The Astronomy Book (Big Ideas Simply Explained)

NEW WINDOWS ON THE UNIVERSE 249

CM/SM CSM/LM docking CSM and LM separation,
separation LM jettison

Touchdown

CSM/
LM
undock

Saturn V CSM reoriented LM descent
launch to dock with LM orbit insertion

Apollo 11’s command and service module docked LM = Lunar module Outbound
with the lunar module in orbit before heading for the moon. CM = Command module Inbound
Before touchdown, the service module was jettisoned, and SM = Service module
only the command module returned to Earth. CSM = Command service module

scientific priorities changed, Apollo contracts also nurtured today’s Silicon Valley. But perhaps
politicians worried about costs, nascent industries, such as Apollo’s real legacy is the idea
and human space travel has not computing and semiconductors. of Earth as a fragile oasis of life
ventured farther than Earth’s orbit. Several employees of the California- in space. Photos taken from orbit,
based Fairchild Semiconductors such as the “Blue Marble” and
For many, the decision to end went on to found new companies, “Earthrise” (p.247), fed into a
manned moon missions was a including Intel, a technology giant. growing awareness of planet
wasted opportunity, caused by a The Santa Clara area where these Earth as a single entity, and the
lack of imagination and leadership. firms were based has become need for careful stewardship. ■
However, the end of the acute Cold
War competition that gave rise to
the Apollo program heralded a new
era of international cooperation for
NASA, with Skylab, Mir, and the
International Space Station.

Gene Cernan, the last man
on the moon, predicted that it
could be another 100 years before
humankind appreciates the true
significance of the Apollo missions.
One result could be that it may have
made the US smarter—the intake
for doctoral degrees at American
universities tripled during the 1960s,
particularly in the field of physics.

On the final three Apollo missions,
astronauts explored the surface of
the moon on lunar rovers. The rovers
were abandoned and can still be seen
where they were left behind.

250

AFTRNHODEMPDLUAASDNTIESTKSOFFOGRAMSED

THE NEBULAR HYPOTHESIS

IN CONTEXT F or centuries, astronomers A version of this hypothesis
have proposed various was put forward by Frenchman
KEY ASTRONOMER models to explain how the Pierre-Simon Laplace in 1796.
Viktor Safronov (1917–1999) sun and planets formed. During
the 18th and 19th centuries, the In the late 1960s, Viktor Safronov
BEFORE nebular hypothesis came to was working in Moscow on how
1755 German philosopher prominence. This proposed that the planets could form in a nebula. He
Immanuel Kant argues that the solar system emerged from a giant wrote an important paper in 1969,
solar system formed out of a cloud of gas and dust that collapsed which was unknown outside the
large gas cloud that collapsed. and started rotating. Most of the Soviet Union until an English version
material collected in the center, was published in 1972. Safronov’s
1796 Pierre-Simon Laplace forming the sun, while the rest theory, which today is known
develops a model of solar flattened into a spinning disk of as the solar nebular disk model
system formation that is material from which the planets (SNDM), was essentially a modified,
similar to Kant’s. and smaller objects condensed. mathematically more fully formed
version of the nebular hypothesis.
1905 The American geologist
Thomas Chrowder Chamberlin In the disk of material In these collisions,
first proposes that planets orbiting the early some slower-moving
develop from particles that sun, particles
he calls “planetesimals.” particles stuck
occasionally collided. together, forming
AFTER
1980s Several apparently young larger particles.
stars, such as Beta Pictoris,
are found to be surrounded Over time, larger planetesimals formed. These aggregated into
by disks of cool dust. a few large bodies, leading to the emergence of the planets.

1983 The Infrared Astronomical The planets formed from a disk of gas and dust.
Satellite is launched. It observes
that many stars have an excess
of infrared radiation that could
be explained if they were orbited
by disks of cooler material.

NEW WINDOWS ON THE UNIVERSE 251

See also: Gravitational disturbances 92–93 ■ The Kuiper belt 184 ■ The Oort cloud 206 ■
Inside giant molecular clouds 276–79

Viktor Safronov research. From the 1950s to the In Safronov’s model, the
1990s, he worked on modeling planets formed out of dust and
Viktor Sergeevich Safronov the idea that the planets formed ice particles, which stuck together
was born in Velikiye Luki near within a disk of gas and dust. within a disk of material spinning
Moscow in 1917, and graduated Today, Safronov’s planetesimal around the newly formed sun.
from Moscow State University hypothesis of planet formation
Department of Mechanics and is widely accepted, although 1 A large cloud of gas and dust starts
Mathematics in 1941. In 1948, alternative theories exist. After contracting and slowly rotating.
he obtained a doctorate degree. the fall of the Soviet Union in
Safronov spent a considerable 1991, he had the opportunity 2 The cloud flattens out into a
part of his career working at to explain his ideas in the West. spinning disk with a denser, hotter
the Schmidt Institute of the center, which forms the sun.
Physics of the Earth, part of Key work
the Academy of Sciences in 3 Solar radiation heats up
Moscow. There, he met his wife, 1972 Evolution of the the inner solar system.
Eugenia Ruskol, who for a time Protoplanetary Cloud and
collaborated with him in his Formation of the Earth 4 Planetesimals rich in iron
and silicate dust begin to form.
Up to the 1940s, astronomers orbiting the early sun. Safronov’s
considered that the nebular breakthrough came when he 5 The solar system forms.
hypothesis contained a major flaw calculated the effect on such a
known as the “angular momentum system of some particles colliding. models, which suggested that
problem.” They calculated that if He figured out the speeds at systems of particles orbiting the
the solar system had formed out which they would collide. Particles early sun could indeed have formed
of a contracting, rotating cloud the traveling at fast speeds would into a handful of planets. Recent
sun should be spinning much faster simply bounce off each other. But observation of disks of cool dust
than it actually is. During the first slower-moving particles would surrounding apparently young stars
half of the 20th century, a number stick together, resulting in larger lend further support to the SNDM. ■
of alternative hypotheses competed particles. As they grew bigger,
with the nebular hypothesis. One the gravity of each particle would
suggested that planets might have cause them to coalesce, forming
formed from material pulled out of larger objects called planetesimals.
the sun by a passing star; another
that the sun passed through a The larger objects would
dense interstellar cloud and emerged attract more mass, and the largest
enveloped in the gas and dust from planetesimals would grow larger
which planets coalesced. Eventually, and larger, until they had gathered
solid reasons emerged for rejecting everything that lay within their
these alternatives. gravitational reach. After a few
million years, only a few planet-
Safronov’s theory develops sized bodies would remain.
Undeterred, Safronov studied in
detail how planets might have By the 1980s, there was wide
formed in the disk of material agreement over the SNDM. One
proposed by Laplace. This researcher suggested that the
disk would have consisted of angular momentum problem could
a collection of dust grains, ice be solved by dust grains in the
particles, and gas molecules, all original disk slowing down rotation
in the center. Others incorporated
Safronov’s ideas into computer

252

DCSWEOAITNLTEHAOCRNATNLOVYEREUBRTYERLSINAEORENSGE

THE HOMESTAKE EXPERIMENT

IN CONTEXT If the sun obtains energy from nuclear fusion, fast-moving
low-mass particles called neutrinos should be produced.
KEY ASTRONOMER
Ray Davis (1914–2006) The detection rate in Neutrinos barely
the interaction is likely interact with other
BEFORE particles, but they may
1930 Austrian physicist to be very low. interact in a form of
Wolfgang Pauli proposes radioactive decay.
the existence of neutrinos.
A very large detector is required.
1939 Hans Bethe outlines
two main processes by which D uring the first half of the Although this theory was accepted
stars obtain their energy. 20th century, scientists by the 1950s, it was not proven. In
figured out a process by 1955, an American chemist named
1956 American physicists which the sun produces energy Ray Davis set out to show that the
Clyde Cowan and Frederick by fusing hydrogen into helium. sun produces energetic neutrinos
Reines confirm the existence In the sun’s core, four hydrogen by detecting just a few of them. He
of the antineutrino, the nuclei, which are single protons, faced a huge problem in achieving
antiparticle of the neutrino. are changed into a helium nucleus, this goal, however. Apart from
two positrons (also called anti- the fact that their existence was
AFTER electrons), and two tiny ghostlike uncertain, scientists thought that
1989 The Kamiokande II particles called neutrinos, with neutrinos had zero electric charge
experiment in Japan, the release of energy. The neutrinos and a tiny mass (if any at all), and
organized by Masatoshi produced were envisaged to escape very rarely interacted with other
Koshiba, demonstrates easily from the sun. particles. If the sun fuses hydrogen,
conclusively that the sun
is a source of neutrinos and
confirms Davis’s abnormally
low detection rate.

NEW WINDOWS ON THE UNIVERSE 253

See also: Cosmic rays 140 ■ Energy generation 182–83 ■ Gravitational
waves 328–31

Neutrino physics is largely and, from this, the rate at which Ray Davis
an art of learning a great argon-37 should be produced
deal by observing nothing. in the tank. Davis began counting Raymond Davis was born
the actual numbers of argon-37 in Washington, D.C., in 1914.
Haim Harari atoms produced. He earned a Ph.D. in physical
chemistry from Yale University
Israeli physicist Although Davis’s experiment in 1943. Davis spent the later
showed conclusively that the years of World War II in Utah,
scientists thought, billions of sun does produce neutrinos, only observing the results of
neutrinos should be passing through about one-third of the number chemical weapons tests.
every square centimeter of Earth’s of argon-37 atoms predicted From 1946, he worked at a
surface every second, but perhaps by Bahcall were detected. The laboratory in Ohio, carrying
only one in one hundred billion discrepancy between the number out research on radioactive
might interact with atomic matter. of neutrino interactions predicted chemical elements. In 1948,
and those detected became known he joined Brookhaven National
Davis thought that neutrinos as “the solar neutrino problem.” Laboratory, on Long Island,
might be detectable through their which was dedicated to
involvement in a type of radioactive Building on his 1989 work, finding peaceful uses for
decay called beta decay. In theory, in 1999, Masatoshi Koshiba nuclear power. He spent the
an energetic neutrino should be discovered the cause of the rest of his career studying
able to convert a neutron in an discrepancy at Japan’s huge Super- neutrinos. Davis retired from
atomic nucleus into a proton. In Kamiokande neutrino detector. Brookhaven in 1984, but
his experiments, Davis found Neutrinos were found to oscillate continued his involvement
that, on very rare occasions, a between three different types— in the Homestake experiment
neutrino passing through a tank the electron neutrino, the muon until it ended in the late 1990s.
of a chlorine-containing substance neutrino, and the tau neutrino—
would interact with the nucleus of while traveling through space. Davis met his wife, Anna,
a stable chlorine atom to produce Davis’s experiment had detected at Brookhaven Laboratory
a nucleus of an unstable isotope only the electron neutrinos. ■ and together they had five
of argon called argon-87. children. He shared the
Nobel Prize in Physics in
Homestake experiment Davis’s neutrino detector was 2002 with Masatoshi Koshiba
In 1964, in what was called the placed deep underground to protect for pioneering contributions
Homestake experiment, Davis it from cosmic rays (another possible to astrophysics. He died
began a trial using a large tank source of neutrinos). in Blue Point, New York,
of a chlorine-containing chemical age 91 in 2006.
as a detector. An acquaintance of
Davis, astrophysicist John Bahcall Key work
calculated the theoretical number
of neutrinos of different energies 1964 Solar Neutrinos II,
that the sun should be producing Experimental

254

CAOSUTLADRNT’THASTEEWE

  DISCOVERING BLACK HOLES

IN CONTEXT B lack holes are invisible. was a strong source close to active
They allow no matter star-forming regions of the Milky
KEY ASTRONOMERS to escape, and, with the Way, in the constellation of Cygnus.
Louise Webster (1941–1990) exception of low-level Hawking In 1973, Australian Louise Webster,
Paul Murdin (1942–)  radiation at the event horizon, Briton Paul Murdin, and American
Tom Bolton (1943–) even swallow up electromagnetic Tom Bolton independently took
light energy. Due to the difficulty measurements of the blue supergiant
BEFORE of detecting an invisible object, star HDE 226868. They revealed that
1783 English clergyman John black holes remained purely it orbits an object far too massive
Michell suggests the existence theoretical concepts up to the to be a neutron star. The only
of a star whose gravity is so mid-20th century. However, such candidate for the invisible partner,
strong that not even light can a concentrated mass ought still Cygnus X-1, was a black hole. Black
escape it. to create observable effects. holes were now more than mere
As it is dragged into a black hole, theoretical entities. ■
1964 Cosmic X-rays are matter will be heated to millions
detected by Geiger counters of degrees as it is ripped apart by An artist’s impression shows matter
in sounding rockets. gravitational forces, pouring out flowing from the blue supergiant star
X-rays into space in the process. HDE 226868 into its black hole partner,
1970 Uhuru, the first X-ray Cygnus X-1. The star is losing one solar
observatory satellite, is launched. In the 1960s, astronomers mass of material every 400,000 years.
looked for cosmic X-ray sources
AFTER with a series of balloon- and rocket-
1975 Stephen Hawking makes launched detectors. Many of the
a bet with theoretical physicist hundreds of sources that they
Kip Thorne that Cygnus X-1 is found were assumed to be “X-ray
not a black hole. binaries”—star systems in which
a superdense stellar remnant, such
1990 Hawking concedes the bet as a neutron star, tears material
and buys Thorne a subscription away from its visible companion.
to Penthouse magazine. Among the first of these X-ray
binaries to be discovered, in 1964,
2011 Further observations give
Cygnus X-1 an expected mass See also: Supernovae 180–81 ■ Cosmic radiation 214–17 ■ Hawking radiation 255
of 14.8 suns (14.8 solar masses).

NEW WINDOWS ON THE UNIVERSE 255

BEMLAITCKRAHDOILAETSION

 HAWKING RADIATION

IN CONTEXT T he mathematical theory of Quantum theory predicts that,
black holes was pioneered throughout space, pairs of “virtual”
KEY ASTRONOMER in the 1910s by the German particles and their antiparticles
Stephen Hawking (1942–) physicist Karl Schwarzschild. should continually appear out of
The object described by nothing, then annihilate (cancel
BEFORE Schwarzschild was a non-rotating out back to nothing). One of each
1916 Karl Schwarzschild mass concentrated at a point of pair has positive energy, the other
provides a solution to the field infinite density, called a singularity. negative energy.
equations of general relativity, At a distance from this, known as
allowing him to describe the the Schwarzschild radius, was an Some of these particle–
gravitational field around a imaginary spherical surface called antiparticle pairs will appear just
black holelike object. the event horizon. The gravity on outside the event horizon of a black
the singularity side of this surface hole. It is possible that one member
1963 New Zealand was so great that nothing—not even of the pair could escape—observed
mathematician Roy Kerr light—could escape. In subsequent as an emission of (positive) radiation
describes the properties decades, black hole theory evolved energy—while the other falls into the
of a rotating singularity. in various ways, but black holes black hole. In order to preserve the
continued to be regarded as same total energy in the system,
1965 British mathematician entirely black, emitting no light. the particle that fell into the black
Roger Penrose shows that the hole must have had negative energy.
gravitational collapse of a giant Virtual particles This causes the black hole to slowly
star could result in a singularity. In 1974, a big change occurred in lose mass-energy—a process called
black hole theory. British physicist black hole evaporation.
1967 American physicist John Stephen Hawking proposed that
Wheeler coins the term “black black holes emit particles, known Hawking radiation remains a
hole” for the types of objects today as Hawking radiation. theoretical prediction. If it proves
described by Schwarzschild, Hawking maintained that black correct, it means that black holes
Kerr, Penrose, and others. holes are not completely black, do not last forever, which has
since they emit radiation of some implications regarding the fate
AFTER sort, if not necessarily light. of the universe, since it had been
2004 Stephen Hawking retracts thought that black holes would be
an earlier claim that any object among the last objects in existence. ■
entering a black hole is
completely lost to the universe. See also: Curves in spacetime 154–55 ■ The life cycles of stars 178 ■
The heart of the Milky Way 297 ■ Michell (Directory) 335

OTFHETETCRHI

1975–PRESENT

IUNOMLPOHGY

258 INTRODUCTION

NASA launches US astronomer Vera Rubin The Hubble Space
the two Voyager publishes data showing Telescope enters orbit.
spacecraft on a It provides the best-ever
mission to tour the that the rotational speeds of images in the visible and
galaxies indicate the presence near-visible spectrum.
outer planets.
of invisible “dark matter.”

1977 1980 1990
1979 1986 1995

US cosmologist Alan American Frank Shu The first brown dwarfs
Guth develops the idea and colleagues present are detected, confirming
that the early universe a theoretical prediction
a new model for
experienced a period star formation. made in 1962 by
of rapid inflation. Shiv S. Kumar.

M ost major discoveries mirrors are flexible, but kept (CCD). CCDs are electronic circuits
in astronomy have precisely in shape by a network with light-sensitive pixels that
been made possible by of computer-controlled supports generate electrical charges when
advances in technology. Recent called actuators. light photons land on them. They
developments have provided are far superior to photography for
powerful tools to collect radiation ESO’s choice of Chile was part sensing faint light and recording an
from space and to process vast of the trend for astronomers to
amounts of data, and the pace scour the world, testing for the We’re going to need a
of discovery has accelerated at a best sites where the air is clear, definitive quantum theory
breathtaking rate. Microelectronics still, and dry, and the sky free of gravity, which is part of
and computing capability, in from light pollution. Another a grand unified theory—it’s
particular, have opened up new major center for astronomy was the main missing piece.
possibilities over the last 40 years. established at the summit of the
volcano Mauna Kea on the Big Kip Thorne
Telescopes and detectors Island of Hawaii in 1967. This prime
The New Technology Telescope site is now home to 13 telescopes.
(NTT) opened by the European
Southern Observatory (ESO) in Until the early 1970s, all
the Chilean Andes in 1989 is astronomical imaging was carried
an example of a telescope with out by means of conventional
revolutionary innovations that have photography. Then, in the mid-
subsequently become standard 1970s, a completely new way of
equipment. Its main and secondary recording an image electronically
became a practical reality. This
was the charge-coupled device

THE TRIUMPH OF TECHNOLOGY 259

Swiss astronomers NASA’s Curiosity rover After an 11-year journey,
Didier Queloz and lands on Mars and begins NASA’s New Horizons
spacecraft makes its closest
Michel Mayor to explore its surface. approach to Pluto, revealing
discover the first details of its icy surface.
exoplanet orbiting

a sunlike star.

1995 2012 2015
1998 2013 2016

Cosmic expansion The European Southern The Ligo Scientific
is found to be accelerating, Observatory opens Collaboration announces the
suggesting the existence of a its Atacama Large
mysterious “dark energy.” detection of gravitational
Millimeter Array, a giant waves, confirming Einstein’s
radio telescope in Chile.
general theory of relativity.

object’s brightness accurately, and Computers are indispensable be followed in real time, as the latest
they made visible objects that had to theorists, too. Huge computing images from the Hubble Space
previously been too faint to detect, power makes it possible to gain Telescope or the Curiosity rover on
such as the small, icy worlds of the insight into what observations Mars are made instantly available.
Kuiper belt beyond Neptune. are telling us about the way
the universe works by creating Landmark discoveries
Computing power simulations based on the laws of Of the many discoveries in recent
Fast, reliable computers and physics. For example, computers decades that have had an impact
an immense capacity for storing allow scientists to model ways in on our understanding, three stand
data have been key not only to which the solar system may have out. The surprise finding in 1998
the way that telescopes and their formed and evolved. that the universe’s expansion is
instruments are constructed, accelerating showed that there
but also to making sense of the Space exploration has now is a gap in fundamental theory.
astronomical data they collect. pushed right out to the edge of By contrast, the detection of
One major project alone, the Sloan the solar system, and no region gravitational waves in 2016
Digital Sky Survey, has collected of the planetary system remains confirmed Einstein’s 100-year-old
information about 500 million unexplored on some level. The theoretical prediction. Meanwhile,
celestial objects since it began New Horizons mission passed the discovery of the first extrasolar
in 2000. This database has been Pluto in 2015 and is moving through planet in 1995, and thousands more
used to create a three-dimensional the Kuiper belt, while the Voyager since, has energized the search
map showing how galaxies are spacecraft, launched back in 1977, for alien life. It is impossible even
distributed across the universe, are now sending back data from to speculate where the next 20
revealing its largest structures. interstellar space. With the advent years may lead. ■
of the internet, missions can also

GIANTA GRAND TOUR OF THE

PLANETS

EXPLORING THE SOLAR SYSTEM



262 EXPLORING THE SOLAR SYSTEM

IN CONTEXT more failures than successes, but A Titan 3E rocket lifts off carrying
over the decade robotic spacecraft the Voyager 1 spacecraft as its payload.
KEY ORGANIZATION began sending back close-up images The Titan 3E was the most powerful
NASA—Voyager mission of Venus and Mars. The NASA craft launch vehicle of its time.
(1977–) were part of the Mariner program,
run largely from the Jet Propulsion outer planets and discovered that,
BEFORE Laboratory (JPL) in California. The in 1978, all the outer planets would
1962 Mariner 2 passes Venus mathematicians at JPL perfected be on the same side of the sun. His
in the first planetary flyby. the art of the “flyby”—sending a calculations revealed that this had
spacecraft on a trajectory that had not happened since 1801, and would
1965 Mariner 4 is the first it fly past a planet close enough to not occur again until 2153.
craft to visit Mars. photograph and observe it, albeit
too quickly to enter its orbit. In Flandro saw the opportunity
1970 Venera 7 makes the 1965, a graduate student named for a Grand Tour of the outer solar
first soft landing on Venus. Gary Flandro, who was working at system, but the distances involved
JPL for the summer, was given the were far beyond the capabilities of
1973 Pioneer 10 is the first task of figuring out routes to the the spacecraft of the day. In 1965,
spacecraft to cross the asteroid Mars’s alignment made it the
belt en route to Jupiter.

1976 Viking 1 sends pictures
from the surface of Mars.

AFTER
1995 The Galileo spacecraft
goes into orbit around Jupiter.

1997 Sojourner is the first
rover to land on Mars.

2005 The Cassini orbiter
releases the Huygens probe,
which touches down on Titan.

2015 New Horizons makes the
first flyby of Pluto and Charon.

O n August 20, 1977, the
Voyager 2 spacecraft
was launched from Cape
Canaveral in Florida. Two weeks
later, its sister craft Voyager 1
was launched. Thus began the
most ambitious exploration of the
solar system ever. The launch
was the culmination of more than
a decade’s work. The core mission
would run for 12 years, but an
interstellar mission continues.

Going interplanetary
By the early 1960s, both the Soviet
and US space agencies were sending
missions to other planets. There were

THE TRIUMPH OF TECHNOLOGY 263

See also: Life on other planets 228–35 ■ The nebular hypothesis 250–51 ■ Exoplanets 288–95 ■
Understanding comets 306–11 ■ Studying Pluto 314–17

closest planet to Earth at that time The best way to learn All the outer planets
at 35 million miles (56 million km), about planets is to send will be close together
but Neptune was 2.5 billion miles
(4 billion km) away, and a journey robotic spacecraft for a short period.
to it would take several years. to them.

Planetary slingshots A Grand Tour could send probes to
A Grand Tour spacecraft would study them throughout this period.
have to change course several times
in order to fly past all its planets. The Voyager program makes a
Flandro’s plan had to use gravity Grand Tour of the giant planets.
assists to fling the craft from
planet to planet. Also known as a from the planet’s orbital motion. planet having added approximately
gravitational slingshot or swing-by, The planet’s gravity would speed twice the speed of the planet to
a gravity assist had first been used up the craft as it made a loop its own velocity. A slingshot would
by the Soviet Luna 3, which had around the planet. It would then not only redirect the craft, but also
swung around the far side of the slow down again as it headed off accelerate it on to its next target.
moon in 1959, photographing as into space, having done an about
it went. It had never been used to turn. If the motion of the planet Taking a Grand Tour
guide spacecraft as far from Earth were ignored, the craft’s escape In 1968, NASA set up the Outer
as the outer planets. The planned speed would be more or less equal Planets Working Group. It proposed
slingshot required the craft to to its approach speed. However, the Planetary Grand Tour mission,
approach the planet head-on, taking the motion of the planet into which would send one spacecraft
traveling in the opposite direction account, the craft would leave the to visit Jupiter, Saturn, and Pluto, ❯❯

An artist’s impression shows
Voyager 1 in space. This craft and
its twin, Voyager 2, communicate
with Earth via radio waves transmitted
and received by a 12-ft (3.7-m) dish.

It was a chance that came
around once in every 176 years
and we prepared for it. Out of

that emerged the greatest
mission of planetary
exploration to this day.
Charles Kohlhase

264 EXPLORING THE SOLAR SYSTEM

Jupiter’s moon Europa was
photographed by Voyager 2. It is
covered in a thick crust of ice, which
has been fractured and filled in with
material from the moon’s interior.

and a second one toward Uranus moon known to have its own all the criteria and would take the
and Neptune. The plan required atmosphere. This change meant spacecraft as close to as many
a new long-range spacecraft and that the mission would be points of interest as possible.
costs grew steadily. Then in 1971, budgeted as an exploration of the
NASA canceled the Grand Tour as two gas giants, not a Grand Tour. Neither Kohlhase nor anyone
it needed cash to fund the Space However, the spacecraft, code- else working on MSJ77 liked
Shuttle program. named JST, was to have a back-up, the name. As the launch date
JSX. Its mission would also include approached, a competition for a
The exploration of the outer Jupiter and Saturn if JST failed. new name was organized. Nomad
planets was handed back to the The X represented an unknown and Pilgrim made the shortlist,
Mariner program. The mission was quantity. If required, JSX would but by the time the two identical
named Mariner Jupiter–Saturn, or go to Titan, but if JST achieved spacecraft were ready, they were
MJS77 for short—77 referred to the its mission, then JSX would be known as Voyager 1 and Voyager 2.
launch year. To reduce costs, Pluto sent to Uranus and Neptune. At 1,590 lb (720 kg), the two
was removed from the tour Voyager spacecrafts were nearly
itinerary. Instead, one craft was Mission profile 50 percent heavier than any
to visit Jupiter, Saturn, and finally In 1974, mission design manager previous flyby craft. About 220 lb
Saturn’s huge moon Titan. Titan Charles Kohlhase began to make (100 kg) of that was scientific
was considered more intriguing a master plan for the MJS77 equipment, comprising two
than distant Pluto. It was larger mission. He had to consider every cameras, magnetic field sensors,
than Mercury, and thought at the aspect, from the spacecrafts’ spectrometers that would analyze
time to be the largest moon in the design, size, and launch system light and other radiation to show
solar system. It was also the only to the many variables they would which chemicals were present in
encounter along their routes—the atmospheres, and particle detectors
Voyager 1 captured a 100-mile- radiation levels, light conditions, for investigating cosmic rays.
(150-km-) high eruption on Jupiter’s and contingencies for altering the In addition, the radio system
moon Io. Strongly affected by Jupiter’s missions. It took Kohlhase and his could be used for a variety of
gravity, Io is the most volcanically team eight months to eventually experiments, such as probing
active place in the solar system. settle on two trajectories that met atmospheres and Saturn’s rings.
The spacecrafts’ trajectories were

THE TRIUMPH OF TECHNOLOGY 265

to be controlled by 16 hydrazine the compositions of the clouds and Titan in size, while images of
thrusters. However, it would be measured the planet’s immense Europa’s eerily smooth yellowish
too dark beyond the asteroid belt magnetic field. It also showed that disk had astronomers puzzled.
for solar panels to generate enough Jupiter had a faint ring system. Its Voyager 2 arrived at Jupiter more
electricity for the spacecraft, and most memorable discoveries came than a year later, and did not
batteries would not last nearly long from the flybys of the Galilean approach as close as Voyager 1, but
enough. The answer was nuclear moons. These were not sparse, it took some of the mission’s most ❯❯
power in the form of radioisotope cratered balls but active worlds.
thermoelectric generators (RTG), Photographs of Io showed the Voyager 2’s images of Saturn’s
held out on a boom to isolate largest volcanic eruptions ever rings revealed a complex structure
them from sensitive equipment. seen, spurting ash clouds into orbit. made of small satellites, none of
Each RTG contained 24 balls of Fresh measurements of Ganymede which were larger than 3–6 miles
plutonium that gave out heat, revealed that it superseded even (5–9 km) across.
which was converted into an
electric current by thermocouples.
The power supply was built to
last for nearly 50 years.

Jupiter and its moons
By December 1977, Voyager 1 had
overtaken Voyager 2, which was
taking a more circular trajectory.
It reached the Jupiter system in
January 1978. Most of Voyager 1’s
important discoveries were made
in a frenetic 48-hour period around
March 5, when it made its closest
approach, coming within 217,000
miles (349,000 km) of the planet’s
cloud tops. In addition to sending
back images, Voyager 1 analyzed

The latter half of the next
decade abounds in interesting
multiple planet opportunities.

Of particular interest is the
1978 “grand tour” which would

make possible close-up
observation of all planets of

the outer solar system.
Gary Flandro

266 EXPLORING THE SOLAR SYSTEM

Voyager 2 sent back this image of
Neptune’s ice moon Triton. During the
craft’s flyby, only the southern ice cap
was visible. Highly reflective, it is
made of frozen nitrogen and methane.

The Titan trajectory then sent
the craft over Saturn’s pole
and away to the edge of the
solar system.

Voyager 2 arrived at Saturn
in August 1981, and was able
to study the planet’s rings and
atmosphere in more detail, but
its camera failed during much
of the flyby. Fortunately, it was
restored, and the order was given
to continue to the ice giants.

iconic images of Io transiting Saturn to prevent any damage Uranus and Neptune
Jupiter. Voyager 2 also got a closer caused by Saturn’s atmosphere Voyager 2 is the only craft
look at Europa, showing that it and rings from endangering this to have visited the ice giants
was covered in a crust of water crucial phase. The spacecraft Uranus and Neptune. It took 4.5
ice riven by cracks. Later analysis swung behind Titan so the sun’s years to travel from Saturn to
revealed that these cracks were light shone through the atmosphere, Uranus, where the craft passed
caused by upwellings in a liquid allowing measurements of its 50,500 miles (81,500 km) above
ocean under the crust, an ocean thickness and composition. the pale blue atmosphere. It
that is estimated to hold at least looked at the planet’s thin rings
twice as much water as Earth and and discovered 11 new moons,
which is thought by scientists all of which are now named after
to be a prime candidate for the Shakespearian characters, as
presence of alien life. is the rule for Uranus. The most
curious thing to be examined
Titan and Saturn
By November 12, 1980, Voyager 1 Charles Kohlhase 2005. In the late 1970s, he
was skimming 77,000 miles worked with computer artists
(124,000 km) above the atmosphere Charles “Charley” Kohlhase to create accurate animations
of Saturn. On the approach, and was born in Knoxville, of space missions for advancing
despite some instrument failures, Tennessee, and graduated with the public understanding of
it revealed details of the rings, a degree in physics. He briefly NASA’s work. Now retired,
which were made of billions of served in the US Navy before Kohlhase remains involved in
chunks of water ice and were joining JPL in 1960, where he several projects that blend art
as thin as 30 ft (10 m) in places. turned his life-long fascination and space science, aiming to
Kohlhase had sent Voyager 1 to with exploration into work on educate and inspire the next
visit Titan before approaching the Mariner and Viking projects, generation of rocket scientists
before joining the Voyager team. and interplanetary explorers.
In 1997, Kohlhase left Voyager
to design the Cassini–Huygens Key work
mission to Saturn, which
succeeded in dropping a lander 1989 The Voyager Neptune
onto the surface of Titan in travel guide

THE TRIUMPH OF TECHNOLOGY 267

The golden records carried by the times a year, the craft spin around The spacecraft will be
Voyager spacecraft included a selection to measure the cosmic rays around encountered and the record
of music, greetings in 55 different them. This data shows that the
languages, and images of humans, craft are approaching the edge of played only if there are
animals, and plants. the heliosphere, the region of space advanced spacefaring
that is influenced by the sun. Soon civilizations in interstellar
on this otherwise relatively quiet they will enter interstellar space space. But the launching
planet is the tilt of its axis, which and measure the cosmic wind from of this “bottle” into the
is roughly 90°. As a result, Uranus ancient stellar explosions. cosmic “ocean” says
does not spin as it orbits, but something very hopeful
“rolls” around the sun. In 2025, the two spacecraft will about life on this planet.
power down and go quiet forever,
The final port of call was but their mission may still not yet Carl Sagan
Neptune, reached in August 1989. be complete. A committee chaired
This deep-blue planet was found by Carl Sagan selected content for will never be found by intelligent
to have the strongest winds in a gold-plated phonograph record life, but the golden records are a
the solar system, up to 1,500 mph (its analog groove would be easier symbol of the hope with which the
(2,400 km/h)—nine times stronger to read than a digital format). They two interplanetary spacecraft were
than anything experienced on included greetings from the world, sent on their way. ■
Earth. The Voyagers’ mission the sounds and sights of Earth,
controllers were able to abandon and even human brain waves. Voyager 1 Heliosheath
caution as the planetary mission The record is a calling card from
drew to an end. Without regard humankind to an alien civilization. Solar wind
for the safety of its final trajectory, The Voyagers are not heading for Earth
Voyager 2 was redirected to fly any star systems; the closest they
past Neptune’s moon Triton. The will get is when Voyager 1 passes
images of the huge ice moon showed 1.6 light-years from a star in 40,000
geysers blasting fountains of slush years’ time. In all likelihood, they
from the surface.
By 2005, the Voyagers had
reached the termination shock,
where the solar wind slows
and becomes turbulent as it
mixes with the interstellar
medium (matter in the
space between star
systems), entering
the heliosheath
region. By 2016,
they were nearing
the heliopause,
where the solar
wind is stopped
by the interstellar
medium.

Continuing mission shock
The Voyager program continues VoyagTere2rmination
and the two craft are still in touch Heliopause
with NASA. As of 2016, Voyager 1
was 12.5 billion miles (20 billion
km) and Voyager 2 was 10 billion
miles (16 billion km) away. Six

268 IN CONTEXT

UMMNIOSISVSTEINROGSFETIHSE KEY ASTRONOMER
Vera Rubin (1928–)
 DARK MATTER
BEFORE
1925 Bertil Lindblad
calculates the likely
shape of the Milky Way.

1932 Jan Oort finds that
the rotational speeds of the
Milky Way galaxy do not
match the presumed mass.

1933 Fritz Zwicky suggests
that a majority of the
universe is made up of
dark, invisible matter.

AFTER
1999 It is discovered that
dark energy is accelerating
the expansion of the universe.

2016 The LIGO experiment
detects gravitational waves,
which offer a new method to
map the distribution of dark
matter across the universe.

I saac Newton’s universal law
of gravitation is adequate for
making calculations to launch
a satellite into orbit, land a crew on
the moon, and send a spacecraft
on a grand tour of the planets.
Newton’s clear mathematics works
well enough for most things on
the solar system scale, but not
on grander scales of the universe,
where Einstein’s relativistic theory
of gravity is needed (pp.146–53).
Nevertheless, Newton’s law of
gravitation was all that was required
to reveal one of the biggest—and
as yet unsolved—mysteries of
astronomy: dark matter. In 1980,
American astronomer Vera Rubin
presented clear evidence that dark

THE TRIUMPH OF TECHNOLOGY 269

See also: Gravitational theory 66–73 ■ Gravitational disturbances 92–93 ■ The shape of the Milky Way 164–65 ■
Supernovae 180–81 ■ The Oort cloud 206 ■ Dark energy 298–303

We became astronomers her to stay in her home city of The outer regions of
thinking we were studying the Washington, D.C., and raise her four galaxies move much more
children. She chose to study the
universe, and now we learn rotation of galaxies, specifically quickly than expected.
that we are just studying the looking at the anomalous behavior To stop spinning
5 percent that is luminous. of the outer regions of galaxies.
galaxies from disintegrating,
Vera Rubin Spinning spirals they must contain a lot more
The problem Rubin tackled was
matter exists. Thanks to Rubin, the the fact that huge disks of stars mass than can be seen.
general public learned that most of in nearby galaxies did not move This mass comes from
the universe appears to be missing. in a way that was consistent with invisible dark matter—
Newton’s law of gravity: their there is six times more dark
Throughout the 1960s and outer regions moved too quickly. matter in the universe than
1970s, the science of astronomy This curiosity was not new, but it
was dominated by projects on a had previously been largely ignored. ordinary matter.
grand scale, as researchers used
massive instruments, often in Since the 1920s, when Bertil Most of
remote parts of the world, to search Lindblad and others showed that the universe
for exotic objects, such as black the Milky Way—and by extension is missing.
holes, pulsars, or quasars. By many other galaxies—were disks
contrast, Rubin was looking for of stars moving around a central
a research area that would allow point, it was assumed that galaxies
were orbital systems just like any
other. In the solar system, near
objects orbit at a faster speed than
distant ones, so Mercury is moving
much more rapidly than Neptune.
This is because, following Newton,
gravity decreases with a square of
the distance. When the velocities ❯❯

Vera Rubin Born Vera Cooper in Philadelphia, concluded that galaxies would
Rubin earned her first degree from clump together, a fact that was
Vassar College in upstate New not fully explored until the work
York, and then applied to go to of John Huchra in the late 1970s.
Princeton. Her application was After teaching at a college in
ignored because women were Maryland, Rubin returned to
barred from joining the university’s Georgetown, and then moved
graduate astronomy program until to the Carnegie Institution of
1975. Instead Rubin pursued her Washington in 1965. It was here
studies at Cornell University, that she conducted her work on
where she studied under greats galactic rotation, and she has
such as Richard Feynman and remained there ever since.
Hans Bethe. She subsequently
earned a Ph.D. from Georgetown Key work
University in Washington, D.C.,
supervised by George Gamow. 1997 Bright Galaxies,
Her thesis, published in 1954, Dark Matters

270 DARK MATTER

of the planets are plotted against Oort’s early measurements were No observational problem will
their distance from the sun, the data inaccurate, while Zwicky’s initial not be solved by more data.
forms a smooth downward “rotation assessment was that “dark matter”
curve.” It followed that plotting the was 400 times more abundant than Vera Rubin
orbital speeds of stars at different the matter of visible material—a
distances from the galactic center huge overestimate. This meant redshift and blueshift of objects,
should produce a similar curve. that their findings were dismissed and calculate their relative speeds
as measurement errors. In 1939, away from and toward Earth.
In 1932, Dutch astronomer Jan American Horace Babcock again
Oort was the first person to provide found anomalies in the rotation of After several years of slow but
observational proof that the galaxy Andromeda and suggested that there careful work, Rubin had enough
was a single orbital system made was some mechanism by which data to plot a rotation curve for
up of a swirling spiral of stars, light from the missing matter was the galaxy. Instead of swooping
in which the sun completed a being absorbed in the galactic core. downward like the curve of the
225-million-year orbit. However, in solar system, the speed data
the course of his calculations, Oort Galactic rotation curve of the galactic curve stayed
found that the motion of the galaxy More than 20 years later, Rubin relatively level with distance.
suggested that it was twice as returned to the problem of galactic This meant that the outer regions
massive as the total mass of visible rotation. Like Babcock, she chose of Andromeda were moving at the
stars. He concluded that there must to focus on the rotation of the same speed as the areas nearer to
be some hidden source of mass. Andromeda galaxy, the Milky Way’s the center. If the galaxy’s mass was
A year later, the Swiss−American nearest galactic neighbor. She limited to what could be observed
Fritz Zwicky was studying the worked with her colleague Kent using telescopes, the outer regions
relative motion of galaxies in the Ford at the Carnegie Institution of Andromeda would be moving
Coma cluster. He found, again, of Washington to measure the faster than escape velocity, and
that their motion suggested the velocities of objects in the outer they should simply fly off into
mass of what could be seen was region of the galaxy. They did this space. However, they were clearly
not the only stuff there. He named using a sensitive spectrograph, being held in place by the galaxy’s
the missing material dunkle which allowed them to detect the overall mass. Rubin calculated that
Materie or “dark matter.” the total galactic mass required
to hold the outer regions in orbit
150 Range was about seven times greater
of error than the visible mass. The ratio
Observation of matter to dark matter is today
thought to be around 1:6.
VELOCITY (KM PER SECOND) in each
observation What is dark matter?
Rubin’s galactic rotation curve,
100 widely disseminated in 1980, was
Observed rotation curve the visual proof that dark matter
existed. As further evidence
Rotation curve expected mounted, the mystery as to what
from visible matter it might be remained. Dark matter
50

0
0 10 20 30 40 50 60
DISTANCE FROM CENTER (x 100 LIGHT YEARS)

In the absence of dark matter, the velocities of objects in the outer regions of
galaxies would be slower than the observed values. Here, the observed rotation
curve is plotted against the expected curve from visible matter alone.

THE TRIUMPH OF TECHNOLOGY 271

Dark matter could be evidence that
the universe is one of many that exist
next to one another, in separate spatial
dimensions, in a bubblelike multiverse.

cannot directly be observed; only
its effects are detectable, and the
only effects that can be detected
are from its gravity. It does not
interact with the electromagnetic
force, meaning that it does not
absorb heat, light, or other radiation,
nor does it emit any. Dark matter
may be completely invisible.

Possible sources is the WIMP—Weakly Interacting However, the gravitational effects
The simplest solution to the dark Massive Particle. This concept is of the matter in the hidden universes
matter problem is the most literal. based largely on an idea in particle leaks through into this one through
It comprises ultra-dense bodies of physics called supersymmetry. the warping of spacetime.
ordinary matter that are too dark to It proposes a new explanation of
observe. Astronomers have dubbed energy and ordinary matter. Energy Providing an explanation for dark
these MACHOs, which stands for and matter form two distinct matter remains one of the biggest
Massive Compact Halo Objects. groups of subatomic particles, prizes in astronomy. However, in
MACHOs include objects like black and supersymmetry proposes that 1999, a possibly even more puzzling
holes, neutron stars, and white and these groups interact thanks to the phenomenon was uncovered. It was
brown dwarfs. They occupy the actions of “super particles,” or discovered that 68 percent of the
galactic halo, a dark and diffuse sparticles. Dark matter WIMPs universe was neither matter nor
region that surrounds the main, might be sparticles that escaped dark matter, but so-called dark
shining disk of a galaxy—and this from their partners in the early energy. Dark matter makes up 27
is why it is difficult to see them. period of the universe, or they may percent; visible matter comprises
MACHOs are clearly out there, but be objects that are there all along. a mere 5 percent. ■
by current estimates they would
only account for a tiny proportion Finally, dark matter might be For the moment we
of dark matter. An alternative idea the observable effect of another might very well call them
universe, or perhaps several, DUNNOS (for Dark Unknown
that exist in a spatial dimension Nonreflective Nondetectable
different from this universe. Their
matter could be very close, a few Objects Somewhere).
centimeters away, but because Bill Bryson
the radiation from each universe is
trapped inside its own spacetime,
one universe can never see another.

A huge ring of dark matter, which
formed long ago in the collision between
two massive galaxy clusters, is shown
around the edge of this Hubble Space
Telescope image in lighter blue.

272

   NPGRERGOADAVTIUTICVYEE RPERPEUSLSSUIRVEES

COSMIC INFLATION

IN CONTEXT B y the 1970s, cosmologists (particles with only one magnetic
were grappling with a variety pole). There are, however, none to be
KEY ASTRONOMER of puzzles thrown up by the found, which suggests the universe
Alan Guth (1947–) Big Bang theory. In an attempt to cooled faster than expected.
solve them, Alan Guth proposed a
BEFORE stage of rapid inflation in the early A second problem arose from
1927 Georges Lemaître universe, caused by the effects the way space is amazingly “flat,”
proposes that the universe predicted by quantum theory. meaning that it expands according
arose from a single primordial to “normal” Euclidean geometry (see
atom. This is later named the The puzzles diagram opposite). A flat universe
Big Bang theory. One of the problems with the would only have arisen if the density
Big Bang theory came from the of the early universe matched a
1947 George Gamow and Grand Unified Theory (GUT), certain critical figure. Varying it
Ralph Alpher describe how which describes how the forces slightly one way or the other would
the elements hydrogen and of the universe (aside from gravity) have resulted in curved universes.
helium were formed in the arose a fraction of a second after
early universe. the Big Bang. The GUT predicted The final issue was the horizon
that high temperatures at this time problem. Light from the edge of the
1964 The cosmic microwave would create bizarre features, such observable universe has taken the
background is discovered to as so-called magnetic monopoles entire life of the universe to reach
be a remnant of the Big Bang. Earth. As light’s speed is constant,
scientists know that it has not
AFTER
1999 Dark energy is found to The Big Bang theory The first stage of the
be accelerating the expansion predicts features universe after the Big
of the universe. that are not seen in Bang may have been a
the current universe. period of rapid expansion
2014 BICEP2 withdraws claims
of finding evidence of inflation. called inflation.

2016 LIGO detects Inflation explains many of the features of the
gravitational waves, offering universe, but there is no evidence that it is correct.
a new way to observe the
structure of spacetime.

THE TRIUMPH OF TECHNOLOGY 273

See also: The birth of the universe 168–71 ■ The primeval atom 196–97 ■
Searching for the Big Bang 222–27 ■ Gravitational waves 328–31

Flat

Curved

Saddle-shaped It is not possible to visualize Alan Guth
warped three-dimensional space,
but if one dimension were removed, Born in New Jersey, Alan
the geometry of space could be Guth received his doctorate
shown as curved (where the internal in 1972 and specialized in
angles of a triangle would add to more particle physics, pursuing
than 180°); saddle-shaped (where the research into quarks
angles would be less than 180°); and (elementary particles). By
flat (Euclidean), where a triangle with the late 1970s, he had worked
angles of 180° would be drawn. at MIT, Princeton, Columbia,
Cornell, and Stanford, as he
had time to shine to the opposite cooled the universe rapidly, thus searched the country for a
edge of the universe. So if no light, solving the GUT problem, and long-term academic position.
energy, or matter has ever passed locked in the uniformity seen today. While at Columbia, Guth
between the edges of the universe, Finally, the inflation ended as the became interested in the
this leaves a puzzle as to why space density of the universe evened out Grand Unified Theory (GUT),
appears so similar in every direction. at a value needed for a flat universe. which had been proposed in
In 2014, BICEP2, an experiment 1974. He began developing
The solution at the South Pole, reported ripples his inflation theory in 1978
Guth’s theoretical solution to in space consistent with cosmic while at Cornell, after hearing
these problems was to inflate the inflation. However, the claim was about the problem of the
early universe using a quantum soon withdrawn. Cosmic inflation flatness of the universe,
effect called a false vacuum, where remains unproven, but it is the best and shortly afterward, the
positive matter energy was created current theory for the Big Bang. ■ problems associated with
as space expanded, equally GUT. While at Stanford,
balanced by an increase in gravity The recent developments in he then came across the
(a form of negative energy). In the cosmology strongly suggest horizon problem, and went
first 10–35 seconds after the Big on to publish his famous
Bang, space doubled in size 100 that the universe may be theory in 1981. He is now
times over, going from a billionth the ultimate free lunch. a professor at MIT, where
of the size of a subatomic particle he helps with the search for
to the size of a marble. This means Alan Guth evidence of cosmic inflation.
that, at the very beginning, the
edges were close enough to mix Key works
and become uniform, thus solving
the horizon problem. During 1997 The Inflationary
inflation, space expanded faster Universe: The Quest for a
than the speed of light. (The New Theory of Cosmic Origins
speed of light is only a speed 2002 Inflation and the New Era
limit through space.) The inflation of High-Precision Cosmology

274

   BBG  EUALBOABNXLTIEEHLSIEKASEPUPSRETFARARUCCTETOSUORFES

REDSHIFT SURVEYS

IN CONTEXT S ince the 1920s, the study space. Geller and Huchra’s work
of the redshift of distant provided valuable clues to the
KEY ASTRONOMERS galaxies has revealed the nature of the very early universe.
Margaret Geller (1947–) scale of space and the way in
John Huchra (1948–2010) which the universe is expanding A redshift survey uses a wide-
in all directions. Redshift occurs angle telescope to select target
BEFORE when a light source is moving away galaxies, generally millions of
1842 Christian Doppler from the observer (p.159). In the light-years away. Astronomers
describes how wavelengths can 1980s, redshift surveys made by compare the light from each galaxy
change due to relative motion. American astronomers Margaret with benchmark wavelengths to
Geller and John Huchra, working determine the redshift, and thus
1912 Vesto Slipher discovers at the Harvard–Smithsonian Center the distance the light has traveled,
that galaxies are redshifted for Astrophysics (CfA), gave an allowing them to plot the positions
by the Doppler effect. even clearer picture of the universe, of many galaxies. Huchra started
showing that galaxies cluster the first redshift survey in 1977;
1929 Edwin Hubble uses around great voids of empty by its completion in 1982, he had
redshift to show that distant mapped 2,200 galaxies.
galaxies are moving away
faster than nearer ones. Margaret Geller Margaret Geller earned a Ph.D.
from Princeton in 1975, and
1980 Alan Guth proposes that a took various fellowships before
rapid expansion, called cosmic joining the Harvard–Smithsonian
inflation, shaped the universe. Center for Astrophysics in 1983.
She worked there with John
AFTER Huchra, analyzing the results of
1998 The Sloan Digital Sky his redshift survey. Geller went
Survey finds walls, galaxy on to lead the second (CfA2)
sheets, and filaments many redshift survey. She is a frequent
hundreds of light-years long. public speaker and has made
several films about the universe,
1999 A redshift survey of including Where the Galaxies
supernovae reveals that the Are, which takes viewers on
universe’s expansion is a graphical voyage around
speeding up. the large-scale objects of the
observed universe.

THE TRIUMPH OF TECHNOLOGY 275

See also: Spiral galaxies 156–61 ■ Beyond the Milky Way 172–77 ■ Cosmic inflation 272–73 ■
A digital view of the skies 296

Galaxies form clusters and superclusters that fill wide, and 16 million thick. It was
narrow bands of space around vast empty voids. the first of several supersized
structures now known.
These voids are too large ever to have contained matter.
The size of the voids puzzled
They must have been present in the very early universe. astronomers. They were too large
to have been emptied completely
Before Huchra began his survey, walls enclosing vast voids, like the by the gravitational collapse of
it was known that galaxies surface film of a bubble. She found material that formed the stars and
existed in clusters. For example, the first “great wall” of galactic galaxies, which meant that they
the Milky Way is one of at least superclusters in 1989. The exact must have been empty since
54 galaxies in a cluster called the size of CfA2 Great Wall is still the beginning of the universe.
Local Group, which is about 10 unclear, but it is estimated at 700 Cosmologists theorize that the
million light-years wide. It was million light-years long, 250 million large-scale order of superclusters
assumed that clusters were evenly and voids is the legacy of quantum
spread. However, by 1980, Huchra fluctuations during the inflationary
had shown through his redshift epoch of the universe. Quantum
survey that dozens of clusters fluctuations are fleeting changes
form superclusters hundreds of in the amount of energy at points
millions of light-years wide. in space. These small but highly
The Local Group is part of the significant irregularities were
Laniakea Supercluster, which locked into the fabric of the
contains 100,000 other galaxies. universe in the first fraction
of a second of its existence, and
Walls of galaxies remain today. They are now the
In 1985, Geller began the CfA2 vast areas of void permeated by
Redshift Survey, taking 10 years a tangled pattern of matter. ■
to map 15,000 galaxies. Her survey
confirmed that superclusters were
themselves arranged in sheets and

This computer simulation of a
portion of the universe shows the
distribution of 10,000 galaxies, which
cluster in long filaments and “walls,”
in between vast empty voids.

276 IN CONTEXT

IFSNRTSOAIRMDSETFOHOUERTM KEY ASTRONOMER
Frank Shu (1943–)
 INSIDE GIANT MOLECULAR CLOUDS
BEFORE
1947 Bart Bok observes dark
nebulae and suggests that
they are sites of star formation.

1966 Frank Shu and
Chia-Chiao Lin develop
the density wave theory
to explain spiral arms in
the Milky Way.

AFTER
2003 The Spitzer Space
Telescope, an infrared
observatory, is launched.
It produces the best view
yet of stellar nurseries.

2018 First light on the
James Webb Space Telescope
will allow astronomers
to study protostars inside
dark Bok globules.

S tars form inside dark globules
of dust and gas that are
called giant molecular clouds
(GMCs). However, the process by
which a cloud of gas transforms
into an embryonic star, or protostar,
has never been observed, partly
because the process must take
millions of years, and partly because
even the most advanced telescopes
find it difficult to penetrate the dark
density of the cloud.

Without observational evidence,
astrophysicists must construct
mathematical models for what they
think is happening inside those
dark globules. The most consistent
model of star formation was derived
by US mathematician Frank Shu.

THE TRIUMPH OF TECHNOLOGY 277

See also: Stellar composition 162–63 ■ Nuclear fusion within stars 166–67 ■ Energy generation 182–83 ■
Dense molecular clouds 200–01 ■ Studying distant stars 304–05 ■ Jeans (Directory) 337 ■ Ambartsumian (Directory) 338

The Pillars of Creation are vast clouds
of gas and dust where new stars are
made. This famous image was captured
by the Hubble Space Telescope in 1995.

Shu and his colleagues Fred
Adams and Susana Lizano at the
University of California at Berkeley
presented their model in 1986 after
20 years of work.

The inside-out model GMCs are vast regions of the In the mid-1960s, Shu and the
Shu’s system is called the “singular galaxy filled with hydrogen atoms renowned Chinese−American
isothermal model,” or the “inside- and molecules mixed with specks mathematician Chia-Chiao Lin
out model.” It is built from the of dust and ice. Typically, a GMC modeled the rotation of a spiral
complex mathematics that define contains 100,000 solar masses of galaxy, and showed that the arms
the dynamics of gas clouds, taking material, which is a mixture of are located at density waves—
into account factors such as primordial gases produced by the “traffic jams” of stars. Such density
temperature, density, electrical Big Bang and the remnants of long- waves sweep up interstellar
charge, and magnetism. Shu’s dead stars. GMCs are mostly found material into GMCs, and this
model works by making the process in the spiral arms of a galaxy. triggers the formation of stars. ❯❯
self-similar. A starting condition
that causes some of the gas cloud
to contract into a dense core will
result in the same—or similar—
conditions, which cause more
gas to join the core, and so on.
This process was found to be
stable enough to keep the young
star together as it grew. Earlier
models had failed because they
could not find a way to balance
the mechanisms that were pulling
the gases in and pushing heat out;
as a result, these models ended
with the young star disintegrating.

Stars are dense balls of They must have formed Material from near
superhot hydrogen. from clouds of hydrogen the middle contracted

gas in interstellar space. first, and then drew
in the outer regions.

Stars form from the inside out.

278 INSIDE GIANT MOLECULAR CLOUDS

Frank Shu’s inside-out model The impact of a density wave, or and the contracting gravitational
describes the four-stage formation something more violent such as force in the core wins out over
of a star from a giant molecular cloud. a blast from a nearby supernova, the outward pressure.
1 Cores form within GMCs as magnetic creates turbulence inside a GMC.
forces and turbulence calm. However, highly tangled magnetic The inner region of the cloud
fields run through the cloud, and core contracts to form a dense ball
2 A protostar with a surrounding nebular these stop the turbulence from of gas at the center. This is the
disk forms at the center of a cloud core, ripping the cloud apart. The protostar. Protostars do not form
collapsing from inside out. magnetism also acts to prevent in a rapid process, but take millions
the cloud from collapsing in on of years, and millions more to
itself under its own gravity. grow into a full-fledged star. The
protostar is also surrounded by
Cloud cores a disk of material formed by the
Over millions of years, the magnetic system’s rotation, and wave upon
pressure and turbulence in the wave of material is pulled in from
gases dissipate, creating regions the surrounding envelope of gas.
of calm, where slowly rotating With each wave, the mass of the
“cloud cores” form. On closer protostar and its more diffuse disk
inspection, GMCs are not uniform, grows, and its gravity grows with
but made up of dark fragments it. The increasing gravity steadily
or clumps of denser material, pulls in material from farther
known as Bok globules. Each away, hence the description of the
globule is thought to contain process as an “inside-out collapse.”
several cloud cores.
The star gathers mass
Shu’s model supposes that the The protostar warms up as it
core becomes a single isothermal becomes denser, but it is still too
(equal-temperature) sphere, or small and cold to produce energy
something very close to it. This by fusing hydrogen in its core.
means that the gravity pulling the The force of all of the new material
ball of gas together is balanced by landing on its surface also adds
the outward pressure of the moving to the heat signature given out by
gas and its magnetic forces. Such the protostar. At this stage, it is
a state can never persist for long, giving out only faint infrared and

3 A stellar wind breaks out along the Frank Shu model while at Berkeley and
rotational axis of the system, creating was the head of the astronomy
a bipolar flow. Born in Kunming, China, department there by the time
4 The infall of material ends, revealing a Frank Shu moved to the United he presented the full review of
newly formed star with a circumstellar disk. States when he was six to his isothermal sphere model in
join his father, an academic 1986. Today, Shu holds tenure
mathematician, who was at Berkeley. In recent years he
beginning research at MIT. has used his knowledge of
Frank followed his father to astrophysics to tackle climate
MIT, where he completed change. He often works in
a degree in physics in 1963. collaboration with his graduate
While there, Shu worked students, who are collectively
on the density wave theory known as the “Shu Factory.”
of spiral arms. He later moved
to Harvard to complete his Key work
doctorate in astronomy in 1968.
Shu worked on his protostar 1981 The Physical Universe

THE TRIUMPH OF TECHNOLOGY 279

microwave radiation, which makes full-fledged stars destined to An infant star sits at the center of
it hard to see. Eventually, however, live short, bright lives. However, two nearly symmetrical jets of dense
the protostar gathers enough mass smaller stars—those less than 8 gas. Known as CARMA-7, the star is
for fusion to begin, but initially only solar masses—have not begun a about 1,400 light-years from Earth.
the deuterium, a heavy isotope of full fusion process and so are known
hydrogen, begins to burn. Unlike as pre-main-sequence (PMS) stars. form from PMS stars that are
an “adult” star, a protostar releases less than 2 solar masses. They are
its heat entirely by a process of A PMS star still has a disk of considerably wider and less dense
convection. Heat from its core rises material spinning around it. Some than their adult forms, and appear
up to the surface in the same way of that will be dispersed by the much brighter as they give out
that hot water in a pan rolls around stellar wind into the wider GMC. light from larger surface areas,
as it boils. The convection and What remains, around the smaller frequently punctuated by high-
rotation of the star create a strong stars especially, is likely to form energy outbursts of X-rays.
magnetic field, which pushes out into gas giant planets, and perhaps This energy is the product of
from each pole, clearing a narrow later, rocky ones as well. gravitational contraction, not
hole in the envelope of gas and Final ignition nuclear fusion. It takes about
dust. The growing protostar’s heat The final phase of star formation is 100 million years for the PMS star
and a stellar wind of plasma are a contraction of the fast-spinning to compress itself enough to begin
directed away from the star via PMS star. Red, orange, and yellow burning hydrogen, and by that
these polar jets. These features, dwarfs (M, K, G, and F type stars) time, it will have lost half to three-
explained by Shu’s model, have quarters of its initial mass. Larger
been confirmed by observations. A protoplanetary disk surrounds PMS stars (those between 2 and 8
the young star HL Tauri in the solar masses) take a different route
Becoming a star, almost constellation Taurus. The dark patches to achieving fusion and form rare
A star with the mass of the sun are thought to represent the possible blue dwarfs (A and B type stars).
spends about 10 million years as positions of newly forming planets.
a protostar. As its mass increases, PMS stars are the earliest
the angle of its polar jets widens, stage of star formation that have
pushing away more of the gas been seen clearly. Infrared space
cloud. Eventually, the protostar’s telescopes such as Spitzer and
stellar wind blasts out from the Hubble have given faint glimpses
entire star’s surface, and it clears of protostars but mostly they are
its gas cloud away completely. At too heavily shrouded by the dark
this point, the young stellar object dust clouds. NASA’s new infrared
is revealed for the first time. Giant James Webb Space Telescope is
stars (above 8 solar masses) have designed to be sensitive enough
already started burning hydrogen to see through that dust, so perhaps
by this point and have become soon the moment when a star is
born may at last be observed. ■

IWNRITNIKMLEES

OBSERVING THE CMB



282 OBSERVING THE CMB

IN CONTEXT T he Cosmic Microwave The Cosmic Microwave
Background, or CMB, Background Explorer (COBE)
KEY ASTRONOMERS was discovered in 1964. spent four years in space collecting
George Smoot (1945–) This is the afterglow of the Big information about the CMB, scanning
John Mather (1946–) Bang and it is as near as scientists the celestial sphere every six months.
can get to observing the event
BEFORE that brought the universe into to the inflationary model of the
1964 The cosmic microwave existence, 13.8 billion years ago. early universe proposed by
background—an echo of the Linking the structures observed American Alan Guth.
Big Bang itself—is discovered. in the universe to the features
1981 Alan Guth proposes discerned in the CMB remains The CMB is a flash of radiation
cosmic inflation, a theory in a key challenge for cosmologists. that was released about 380,000
which fluctuations of energy years after the Big Bang, at
density were locked into space Wrinkled time the time the first atoms formed
during the Big Bang. The first great breakthrough (pp.196–97). The expanding
1983 Redshift surveys show came from the Cosmic Microwave universe had cooled enough for
that galaxies are clustered Background Explorer, known as stable ions (positively charged
around voids of nothingness. COBE, a NASA satellite launched nuclei) of hydrogen and helium
in 1989. The detectors on COBE, to form, and then, after a little
AFTER designed and run by George more cooling, the ions began to
2001 Wilkinson Microwave Smoot, John Mather, and Mike collect electrons to make neutral
Anisotropy Probe is launched Hauser, were able to find the atoms. The removal of free electrons
to refine the map of the CMB. oldest structures in the visible from space led to the release of
2015 The Planck observatory universe, described by Smoot as photons (particles of radiation).
studies the CMB to refine the “wrinkles in time.” These wrinkles
age of the universe to 13.813 in otherwise uniform space were Those photons are visible now
billion years +/- 38 million years. once dense regions containing the as the CMB. The CMB comes from
Combining this with other data, matter that would form stars and the whole sky, without exception.
the latest estimate is 13.799 galaxies. They correspond to the It has redshifted (the wavelengths
billion years +/- 21 million years. large-scale galaxy superclusters have stretched), and it now has
and great walls seen in the wavelengths of a few millimeters,
I always think of universe today, and add weight while the original radiation’s
space-time as being the wavelengths would be measured
real substance of space, in nanometers (billionths of a meter).
and the galaxies and the
stars just like the foam

on the ocean.
George Smoot

THE TRIUMPH OF TECHNOLOGY 283

See also: The birth of the universe 168–71 ■ Searching for the Big Bang 222–27 ■ Cosmic inflation 272–73 ■
Redshift surveys 274–75 ■ Tegmark (Directory) 339

One of the key observations of The cosmic microwave background
the CMB came in the 1970s, and is a flash of radiation produced
removed any doubt that it was an 380,000 years after the Big Bang.
echo of the Big Bang. This was
the discovery that the thermal The CMB’s wavelength The CMB is not
spectrum of radiation from the shows how hot the smooth and uniform, but
CMB tallied very closely with that contains tiny fluctuations
of a theoretical black body (p.225). universe was when the
CMB was emitted. in temperature.
Black bodies
Black bodies do not really exist— These fluctuations, or
they cannot be made and no object “wrinkles in time”, are the oldest structures
observed in the universe functions
as black bodies do in theory. ever found and represent the formation
However, the CMB is the closest of the first stars and galaxies.
match that has ever been found.
heated. Heating it more makes it other astronomical objects are a
A black body absorbs all orange, and eventually the bar will much closer match to a black body,
radiation that hits it. Nothing is glow “blue hot.” Metalworkers learn and so the color, or wavelengths of
reflected. However, the absorbed to roughly judge the temperature of their emissions, can be compared
radiation adds to the thermal iron by its color. The metal is not to the thermal spectrum of a
energy of the object, and this particularly close to a black body in theoretical black body to give
is released as radiation. In 1900, the theoretical sense, but stars and a relatively precise temperature. ❯❯
German Max Planck, the founding
figure of quantum physics, showed
that the spectrum of radiation
released by a black body is entirely
dependent on temperature.

In an everyday example of
radiation varying with temperature,
an iron bar glows red when first

George Smoot After a childhood in Florida and Time with Keay Davidson
Ohio, Smoot began his career as to explain the discovery.
a particle physicist working at Smoot won the Nobel Prize in
MIT. His interests switched to 2006, along with John Mather,
cosmology and he moved across for his work on COBE. He
the country to the Lawrence reportedly gave his prize
Berkeley National Laboratory. money to charity. However,
It was there that Smoot studied three years later, Smoot won
the CMB and developed ways an even greater sum when he
of measuring its radiation. bagged the $1 million jackpot
on the TV game show Are You
Smoot’s early work involved Smarter Than a 5th Grader?
fitting detectors to high-altitude
U2 spyplanes, but in the late Key work
1970s, he became involved in the
COBE project to take his detector 1994 Wrinkles in Time (with
into space. After his success with Keay Davidson)
COBE, Smoot cowrote Wrinkles in

284 OBSERVING THE CMB

The temperature of the CMB today is in places. These denser areas, or [COBE has made] the
a chilly 2.7 K. The thermal spectrum anisotropies, were where the stars greatest discovery of the
at that temperature contains no and galaxies formed. COBE was century, if not of all time.
visible light, which is why space looks sent into space to take a close look
black to human eyes. However, the at the CMB to see if it could find Stephen Hawking
spectrum has redshifted (stretched) any anisotropies, to find out whether
over time as the universe has the CMB changed, however slightly, 2 K (colder than space itself) using
expanded. Extrapolating back to depending on where it looked. 100 gallons (650 liters) of liquid
the moment the CMB was emitted helium. George Smoot ran the
gives an original temperature COBE’s mission Differential Microwave Radiometer
of about 3,000 K. The color of A mission to study the CMB from (DMR), which mapped the precise
radiation at this temperature is space had been in the planning wavelengths of the CMB, while
orange, so the CMB started out stages since the mid-1970s. John Mather was in charge of
as a flash of orange light that shone Construction of COBE began in 1981. FIRAS, the Far-InfraRed Absolute
out from every point in space. It was initially designed to enter Spectrophotometer, which collected
polar orbit (its orbit passing over both data on the spectra of the CMB.
Smooth signal poles). However, the Challenger These two experiments were
The early observations of the CMB disaster of 1986 grounded the shuttle looking for anisotropies. The third
suggested that it was isotropic, fleet, and the COBE team had to look detector on COBE had a slightly
which means that its spectrum is for another launch system. In 1989, different goal. The Diffuse Infrared
the same everywhere. In cosmology, the satellite was launched using Background Experiment, run
the terms density, energy, and a Delta rocket, and it was placed by Mike Hauser, found galaxies
temperature are somewhat in a sun-synchronous geocentric that were so ancient and far
synonymous when discussing orbit—orbiting in a way that saw away that they are only visible by
the early universe. So the isotropic it pass over each place on Earth at their heat radiation (or infrared).
nature of the CMB suggested that, the same time of day. This worked
in those early days, space had a just as well as a polar orbit in that COBE’s instruments created
uniform density, or spread of energy. it allowed COBE to point away from the most accurate map of the
However, this did not tally with the Earth and scan the entire celestial CMB to date. However, it was not
developing theories of the Big Bang, sphere, strip by strip. a simple surveying job. Smoot and
which demanded that matter and Mather were interested in primary
energy were not evenly spread The spacecraft carried three anisotropies—that is, the density
through the young universe, but instruments, all protected from differences that were present at
had been concentrated together the sun’s heat and light by a cone- the time the CMB formed. To find
shaped shield, and chilled to these, they needed to filter out the
secondary fluctuations caused by
obstacles that lay between COBE

The full-sky map produced
by WMAP in 2011 showed many
fine details of the isotropy of the
CMB. Colder spots are blue, while
hotter spots are yellow and red.

THE TRIUMPH OF TECHNOLOGY 285

In addition to mapping the CMB,
WMAP measured the age of the
universe as 13.77 billion years, dark
matter as 24.0 percent of the universe,
and dark energy as 71.4 percent.

and the edge of the universe. Dust
clouds and the effects of gravity
had interfered with the radiation on
its long journey to Earth. The data
from the three instruments were
used to detect and correct these
so-called secondary anisotropies.

Tiny fluctuations Since COBE, two subsequent However, no known galaxy can
After 10 months in space, COBE’s missions have added detail to be seen forming in the CMB. The
helium ran out, which limited the picture of the CMB. Between CMB radiation detected today has
the function of the two infrared 2001 and 2010, NASA’s Wilkinson traveled from near the edge of the
detectors, but the DMR continued Microwave Anisotropy Probe observable universe over the course
working until 1993. By 1992, the (WMAP) mapped the CMB to of most of the age of the universe.
COBE team’s analysis had shown a higher resolution than COBE. Astronomers can only see 13.8 billion
what they were looking for. The Then, from 2009–2013, the ESA’s light-years away, but most of the
CMB, and thus the early universe, Planck Observatory produced the universe now lies farther away
was not a uniform blob of energy. most accurate map to date. than that. The galaxies forming in
Instead it was riddled with tiny the CMB are now far beyond what
but significant fluctuations. The Every wrinkle on the map is the can be observed, and are receding
differences were minute, with seed from which an entire galaxy faster than the speed of light. ■
density variations of about 0.001 formed about 13 billion years ago.
percent. However, the pattern was
enough to explain why the contents
of the universe are clustered
together, while the rest of space
is made from vast empty voids.

Improving resolution of the CMB

COBE’s imaging of the CMB shows WMAP’s map of the CMB shows Planck’s resolution is 2.5 times greater
slight variations in a 10-sq-degree greater detail within the same panel, than that of WMAP, showing features as
panel of its all-sky map, proving that revealing smaller-scale features that small as 1⁄ 12 of a degree. This is the most
the CMB is not uniform. COBE could not identify. detailed map of the CMB to date.

286

BTHELETKIUSIPREERAL

EXPLORING BEYOND NEPTUNE

IN CONTEXT The outer solar system contains the
leftover material from the formation of the planets.
KEY ASTRONOMERS
David Jewitt (1958–) Some of the material travels Short-period comets
Jane Luu (1963–) from the edge of the solar must come from a
nearer source.
BEFORE system in the form of
1930 American astronomer long-period comets.
Clyde Tombaugh discovers Pluto
orbiting beyond Neptune. It is The Kuiper belt, a theoretical reservoir of icy bodies beyond the
initially identified as the ninth orbit of Neptune, could be the source of short-period comets.
planet but is later reclassified.
I n 1950, Dutch astronomer Jan belt beyond Neptune. But the
1943 Kenneth Edgeworth Oort proposed that a spherical Dutch−American astronomer
suggests that Pluto is just shell of potential comets Gerard Kuiper argued in 1951 that,
one of many objects in the surrounds the solar system half a although there was once such a
outer solar system. light-year away. The so-called Oort belt, it would have been scattered
cloud was the source of long-period away by the gravity of the outer
1950 Fred Whipple describes comets, which took millennia to planets. It was a puzzle, and comet
the icy nature of comets as orbit the sun. But the source of nuclei that far away would be too
“dirty snowballs.” the short-period comets that orbit faint for even the best telescopes.
the sun every few centuries must
AFTER be nearer. In 1943, Irish scientist In the 1980s, sensitive new CCD
2003 Sedna is discovered Kenneth Edgeworth speculated (charge-coupled device) detectors
orbiting 76 AU−1,000 AU from that the comet reservoir was a became available. With these,
the sun, beyond the outer edge astronomers at last had a chance
of the Kuiper belt.

2005 Eris is seen in the disk
beyond the Kuiper belt.

2008 Two Kuiper Belt Objects
are classified as dwarf planets
along with Eris, Pluto, and Ceres.

THE TRIUMPH OF TECHNOLOGY 287

See also: The Kuiper belt 184 ■ The Oort cloud 206 ■ Studying Pluto 314–17

The egg-shaped dwarf planet Haumea
hangs in the sky above one of its two
moons, Namaka. Haumea, discovered
in 2004, is the third-largest dwarf planet.

of spotting small icy objects beyond
Neptune. Americans David Jewitt
and Jane Luu were among the
astronomers who set about the
difficult task. After five years of
searching, in 1992, Jewitt and
Luu discovered an object formally
designated 1992 QB1, the first body
to be found beyond Neptune since
Pluto, and the first evidence that
the Kuiper belt was real.

Cubewanos and plutinos 45 AU from the sun. These KBOs this region is the source of short-
More than 1,000 Kuiper Belt Objects are sometimes called “cubewanos.” period comets. In 2006, Eris was
(KBOs) are now known and there Closer in, at around 40 AU, the designated a dwarf planet along
are probably thousands more. gravity of Neptune has thinned with Pluto. Since then, two more
They are designated as asteroids, out the Kuiper belt, leaving a family cubewanos, Makemake and
but unlike most asteroids, KBOs of objects (including Pluto itself) Haumea (Haumea is orbited by two
are typically a mixture of rock called “plutinos,” in orbits that are small moons), have been classed
and ices. The largest are several unaffected by Neptune’s gravity. as dwarf planets, with many more
hundred miles across and many Beyond the main Kuiper belt lies KBOs listed as candidate dwarf
of them have moons. a region called the “scattered disk,” planets. Scientists believe that
which includes the large objects Eris these KBOs resemble the primitive
1992 QB1 is typical of the KBOs and Sedna. It is now believed that bodies that formed the planets. ■
in the most densely populated
middle part of the Kuiper belt, about

Gerard Kuiper Gerard Kuiper was born surrounded the young sun
in the Netherlands in 1905. changed scientists’ view
At a time when few other of the early solar system.
astronomers were interested
in the planets, Kuiper, working In the 1960s, Kuiper
mostly at the University of helped identify landing sites
Chicago, made many discoveries on the moon for the Apollo
that changed the course of program and cataloged
space science: he found that the several binary stars. He died
Martian atmosphere was mostly of a heart attack in 1973, at age
carbon dioxide, that Saturn’s 68. Since 1984, the Kuiper Prize
rings comprised billions of chunks has been awarded annually by
of ice, and that the moon was the American Astronomical
covered in a fine rock dust. In Society to recognize achievement
1949, Kuiper’s idea that the in planetary science, a field
planets were formed from of astronomy in which many
a cloud of gas and dust that consider Gerard Kuiper to
have been the pioneer.

MOST STARS

PARLE AORNBITEETD BSY

EXOPLANETS



290 EXOPLANETS I n 1995, two Swiss astronomers, For more than 2,000 years,
Michel Mayor and Didier people have dreamed of
IN CONTEXT Queloz, researching at the
Observatoire de Haute-Provence finding other habitable worlds.
KEY ASTRONOMERS near Marseille, found a planet Michel Mayor
Michel Mayor (1942–) orbiting 51 Pegasi, a sunlike
Didier Queloz (1966–) star 60 light-years away in the The discovery of 51 Pegasi b
constellation of Pegasus. This marked the final milestone in a
BEFORE was the first confirmed observation process that has forced astronomers
1952 US scientist Otto Struve of a true extrasolar planet, or to abandon any lingering notion
proposes the radial velocity exoplanet—a planet beyond the that Earth occupies a privileged
method to find exoplanets. solar system. It was orbiting place in the universe.
a main sequence star, and was
1992 The first such planet is therefore assumed to have formed Copernican principle
found, orbiting a pulsar and by the same process as that In the 1950s, the Anglo−Austrian
not a main sequence star. which created the solar system. astronomer Hermann Bondi had
described a new way for humans
AFTER Mayor and Queloz named the to think about themselves, which
2004 Construction begins new planet 51 Pegasi b, but it is he called the Copernican principle.
on the James Webb Space unofficially known as Bellerophon According to Bondi, humankind
Telescope, which will be after the hero who rode Pegasus, could no longer regard itself as
able to image exoplanets. the winged horse of ancient Greek a unique phenomenon of central
myth. Its discovery prompted a importance to the universe. On
2005 The Nice model offers a major hunt to find more exoplanets. the contrary, humans should now
new idea for the evolution of the Since 1995, several thousand
solar system that places the exoplanets have been found,
giant planets closer to the sun. many in multiple star systems.
Astronomers now estimate that
2014 The construction of the there is an average of one planet
European Extremely Large around every star in the galaxy,
Telescope begins. although this is probably a very
conservative figure. Some stars
2015 Kepler 442-b, an Earth- have no planets, but most, like
sized rocky exoplanet around the sun, have several.
an orange dwarf, is discovered.
Michel Mayor was born in sensitive enough to spot giant
Michel Mayor Lausanne, Switzerland, and has planets as well, and, following
spent most of his career working their 1995 discovery, Mayor is
at the University of Geneva. His currently the chief investigator
interest in exoplanets arose from at the HARPS program
his earlier study of the proper for the European Southern
motion of stars in the Milky Way. Observatory in Chile. His team
To measure this motion more has found about half of all the
accurately, he developed a exoplanets discovered to date.
series of spectrographs, which In 2004, Mayor was awarded
eventually culminated in ELODIE. the Albert Einstein medal.
The ELODIE project with Didier
Queloz was initially intended to Key work
search for brown dwarfs—objects
that were bigger than planets 1995 A Jupiter-mass Companion
but not quite large enough to be to a Solar-type Star (with
stars. However, the system was Didier Queloz)

THE TRIUMPH OF TECHNOLOGY 291

See also: The Copernican model 32–39 ■ Radio telescopes 210–11 ■ Studying distant stars 304–05 ■
Looking farther into space 326–27 ■ Kumar (Directory) 339

understand that their existence Unseen Star wobbles from one
is insignificant in the context planet position to another
of the universe.
Longer wavelength
The principle is named after indicates the star
Nicolaus Copernicus, who changed is retreating.
the way humankind saw itself by
relegating Earth from the center of Shorter wavelength indicates
the solar system to one of several the star is advancing.
planets that orbited the sun. By
the late 20th century, successive When a large Jupiter-like planet orbits its
discoveries had moved the solar star, it exerts a gravitational pull on the star.
system from the center of the Both star and planet revolve around a common
universe to a quiet wing at center of gravity. The “wobble” in the star’s
the edge of a galaxy containing orbit allows the planet to be detected.
200 billion other stars. The galaxy
was not special either, simply one In 1952, US astronomer Otto Struve That detector was a spectrograph
of at least 100 billion arranged in had suggested that this kind of named ELODIE developed by
vast filaments that extended for star wobble could be detected Mayor in 1993. ELODIE was about
hundreds of millions of light-years. as small fluctuations in a star’s 30 times more sensitive than any
Nevertheless, planet Earth and the spectrum. As the star moved away previous instrument. Even then,
solar system were still regarded as from Earth, its emissions would it was only capable of measuring
very special—since there was no be slightly redshifted from the velocity changes of 7 miles/s
evidence that any other stars had norm. When it wobbled back again (11 km/s), which meant it was
planets, let alone planets capable toward the observer, the light limited to detecting planets
of supporting life. Since Mayor’s would be blueshifted. The theory about the size of Jupiter.
and Queloz’s discovery, however, was solid but detecting the wobble
this idea has also succumbed to required an ultrasensitive detector. Improving the search
the Copernican principle. In 1998, an even more sensitive
We are getting much spectrograph, named CORALIE,
Wobbling light closer to seeing solar was installed at La Silla Observatory
Queloz and Mayor found 51 Pegasi b systems like our own. in Chile, which again was
using a system called Doppler searching for planets using the
spectroscopy. Also known as Didier Queloz radial velocity technique. In 2002,
the radial velocity or “wobble” Michel Mayor began overseeing
method, Doppler spectroscopy HARPS (High Accuracy Radial
can detect an exoplanet by its velocity Planet Searcher) at the
gravitational effects on its host star. same site, using a spectrograph
The star’s gravity is far greater than capable of detecting exoplanets
that of the planet, and this is what about the size of Earth. The wobble
keeps the planet in orbit. However, method of detection was very slow,
the planet’s gravity also has a small so new techniques of spotting
effect on the star, making it wobble exoplanets were developed. ❯❯
back and forth as the planet moves
around it. The effect is tiny: Jupiter
changes the sun’s speed by about
12 miles/s (7.4 km/s) over a period of
11 years, while Earth’s effect is only
0.1 miles/s (0.16 km/s) each year.

292 EXOPLANETS

The most successful method Ssun shade Photometer High gain
was the transit method, which antenna
looked for periodic changes in the
brightness of a star. These changes Radiator
were very small and happened
when a planet transited the star, Avionics
passing between the star and the
observer, and causing it to dim very Solar array
slightly. The best place to look for
exoplanets by the transit method The Kepler observatory Star
was out in space and so, in 2009, looked outward from the plane trackers
the Kepler observatory, named of the ecliptic, so that Earth,
after the man who first described the moon, and the sun Solid state
planetary orbits (pp.50–55), was did not obscure the view. recorder
launched to do just that.
unable to see individual exoplanets, that. Those that were correctly
Staring at one place but could identify stars that were oriented would only transit their star
Kepler was placed in a heliocentric likely to have them. once every orbital period (the planet’s
orbit, trailing behind Earth as it year), so Kepler’s method was better
circled the sun. The craft was Kepler could only detect the at finding planets that orbited close
designed to keep its aperture firmly transits of exoplanets with orbital to their star, taking a few years and
fixed on a single patch of space, paths that crossed the spacecraft’s months (or even weeks and days)
called the Kepler field. This made line of sight. Many exoplanets would to complete each revolution.
up only about 0.25 percent of the be orbiting at the wrong angle for
whole sky, but the spacecraft could
see 150,000 stars in that area. To
find exoplanets, Kelper would have
to concentrate on this single field
of view for years on end. It was

According to the Copernican principle, if the sun has Candidate stars
a planetary system, it is likely that other stars do as well. By the start of 2013, Kepler had
identified about 4,300 candidate
Many exoplanets have Exoplanets can be stars that might have extrasolar
been found by several detected by their effects planetary systems. Unfortunately,
different techniques. the guidance system used to keep
on their host star. Kepler locked on target then failed,
bringing its planet hunt to an end
Statistical analysis of Most stars are about three years sooner than
the data reveals how orbited by planets. expected. However, the data it
common exoplanets are. had collected was enough to keep
researchers busy for years to come.
Kepler’s candidate stars could only
be confirmed as planetary systems
using radial velocity measurements
from ground-based observatories,
such as HARPS in Chile and the
Keck Telescope in Hawaii. (Radial
velocity is the velocity of the star in
the direction of Earth.) So far, about
a tenth of Kepler’s candidate stars

THE TRIUMPH OF TECHNOLOGY 293

have proved to be false positives We were not expecting example, 51 Pegasi b was the first
but, after three years of analysis, to find a planet with a of many “hot Jupiters.” These have
the program had identified 1,284 4-day [orbital] period. No a mass similar to Jupiter’s and a
exoplanets, with more than 3,000 one was expecting this. large size that shows that they are
stars left to examine. The statistics mostly made of gas. 51 Pegasi b is
for the exoplanets in the Kepler field Michel Mayor half as massive as Jupiter, but is
are striking—most stars are part slightly larger. This gas giant orbits
of a planetary system. This means distance, and the temperature its sunlike star every four days.
that the number of planets in the of the star. This tells them what That means it is much closer to
universe is likely to exceed the the planet is probably made of its star than Mercury is to the sun.
number of stars. and allows them to conjecture Such proximity means it is tidally
what the surface conditions are locked to the star—one side always
The amount of dimming during likely to be. faces the scorching stellar surface,
a transit gives an indication of how and the other always faces away.
big an exoplanet might be, but the Hot and super Jupiters Many hot Jupiters have been found.
study of an exoplanet’s size and The exoplanets discovered so far They have confounded scientists,
characteristics is still in its early have added a host of weird worlds who are trying to understand how
stages. The light reflected from to the neat family portrait that is gas planets can exist so close to
a planet is about 10 billion times the sun’s planetary system. For a star without evaporating. Some
fainter than the star it orbits. exoplanets are dozens of times
Astronomers are waiting for the more massive than Jupiter, and
James Webb Space Telescope in are known as “super-Jupiters.” ❯❯
2018 and the European Extremely
Large Telescope in 2024 to image The “super-Jupiter” Kappa
this light directly and analyze the Andromedae b, shown here in an
chemistry of exoplanets. Until then, artist’s render, has a mass of 13 times
they have to speculate using very Jupiter’s. It glows a reddish color, and
little data: an approximate mass may yet be reclassed as a brown dwarf.
of the planet, its radius, the orbital

294 EXOPLANETS 2011-BLG-262, is thought to have that it orbits once an Earth day
a satellite, and could be the first and has a surface temperature
Red dwarfs with rocky exoplanet found with an exomoon. that would melt iron. Life seems
planets could be ubiquitous highly unlikely there, but the
Another class of planet are hunt continues for rocky planets
in the universe. called the super-Earths. These have that might be more hospitable.
Phil Muirhead a mass 10 times that of Earth but
less than that of an ice giant like Astrobiologists—scientists who
Professor of Astronomy Neptune. Super-Earths are not search for alien life—focus on the
Boston University rocky but made from gas and ice: particular conditions that all life
alternative names for them are needs. When choosing likely places
These super-Jupiter planets do mini-Neptunes or gas dwarfs. to look, they assume that alien life-
not appear to grow in size as their forms will require liquid water and
mass increases. For instance, Living planets carbon-based chemicals, just like
Corot-3b is a super-Jupiter that Earth’s solar system has terrestrial life on Earth. Living planets would
is 22 times as heavy as Jupiter planets (planets with a rocky also need an atmosphere to shield
but more or less the same size, due surface), of which Earth is the the surface from damaging cosmic
to its gravity holding its gaseous largest. So far, exoplanet searches rays and to act as a blanket that
contents together. Astronomers have struggled to find many retains some of the planet’s heat
have calculated that the density terrestrial planets, because they during the night.
of Corot-3b is greater than that are generally small and beyond
of gold and even osmium, the the sensitivity of the planet The region around a star where
densest element on Earth. detectors. The first confirmed the temperatures would allow
terrestrial exoplanet was Kepler- planets to have liquid water, carbon
Brown dwarfs and rogues 10b, which is three times the mass chemistry, and an atmosphere,
When a super-Jupiter reaches of Earth and is so close to its star is known as its habitable zone,
60 Jupiter masses, it is no longer also called the “Goldilocks zone”—
regarded as a planet, but as a
brown dwarf. A brown dwarf
is essentially a failed star—a
ball of gas that is too small to
burn brightly through nuclear
fusion. The brown dwarf and
its star are seen as a binary star
system, not a planetary one.
Some super-Jupiters and small
brown dwarfs have broken free of
their star to become free-floating
rogue planets. One, named MOA-

Kepler 10b in the Draco constellation
is shown transiting its star in an
artist’s impression. Its extremely
hot surface temperature and dizzying
orbit mean life there is improbable.

THE TRIUMPH OF TECHNOLOGY 295

A hotter star
The sun

A cooler star

The size of the habitable zone (green) depends on the size of the star. The red zone is too hot, while the
blue zone is too cold. The habitable zone is closer to cooler stars than it is to hotter stars. The size of a planet,
the shape of its orbit, and the speed of its rotation between night and day also affect its habitability.

like Baby Bear’s porridge in the the atmospheric chemistry for signs may throw light on that process.
fairy tale, “not too hot, not too cold.” of life, such as the presence of Even if life is found, it is likely
The size and locations of habitable elevated levels of oxygen, produced that most extraterrestrial natural
zones depend on the activity of by photosynthesizing life-forms. histories will not have moved
the host star. For example, if Earth How life evolved from nonliving beyond microorganisms. As
were orbiting a K-type star, an material on Earth is still a mystery every step toward evolving more
orange dwarf that is considerably but the study of Earth-like planets complex life-forms becomes ever
cooler than the sun (the sun is a more unlikely, so alien civilizations
G-type, or yellow dwarf), it would If we keep working as well and that match humankind’s will be
need to orbit at about one-third its we keep being as enthusiastic a lot less common. However, if
current distance to receive the … the issue about life on other only G-type stars, like the sun, are
same amount of warmth. counted, there are about 50 billion
planets will be solved. in the galaxy. It is estimated that
Of the thousands of exoplanets Didier Queloz 22 percent of them have an Earth-
that have been identified, only like planet in their habitable zones,
a tiny proportion are candidates which equals 11 billion possible
orbiting in their star’s habitable Earths. Adding in other types
zones, with Earth-like conditions of stars such as orange and red
for life—rocky surface with liquid dwarfs, that number rises to 40
water. Typically, they are larger billion. Even if the probability of
than Earth, and very few have good civilizations evolving is one in
prospects for being Earth-like. If a billion, the chances are that
and when Earth-like planets are humankind is not alone. ■
found, astrobiologists will look at

296

  UMTHNAEIPVMEORFOSSTETHEEAVMEBRITIOUS

A DIGITAL VIEW OF THE SKIES

IN CONTEXT S et up to produce “a field A cube section of the SDSS sky map
guide to the heavens,” the shows the intricate distribution of
KEY ASTRONOMER Sloan Digital Sky Survey matter in space. The tangles of light
Donald York (1944–) (SDSS) began operating in 1998. are interconnected galaxies.
The ambitious goal was to make
BEFORE a map of the universe on an onto a spectroscope. From
1929 Edwin Hubble proves immense scale—not just a survey these accurate galactic spectra,
that the universe is expanding. of objects on a celestial sphere, the astronomers can figure out
but a three-dimensional model of how far away each galaxy is. Data
1963 Maarten Schmidt a large portion of deep space. The collection began in 2000 and is
discovers quasi-stellar objects, project was initially headed by US expected to continue until 2020.
or quasars, which turn out to astronomer Donald York, but is The information gathered so far
be young galaxies. now a collaboration between 300 has revealed galaxies in clusters
astronomers from 25 institutions. and superclusters, and even galactic
1999 Saul Perlmutter, Brian SDSS uses an 8-ft 3-in (2.5-m) “walls”—immense structures
Schmidt, and Adam Riess telescope at Apache Point, New containing millions of galaxies,
show that the expansion of Mexico. The telescope’s wide-angle forming a tangled cosmic web
the universe is increasing camera has digitized objects visible with vast voids in between. ■
due to the mysterious effects from the northern hemisphere.
of “dark energy.”
From the 500 million objects
AFTER visible, the brightest 800,000
2004 Construction begins galaxies and 100,000 quasars
on the James Webb Space were selected, and their sizes
Telescope, which will use and positions in the sky accurately
infrared to see the first stars transposed as holes drilled into
that formed after the Big Bang. hundreds of aluminum disks. When
fitted to the telescope, a disk blocks
2014 Approval is given for unwanted light, and feeds the light
the European Extremely Large from each target galaxy into its
Telescope, which will have a own dedicated optical fiber and
128-ft (39-m) segmented main
mirror, making it the most See also: Beyond the Milky Way 172–77 ■ Quasars and black holes 218–21 ■
sensitive optical telescope ever. Studying distant stars 304–05 ■ Looking farther into space 326–27

THE TRIUMPH OF TECHNOLOGY 297

HOCUAENRRTBGROAARLLSABXALYMACASKSHIOVELE

 T  HE HEART OF THE MILKY WAY

IN CONTEXT I n 1935, Karl Jansky found a In 1980, Hawaii’s Keck Observatory
source of radio waves named began measuring the speed of stars
KEY ASTRONOMER Sagittarius A (Sgr A) at the orbiting close to the galactic center.
Andrea Ghez (1965–) center of the Milky Way. Hidden This data made it possible to
from light telescopes by cosmic calculate the mass of the invisible
BEFORE dust, the radio waves emanated object inside Sgr A*. Ghez’s team
1971 British astronomers from several sources. In 1974, radio found that the stars closest to Sgr A*
Martin Rees and Donald telescopes pinpointed the most were orbiting at a quarter of the
Lynden-Bell propose that intense source, named Sagittarius speed of light. Such speed indicated
the radio waves emanating A* (Sgr A*). It was small and an immense gravitational presence:
from Sagittarius A are produced intense X-rays, suggesting a black hole 4 million times heavier
produced by a black hole. that matter at the heart of the than the sun, which must have
galaxy was being ripped apart swallowed up stars and other black
AFTER by a gigantic black hole, emitting holes when the galaxy was young. ■
2004 A smaller black hole X-rays in the process. However, this
is discovered in orbit around remained hypothetical until Andrea An X-ray flare shoots from the black
Sagittarius A*. Ghez, an astronomer at UCLA, hole at the heart of the Milky Way. The
used a method for observing stars discovery suggests all galaxies may
2013 The Chandra X-ray through the dust using infrared. have black holes at their hearts.
Observatory sees a record-
breaking X-ray flare from
Sagittarius A*, perhaps caused
by an asteroid entering the
black hole.

2016 The LIGO experiment
makes the first detection of
gravitational waves, capturing
the moment when two black
holes merged into one.

See also: Radio astronomy 179 ■ Discovering black holes 254 ■
Studying distant stars 304–05

COSMIC

EXPANSION IS

ACCELERATING

DARK ENERGY