiter Declination (2000.0)
Opposition is July 14

40 ASTRONOMY • JULY 2020

This map unfolds the entire night sky from sunset (at right) until sunrise (at left). Arrows JULY 2020
and colored dots show motions and locations of solar system objects during the month.

1

CrB BOÖ PaCnoSmTeAtRRS LMi GEM Callisto 2
LEO Europa 3
SER COM CNC Io 4
Sun 5
VIR
LIB CMi 6
7
Pa(tehcolipf tthice) Sun Ganymede

LUP SEX MON

CRV CRT H YA JUPITER’S 8
ANT MOONS 9
CMa 10
Dots display 11 Jupiter
PYX PUP positions of 12 Europa
VEL Galilean satellites 13
at 11 P.M. EDT on Ganymede
Early evening the date shown. Callisto
South is at the
21 top to match the Io
view through a
telescope.

14

29 28 27 26 25 24 23 22 21 20

15

S Jupiter THE PLANETS IN THE SKY 16
WE 17
These illustrations show the size, phase, and orientation 18
N of each planet and the two brightest dwarf planets at 0h 19
UT for the dates in the data table at bottom. South is at 20
10" the top to match the view through a telescope. 21

Saturn

Ceres 22
23
Uranus Neptune Pluto 24

MARS CERES JUPITER SATURN URANUS NEPTUNE PLUTO 25
26
July 15 July 15 July 15 July 15 July 15 July 15 July 15 27
–0.8 8.3 –2.8 0.1 5.8 7.9 14.5 28
12.7" 0.6" 47.6" 18.5" 3.5" 2.3" 0.1" 29
85% 98% 100% 100% 100% 30
0.736 4.139 100% 100% 31
1.384 2.233 5.156 9.000 20.073 29.377 33.063
0h40.3m 2.981 19h35.1m 10.012 19.794 29.930 34.079
0°34' 23h15.9m –21°58' 20h03.9m 2h31.0m 23h27.0m 19h42.0m
–18°57' –20°37' 14°23' –4°45' –22°23'

WHEN TO

SKY THIS MONTH — Continued from page 37 VIEW THE

Switching places giant by July 31. On July 31, PLANETS

S the magnitude 7.3 dwarf EVENING SKY
planet is only 0.8° northwest Jupiter (southeast)
Europa Jupiter 30" of 3rd-magnitude 88 Aquarii. Saturn (southeast)
W Io
Ganymede Increasing in brightness and MIDNIGHT
July 10, 3:30 A.M. EDT Ganymede’s shadow apparent diameter in the morn- Mars (east)
ing sky, Mars opens the month Jupiter (south)
On July 10, Ganymede’s shadow precedes it across the face of Jupiter. Those at magnitude –0.5 in southwest- Saturn (south)
in western states can catch the moon’s next transit on the 17th. This time, ern Pisces, rising soon after Neptune (southeast)
the shadow follows the moon because Jupiter has passed opposition. midnight local time. It bright-
ens to magnitude –1.1 by July 31 MORNING SKY
and rises just after 11 P.M. local Mercury (northeast)

Venus (east)

of the rings in comparison to 4th-magnitude Phi (φ) Aquarii. time. Its disk grows dramati- Mars (south)

the days leading up to and Grab a pair of binoculars and cally from 12" to 15" this Jupiter (southwest)
trailing opposition. This place Phi at the right side of month, as the distance between Saturn (southwest)
“Seeliger effect” occurs your field of view; in 7x50 or Earth and Mars decreases.
because the shadows of ring 10x50 binoculars, Neptune will Uranus (east)
In the hour before dawn — Neptune (south)

particles are hidden from view, stand about 4° east-northeast of the best time to view it — Mars

and icy particles preferentially the star. On July 10, Neptune stands 30° high in the southeast

reflect sunlight back toward lies nearly 5° north of the gib- in early July. By July 31, the Red Mars starts the month in

the light source (the Sun). bous Moon. A telescope reveals Planet reaches 45° altitude, southwestern Pisces, 17° south

Saturn’s wide-ranging moon its tiny 2"-wide bluish disk. making this apparition more of Algenib in the Square of

Iapetus reaches inferior con- Ceres is also in Aquarius, favorable for Northern Pegasus, and moves through the

junction with the planet on 13.5° due south of Neptune on Hemisphere observers than northwest corner of Cetus the

July 28, as it moves from its July 1 and 16° south of the ice its 2018 perihelic opposition. Whale between July 8 and 26.

fainter eastern elongation early

in the month and brightens to COMET SEARCH I Graying in time
magnitude 11. On July 27 and

28, it lies 1' south of Saturn,

while Titan, Saturn’s largest and BRIGHTER THAN MANY of Comet PanSTARRS (C/2017 T2)
brightest moon, stands 3' west Virgo’s Messier galaxies, Comet

of the planet. You’ll find Titan PanSTARRS (C/2017 T2) will N NGC 4485/90
due north of Saturn July 15 and appear in a 4-inch scope under ` July 1
31, and due south on July 7 and a country sky. Follow the 9th- to URSA
MAJOR

23. A trio of 10th-magnitude 10th-magnitude fuzz across the _5
moons orbit closer to the rings: Milky Way’s north polar region as
Tethys, Dione, and Rhea.
it gradually fades on its return to E CANES 10
Pluto also lies in this region. the Oort cloud. V E NAT I C I
On July 1, the magnitude 14.6 Path of
dwarf planet lies 41' south of On June 30, our ball of ice and ` 15 Comet PanSTARRS
Jupiter. By late July, Pluto stands dust is nearly equal to the inter-
3.2° due east of the fast-moving esting NGC 4485/90 pair. The a
giant. Pluto is hard to see even
with most telescopes, but digital Moon interferes in early July; on 20
cameras will detect it.
the 10th and 11th, leverage the BOÖTES
Neptune in northwestern
Aquarius is a relatively easy two hours of evening darkness to 25
binocular object at magnitude
7.9. It rises soon after local jump between PanSTARRS and Arcturus 30 M53 COMA 5°
midnight July 1 and climbs the nearby galaxies of Coma d BERENICES
to a decent altitude a few Berenices. July is also globular
hours later. In the predawn o NGC 5053 _

cluster season, so compare M53 It’s time for our now-familiar friend to head home. Comet PanSTARRS
and NGC 5053 to the comet for passed its midlife maximum in May and is now fading as it travels past
brightness, shape (morphology), several deep-sky objects this month.

and character. Observe intently

at different magnifications to examine which objects have well-defined edges, bright cores, and fading

flanks. The classic green halo of ionized gas around the coma is powering down as the comet continues

sky the first week of July, to pull away from the Sun, but enough output remains for imagers’ sensitive pixels.

Neptune stands between 11° Perfectly timed for New Moon, the periodic visitor 88P/Howell scoots less than 2° from blue-white

and 15° west of Mars. The Spica. It glows at a faint magnitude 10 to 11 this month, but will pick up to 9 when it arrives at perihelion

closest bright guide star is in late September.

42 ASTRONOMY • JULY 2020

LOCATING ASTEROIDS I

Standing out against the darkness

A morning mythological tableau THE EASIEST TIME to pick out an asteroid is when the
backdrop goes black. The Great Rift splitting our summer Milky
AURIGA TAU R U S Way does just that. The prominent Scutum Star Cloud just below
Aldebaran Aquila sports a metaphorical cliff on its western (right) edge,
where a field chock-full of stars suddenly empties to a couple
Venus of dozen. The dark swath of dust and gas that cloaks the bulge
of our galaxy is so dense it caught the attention of E.E. Barnard,
Castor ORION Rigel pioneer of dark nebula photography.
Betelgeuse 10°
GEMINI Beginning at the fabulous star cluster Messier 11, push your
Pollux scope westward past a small inky circle cataloged as Barnard 103
and onto the near emptiness. 56 Melete starts out the month just
Mercury east of a 6.3-magnitude star that it grazes on the 3rd. Look farther
west to find a cat’s eyes of 8th-magnitude stars — Melete will skim
July 31, 1 hour before sunrise south of the pair on the 13th. Those two nights will be perfect for
Looking east catching the asteroid’s change of position in a three-hour period.

By month’s end, Mercury and Venus are visible before sunrise amidst three You’ll need to be under dark country skies to “aha!” the
easily recognizable constellations: Taurus, Orion, and Gemini. “nothing” of the dark nebula, as well as other Barnard objects
field after field to the south. Melete also crosses a series of lesser-
It crosses the celestial equator Uranus stands 4.8° due north known Lynds dark nebulae — 453, 431, and 424 — which you can
into the northern half of the of a waning crescent Moon. recognize by the lack of faint stars.
sky on July 11. The same morn-
ing, Mars stands 6° northeast At 5 A.M. on July 1, brilliant Spanning some 70 miles in diameter, this main belt space rock
of a waning gibbous Moon. Venus (magnitude –4.6) stands was discovered by Hermann Goldschmidt in 1857 and named
8° high, adjacent to the Hyades after the Greek muse of meditation.
The martian disk swells star cluster. The Pleiades (M45)
from 84 percent lit to 86 per- are 10° directly above the Skirting darkness and stars
cent during July; as it does, fine planet. Venus reaches greatest
detail on the surface is observ- brilliancy (magnitude –4.7) on N
able with smaller telescopes. July 10. On July 12, it passes 1°
It’s always a small disk and north of Aldebaran, the bright- SCUTUM 5 10 SERPENS
atmospheric conditions greatly est star in Taurus the Bull. In July 1 15 CAUDA
affect what you will see. High- the July 16 and 17 predawn sky,
speed video capture is the best a waning crescent Moon adds E Path of Melete 20
way to record fine details. more beauty to the scene, first
Observers should be on the located 5° above the Hyades LDN 453 LDN 431 25 o
lookout for dust storms that and then 3.5° northeast of
can erupt as the martian Venus the next morning. By 30
southern hemisphere July 31, Venus stands 2.3° LDN 424
approaches summer. southeast of Zeta (ξ) Tauri, the j OPHIUCHUS
southern horn of the Bull.
Uranus rises two hours 1°
before dawn on July 1 in Aries Watch the fattening crescent
the Ram, located roughly mid- of Venus through any telescope Melete’s curved path covers only a small region of the sky this month.
way between its brightest star, — it grows from 19 percent The asteroid skims several faint stars and three Lynds dark nebulae.
Hamal, and Menkar, the illuminated on July 1 to 43 per-
brightest star in Cetus. The cent on July 31. Meanwhile, the earlier, on the 19th, it shines at further south along the hori-
planet shines at magnitude 5.8 disk of the planet shrinks from magnitude 0.8 and stands 5° to zon. On July 31, it is easier still
with no other bright stars 43" to 27" in diameter as its the right of the waning crescent to see at magnitude –0.7, form-
nearby. It’s best spotted in bin- distance from Earth increases. Moon, which will help you spot ing a nice trio of objects with
oculars, while a telescope easily the faint planet in brightening Pollux (7° northeast) and
reveals the greenish-colored Mercury reaches greatest twilight. Both rise soon after Castor nearby.
disk spanning 3.5". On July 14, elongation west (20°) at magni- 4:30 A.M. local time and should
tude 0.3 on July 22. Three days be visible above the northeast- Martin Ratcliffe provides
ern horizon 30 minutes later. planetarium development for
GET DAILY UPDATES ON YOUR NIGHT SKY AT Sky-Skan, Inc., from his home
www.Astronomy.com/skythisweek. Mercury’s visibility improves in Wichita, Kansas. Alister
as it brightens to magnitude Ling, who lives in Edmonton,
–0.1 on July 25. It’s in Gemini Alberta, has watched the skies
and the pair of bright stars in since 1975.
the Twins rise with Mercury

WWW.ASTRONOMY.COM 43

Our Sun was born 4.5 billion
years ago, but did not form
alone. Astronomers are sifting
through mounds of data,
searching for stars that
formed with it. NASA/SDO

44 ASTRONOMY • JULY 2020

Astronomers think it’s possible to identify
the stars that formed from the same nebula
as the Sun. BY YVETTE CENDES

omewhere in the galaxy, we Harvard-Smithsonian Center for
have a long-lost family. At Astrophysics. The key to this origin
this very minute, there are is locked in a surprising location:
hundreds to thousands of meteorites.
stars that began to form and
shine in the same dust cloud Meteoroids — as meteorites are
as our Sun, whose current locations are known when still in space — are small
unknown in the sea of other stars. But chunks primarily made of iron, nickel,
what if it were possible to test stars to and trace amounts of other materials.
find our stellar siblings, like a DNA test The composition of most has not
can reveal unknown family members for changed since the solar system formed;
humans on Earth? Astronomers think it’s what’s more, because they are the only
doable — and, what’s more, we may have part of outer space that can be physically
already done it. carried into a laboratory on Earth, they
are well studied. In examining them,
The beginning scientists have discovered elements in
amounts only possible if a supernova
Although the Sun was born billions of occurred just tens of thousands of years
years ago, we know roughly how the pro- before the meteorites formed.
cess happened by studying “stellar nurser-
ies” we see today, called nebulae. Nebula We see further evidence all around us.
means “cloud” in Latin, and each consists The elements in our world (except hydro-
of interstellar gas, primarily hydrogen gen and helium) formed in stars that
and helium with trace amounts of other died before our Sun was born, from the
elements. Many nebulae are inert, with carbon in your cells to the oxygen in
no star formation happening in them, your lungs to the iron in your veins.
their presence betrayed only by the dark These elements were then part of the
regions they form as they block light from material in our parent nebula that ended
more distant stars. In fact, if these dark up forming Earth. As Carl Sagan said,
nebulae did not exist, the Milky Way in “We are made of starstuff.”
our night sky would be much brighter.
On the other hand, nebulae that are home After the supernova’s shock wave
to star formation are positively glowing, passed through the cloud that would
and several are so bright you can spot become the solar system, the dust and
them with the naked eye. gas began to collapse in on itself due to
gravity. More and more material fell
Millions of years before the Sun onto it, forming a dense core, known as
formed, something disturbed the dark a protostar, that would become the Sun,
nebula containing the gas that would and a protoplanetary disk of gas that
become our solar system. Astronomers would eventually become the rest of the
believe they know what caused it: a solar system.
massive explosion from a dying star,
called a supernova. The Sun was not yet shining at this
point. A protostar is not yet fusing hydro-
“A blast wave from a supernova can gen, so no ancient aliens would see our
trigger star formation in the shock front developing Sun, at least in wavelengths of
[the leading edge of the explosion] if the visible light. There would be a lot of heat
material is dense enough,” explains from all the collapsing gas, however, so
Anna Rosen, an astrophysicist at the the system would emit infrared radiation.
Altogether, the Sun probably spent half a

WWW.ASTRONOMY.COM 45

stars will remain behind, an example of
which — the Pleiades — is also familiar
to naked-eye observers. Located in the
constellation Taurus the Bull, the cluster
is dominated by young, hot blue stars
formed within the last 100,000 years.

It is clear from this process that stars
do not begin their lives in isolation and,
in fact, begin their lives with hundreds or
even thousands of companions. But if the
Sun’s journey began with thousands of
stellar siblings, what happened to the rest
of them? And would we recognize those
long-lost family members in a galaxy
with billions of other stars?

The Orion Nebula (M42) is one of the finest deep-sky objects. The Sun and hundreds to thousands of other Separation anxiety
stars formed in a cloud of gas and dust similar to this stellar nursery. TONY HALLAS
The estrangement would have begun
million years as a protostar, although stellar nursery — astronomers have like this: From almost the moment the
accreting gas in the earlier stages could observed about 700 developing stars in Sun and its stellar siblings formed, they
have taken many times longer. various stages of formation and have would have begun to drift apart, with
counted 2,000 in the innermost 20 light- each pulled in different directions by
A great example years. Shocks and bows of gas, formed the gravitational effects of other stars in
by intense stellar winds from new stars, the Milky Way. Within a few hundred
To study how this process would have ripple through the system. million years, our tight cluster would
occurred, astronomers scrutinize nearby become a loose grouping, spreading
nebulae where stars are being born Despite the hive of activity within a apart ever more as the years passed.
today. One popular target is the Orion stellar nursery like the Orion Nebula, we Eventually, you would not know the
Nebula (M42), located in the middle don’t expect this stage to last long, astro- stars had been part of the same cluster.
of the constellation Orion’s sword. nomically speaking. “Young, massive stars
Although it is more than 1,300 light- in a nebula inject energy and momentum A perfect example of this process,
years away, it is such a hotbed of stellar into the system, and blow material away,” familiar to even the most casual star-
formation that it is visible to the naked explains Rosen, who studies this feedback. gazer, exists in our night sky: The Big
eye even under suburban skies. A mod- Simulations estimate that in 100,000 years, Dipper. The dipper is an asterism, a pat-
est amateur telescope will reveal glow- the gas from the Orion Nebula will be tern of stars that is not a constellation. It
ing gas illuminated by four bright stars swept away altogether. belongs to Ursa Major the Great Bear.
called the Trapezium. M42 is a huge For most constellations and asterisms,
When this occurs, a young cluster of the distances to the stars are random,
so they’re not related to one other. In
the Big Dipper, however, astronomers
discovered six of the eight stars (the star
in the handle’s bend is an easily seen
double) moving in the same direction
through space. They call it the Ursa
Major Moving Group. These stars
formed several hundred million years
ago, between 78 and 84 light-years
away, and are now spread over an area
30 light-years in diameter.

On longer time scales, stars are also
pulled apart not just by other nearby
stars and matter, but by the rotation of
the galaxy itself. The Milky Way is
shaped like a disk containing spiral
arms and a barlike center, and all stars
in the Milky Way orbit the central point.
Our Sun has a roughly circular orbit
that takes some 220 million years to
complete. The Sun is about 4.5 billion

46 ASTRONOMY • JULY 2020

As the Sun began to condense out of its primordial nebula, it had lots of company. It formed as a single star within a cluster like the Pleiades (M45), an object easily
visible to the naked eye in the fall and winter from north of the equator. MARK HANSON AND JEFF HAPEMAN

years old, so we’ve orbited the center of opposite side of the galaxy and be spread It turns out that stars are similar to
the Milky Way about 20 times — plenty out quite a bit.” people in this way. First, while stars are
of time for stars already drifting apart to primarily hydrogen and helium, they
become completely estranged. But just because you separated doesn’t also have traces of other elements that
mean you never run into one another were present in the original nebula. If
“Physically nearby stars are not neces- again. I graduated high school and col- you take spectra of the stars in the
sarily siblings,” explains Jeremy Webb, lege many years ago, and I still randomly Trapezium or the Pleiades, you find the
an astronomer at the University of run into some of the same few hundred abundances of those trace elements are
Toronto who searches for solar sibling people with whom I attended school in a the same for stars originating from the
candidates. “Siblings can end up on the city full of otherwise complete strangers. same nebula.

Detailed spectra now exist for hun-
dreds of thousands of stars in our gal-
axy, so astronomers can compare the
elements that appear in them to the
abundances in the Sun. Using this
method, some have suggested certain
stars’ spectra are similar enough to the
Sun that they must be long-lost siblings.
However, other astronomers remain
skeptical, worrying that birth nebulae
might not be sufficiently different from
one another for such chemical tagging.
Are spectra really enough to find such
a needle in a haystack?

As the cluster that contained the Sun formed and began orbiting the center of the Milky Way, the Sifting the data
tremendous gravitational influence of our galaxy started to spread out its members. This process is
currently occurring in the Ursa Major Moving Group. Six of its stars help form the Big Dipper. Only the Recently, a second way to solve the
stars at the end of the handle and the end of the bowl are not part of this group. JEFF DAI puzzle came online. Thanks to the Gaia
space observatory, astronomers can
now study 3D data and obtain precise

WWW.ASTRONOMY.COM 47

The Sun’s spectrum reveals which elements are inside it, along with their abundances. Astronomers comb through spectral data to find stars with similar properties,
hoping that some will have formed from the same nebula as our star. N. A. SHARP/NOAO/NSO/KITT PEAK FTS/AURA/NSF

motions and positions for an astounding Armed with the new Gaia data, Price-Jones, a graduate student working
1.3 billion stars in our galaxy. astronomers have revisited the search with Webb to find solar siblings. “We’re
for the Sun’s siblings. “We really cast the not looking to reproduce the exact
Launched in 2013 by the European widest net possible,” explains Natalie velocity and orbit of the Sun,” she says.
Space Agency, Gaia compiles this
impressive catalog using parallax. It f
takes careful measurements of nearby
stars, recording the minute apparent Vega + HD 162826
shifts in position against more distant _
stars. Using measurements six months HERCULES
apart, astronomers can calculate a star’s /
distance by knowing the size of Earth’s
orbit and using a bit of trigonometry. If el
the star is monitored for years, research-
ers also can discern the motion of the g
star across our line of sight.
LY R A
Gaia is able to take better measure-
ments than are possible from the 2° ¡
ground, and its data release in 2018
means astronomers now have the most In 2014, astronomers at the University of Texas at Austin announced the discovery of the first possible stellar
precise map of the galaxy ever con- sibling of the Sun. HD 162826, which glows at magnitude 6.6, lies in the constellation Hercules about
structed. There have been surprises. 110 light-years away. It’s easily visible through binoculars or small telescopes. ASTRONOMY: RICHARD TALCOTT AND ROEN KELLY
For example, in early 2020 astronomers
announced the discovery of a gigantic
gaseous structure 9,000 light-years long
and 400 light-years from the Sun at its
closest. Known as the Radcliffe Wave,
it is the largest such structure ever seen
in the Milky Way.

48 ASTRONOMY • JULY 2020

That would be impossible. Instead, she

says, “we are looking for general param-

eters and trends in movement. For

example, if we know the angular

momentum of the Sun, lots of orbits can

fit a certain angular momentum.”

In other words, astronomers use

Gaia’s orbital parameters to trace a star’s

motion backward in time to see if it

intersects with the Sun’s.

Studying Gaia’s data is enough to rule

out many stars that may be physically

close to the Sun, or have similar spectra,

but which are unrelated to it. As one such

example, for many years astronomers

hypothesized that the open cluster M67

in the constellation Cancer may be the

parent cluster of our Sun, based on the

fact that it is home to 100 stars similar

to the Sun with roughly the same age. The European Space Agency’s Gaia spacecraft has provided precise distances and motions of more than
However, “we have ruled it out as a a billion stars. Researchers use the data to identify stars with motions similar to the Sun through the Milky
potential solar birthplace because its Way, hoping some will be solar siblings. ESA/D. DUCROS

orbital properties were completely offset,”

explains Webb. A possible sibling field, you would never know our Sun and
There was just no way to simulate

M67 and the Sun’s orbit intersecting By reexamining stellar spectra combined SS1 might be long-lost relatives unless

5 billion years ago when the Sun formed, with the new orbital information, how- you had the clues provided by modern

no matter what permutation was used. ever, the Toronto team discovered a new astronomy at your disposal.

The University of Toronto team has also candidate with the same elemental abun- We don’t yet know for sure if SS1 orig-

discarded several previously suggested dances and orbital parameters as the Sun. inated in the same birth nebula. But as

solar sibling candidates on the grounds Dubbed SS1, short for “Solar Sibling 1,” it time goes on, the orbital calculations

that while they are chemically similar to is an unremarkable star some 1,100 light- used to determine whether it can be

the Sun, their orbits would never overlap years from us, located in the constellation traced back to the same birthplace as

with our own. Cygnus. Looking at the crowded star the Sun should improve.

While Gaia already has revolutionized

a astronomy with the most detailed picture

of the galaxy to date, huge questions

` remain about its structure. How many
pb spiral arms does the Milky Way have?
What is its exact bar shape? Astronomers

still aren’t sure, and knowing these

h answers would help narrow down the
orbital dynamics further for SS1 and any

HD 186302 ij other potential candidates. Luckily, the
Gaia mission is ongoing and the mission

team is planning a data release with

proper motions twice as precise as those

¡ PAVO in its 2018 dataset. The data are expected
/ in late 2020 or early 2021.

It’s a big galaxy out there. And

although we appear to be alone, one day

O CTANS c d we’ll know which stars we began our jour-
ney with, and where we’ve adventured.

2° Yvette Cendes is a postdoctoral fellow
at the Harvard-Smithsonian Center for
The AMBRE project, which searches for stellar siblings, announced another possible solar sibling in Astrophysics. She regularly posts on Twitter
November 2018. HD 186302 lies 184 light-years away in the far-southern constellation Pavo the Peacock. @whereisyvette.
It glows at magnitude 8.8. ASTRONOMY: RICHARD TALCOTT AND ROEN KELLY

WWW.ASTRONOMY.COM 49