Astronomy - The Solar System and Beyond

Nucleus Cell membrane 2a A cell is a tiny factory that uses the DNA code
(information (transport of raw to manufacture chemicals. Most of the DNA
storage) materials and
finished product) remains safe in the nucleus of a cell, and the code

Material is copied to create a molecule of RNA (ribonucleic
storage
acid). Like a messenger carrying blueprints, the

RNA carries the code out of the nucleus to the

work site where the proteins and enzymes are

made.

Manufacture
of proteins
and enzymes

Original DNA

Energy
production

2b A single cell from a human being contains about
1.5 meters of DNA containing about 4.5 billion

base pairs — enough to record the entire works of

Shakespeare 200 times. A typical human contains a

total of about 600 AU of DNA. Yet the DNA in

each cell, only 1.5 meters in length,

contains all of the information to Sign in at

create a new human. A clone is a www.academic.cengage.com

new creature created from the and go to to see

DNA code found in a single the Active Figure called “DNA.”

Explore the structure of DNA.

cell.

3 DNA, coiled into a 3a To divide, a cell must duplicate its
tight spiral, makes up DNA. The DNA ladder splits, and
Copy DNA
new bases match to the exposed bases
the chromosomes that are
of the ladder to build two copies of the
the genetic material in a cell. A
original DNA code. Because the base
gene is a segment of a chromosome
pairs almost always match correctly,
that controls a certain function. When a
errors in copying are rare. One set of the
cell divides, each of the new cells receives a
DNA code goes to each of the two new
copy of the chromosomes, as genetic information
Copy DNA cells.
is handed down to new generations.

Cell Reproduction by Division

As a cell begins to The duplicated The two sets of the cell divides to two cells, each
divide, its DNA chromosomes move chromosomes produce . . . containing a full set
duplicates itself. to the middle. separate, and . . . of the DNA code.

Electrodes

To CH4 ⎫ Spark
vacuum ⎪ discharge
pump NH3 ⎬ Gases
H2O ⎪

H2 ⎭

Cooling water out
Condenser

Cooling water in

Water droplets

Boiling water Water containing b
a organic compounds

Liquid water in trap

■ Figure 20-3

(a) The Miller experiment circulated gases through water in the presence of an electric arc. This simulation of primitive conditions on Earth produced many complex
organic molecules, including amino acids, the building blocks of proteins. (b) Stanley Miller with a Miller apparatus. (Stanley Miller)

Miller and Urey let the experiment run for a week and then Many of these organic compounds would have been able to
analyzed the material inside. They found that the interaction link up to form larger molecules. Amino acids, for example, can
between the electric arc and the simulated atmosphere had pro- link together to form proteins by joining ends and releasing a
duced many organic molecules from the raw material of the ex- water molecule (■ Figure 20-4). That reaction, however, does not
periment, including such important building blocks of life as proceed easily in a water solution. Scientists hypothesize that this
amino acids. When the experiment was run again using different step may have been more likely to happen in sun-warmed tidal
energy sources such as hot silica to represent molten lava spilling pools or on shorelines where organic molecules from the primor-
into the ocean, similar molecules were produced. Even a source dial soup could have been concentrated by evaporation. The
of ultraviolet radiation representing the small amount of UV in production of large organic molecules may have been aided in
sunlight was sufficient to produce complex organic molecules. such semi-dry environments by clay crystals acting as templates
to hold the organic subunits close together.
Scientists are professionally skeptical about scientific find-
ings (see “How Do We Know” 7-3), and they re-evaluated the These complex organic molecules were still not living things.
Miller-Urey experiment in light of new information. According Even though some proteins may have contained hundreds of
to updated models of the formation of the solar system and Earth amino acids, they did not reproduce but rather linked and broke
(Chapters 7 and 8), Earth’s early atmosphere probably consisted apart at random. Because some molecules are more stable than
mostly of carbon dioxide, nitrogen, and water vapor instead of others, and some bond together more easily than others, scien-
the mix of hydrogen, ammonia, methane, and water vapor as- tists hypothesize that a process of chemical evolution eventually
sumed by Miller and Urey. When gases corresponding to the concentrated the various smaller molecules into the most stable
newer understanding of the early Earth atmosphere are processed larger forms. Eventually, according to the hypothesis, somewhere
in a Miller apparatus, lesser but still significant numbers of or- in the oceans, after sufficient time, a molecule formed that could
ganic molecules are created. copy itself. At that point, the chemical evolution of molecules
became the biological evolution of living things.
The Miller experiment is important because it shows that
complex organic molecules form naturally in a wide variety of An alternate theory for the origin of life holds that reproduc-
circumstances. Lightning, sunlight, and hot lava are just some of ing molecules may have arrived here from space. Radio astrono-
the energy sources that can naturally rearrange simple common mers have found a wide variety of organic molecules in the inter-
molecules into the complex molecules that make life possible. If stellar medium, and similar compounds have been found inside
you could travel back in time, you would expect to find Earth’s meteorites (■ Figure 20-5). The Miller experiment showed how
first oceans filled with a rich mixture of organic compounds easy it is to create complex organic molecules from simpler com-
called the primordial soup. pounds, so it is not surprising to find them in space. Although

434 P A R T 5 | L I F E

HH Water
O

H HOHHOH HOH HO H HO HHO H HO

NCCNCCNCC NCC NCC N C C OH H N C C OH

CH3 CH3 CH3 CH3 CH3 CH3 CH3 N HC
H
HC
Growing carbon-chain molecule Amino acid Amino acid

CH 3 O OH
Amino

acid

■ Figure 20-4

Amino acids can link together via the release of a water molecule to form long carbon-chain molecules. The amino acid in this hypothetical example is alanine, one
of the simplest.

■ Figure 20-5 speculation is fun, the hypothesis that life arrived on Earth from
space is presently more difficult to test than the hypothesis that
A piece of the Murchison meteorite, a carbonaceous chondrite (see Chapter 10) life on Earth originated on Earth.
that fell in 1969 near Murchison, Australia. Analysis of the interior of the mete-
orite revealed the presence of amino acids. Whether the first building blocks of Whether the first reproducing molecules formed here on
life originated in space is unknown, but the amino acids found in meteorites Earth or in space, the important thing is that they could have
illustrate how commonly amino acids and other complex organic molecules oc- formed by natural processes. Scientists know enough about those
cur in the universe, even in the absence of living things. (Chip Clark, National processes to feel confident about them, even though some of the
Museum of Natural History) steps remain unknown.

The details of the evolution of the first cells are unknown,
but the first reproducing molecule to surround itself with a
protective membrane would have gained an important survival
advantage. Experiments have shown that microscopic spheres the
size of cells containing organic molecules form relatively easily in
water (■ Figure 20-6), so the evolution of cell membranes is not
surprising.

The first cells must have been simple single-celled organisms
much like modern bacteria. As you learned earlier, these kinds of
cells are preserved in stromatolites (Figure 20-2), mineral forma-
tions produced by layers of photosynthetic bacteria and shallow
ocean sediments. Stromatolites are in rocks with radioactive ages
of 3.4 billion years, and they also still form in some places today.
Stromatolites and other photosynthetic organisms would have
begun adding oxygen, a product of photosynthesis, to Earth’s
early atmosphere (■ Figure 20-7). An oxygen abundance of only
0.1 percent would have created an ozone screen, protecting or-
ganisms from the sun’s ultraviolet radiation and later allowing life
to colonize the land.

Over the course of eons, the natural processes of evolution
gave rise to stunningly complex multicellular life forms with

C H A P T E R 2 0 | L I F E O N O T H E R W O R L D S 435

■ Figure 20-6 their own widely differing ways of life. It is a Common Miscon-
ception to imagine that life is too complex to have evolved from
Single amino acids can be assembled into long proteinlike molecules. When such simple beginnings. It is possible because small variations
such material cools in water, it can form microspheres, microscopic globules can accumulate, although that accumulation requires huge
with double-layered boundaries similar to cell membranes. Microspheres may amounts of time.
have been an intermediate stage in the evolution of life between complex but
nonliving molecules and living cells holding molecules reproducing genetic in- Geologic Time
formation. (Sidney Fox and Randall Grubbs)
Life has existed on Earth for at least 3.4 billion years, but there
is little evidence of anything more than simple organisms until
about 540 million years ago, when life suddenly branched into a
wide variety of complex forms like the trilobites (■ Figure 20-8).
This sudden increase in complexity is known as the Cambrian
explosion, and marks the beginning of the Cambrian period.

If you represented the entire history of Earth on a scale dia-
gram, the Cambrian explosion would be near the top of the
column, as shown at the left of ■ Figure 20-9. The emergence of
most animals familiar to you today, including fishes, amphibians,
reptiles, birds, and mammals, would be crammed into the top-
most part of the chart, above the Cambrian explosion.

If you magnify that portion of the diagram, as shown on the
right side of Figure 20-9, you can get a better idea of when these
events occurred in the history of life. Humanoid creatures have
walked on Earth for about 4 million years. This is a long time by
the standard of a human lifetime, but it makes only a narrow red

■ Figure 20-7

Artist’s conception of a scene
on the young Earth, 3 billion
years ago, with stromatolite
bacterial mats growing near the
shores of an ocean. (National Mu-
seum of Natural History, Peter Sawyer,
© 2009 Smithsonian Institution)
Animated!

436 P A R T 5 | L I F E

■ Figure 20-8 If you watched closely, you might see the first humanoid
forms by late afternoon on New Year’s Eve, and by late evening
Trilobites made their first appearance in the Cambrian oceans. The smallest were you could see humans making the first stone tools. The Stone
almost microscopic, and the largest were bigger than dinner plates. This exam- Age would last until 11:59 pm, after which the first towns, and
ple, about the size of a human hand, lived 400 million years ago in an ocean then cities would appear. Suddenly things would begin to hap-
floor that is now a limestone deposit in Pennsylvania. (Franklin & Marshall Col- pen at lighting speed. Babylon would flourish, the Pyramids
lege/Grundy Observatory/Michael Seeds) would rise, and Troy would fall. The Christian era would begin
14 seconds before the New Year. Rome would fall, the Middle
line at the top of the diagram. All of recorded history would be Ages and the Renaissance would flicker past. The American and
a microscopically thin line at the very top of the column. French revolutions would occur one and a half seconds before
the end of the video.
To understand just how thin that line is, imagine that the
entire 4.6-billion-year history of the Earth has been compressed By imagining the history of Earth as a yearlong video, you
onto a yearlong video and that you began watching this video on have gained some perspective on the rise of life. Tremendous
January 1. You would not see any signs of life until March or amounts of time were needed for the first simple living things to
early April, and the slow evolution of the first simple forms evolve in the oceans; but, as life became more complex, new forms
would take the next six or seven months. Suddenly, in mid- arose more and more quickly as the hardest problems — how to
November, you would see the trilobites and other complex or- reproduce, how to take energy efficiently from the environment,
ganisms of the Cambrian explosion. how to move around — were “solved” by the process of biological
evolution. The easier problems, like what to eat, where to live, and
You would see no life of any kind on land until November how raise young, were managed in different ways by different or-
28, but once life appeared it would diversify quickly, and by ganisms, leading to the diversity that is seen today.
December 12 you would see dinosaurs walking the continents.
By the day after Christmas they would be gone, and mammals Even intelligence — that which appears to set humans apart
and birds would be on the rise. from other animals — may be a unique solution to an evolution-
ary problem posed to humanity’s ancient ancestors. A smart ani-
mal is better able to escape predators, outwit its prey, and feed
and shelter itself and its offspring, so under certain conditions
evolution is likely to select for intelligence. Could intelligent life
arise on other worlds? To try to answer this question, you can
estimate the chances of any type of life arising on other worlds,
then assess the likelihood of that life developing intelligence.

Life in Our Solar System

Could there be carbon-based life elsewhere in our solar system?
Liquid water seems to be a requirement of carbon-based life,
necessary both as the medium for vital chemical reactions and to
transport nutrients and wastes. It is not surprising that life devel-
oped in Earth’s oceans and stayed there for billions of years be-
fore it was able to colonize the land.

Scientists are in agreement that any world harboring living
things must have significant quantities of some type of liquid.
Water is a cosmically abundant substance with properties such as
high heat capacity that set it apart from other common molecules
that are liquid at the temperatures of planetary surfaces. Maybe
there are other liquids that can support the processes of life on
other planets, but water, aside from being common in the uni-
verse, has characteristics that cause Earth scientists to regard it as
special, and not just because they are made of water themselves.

Many worlds in the solar system can be eliminated immedi-
ately as hosts for life because liquid water is not possible there. The
moon and Mercury are airless, and water would boil away into
space immediately. Venus has traces of water vapor in its atmo-

CHAPTER 20 | LIFE ON OTHER WORLDS 437

0 0 Age of humans Age of
Dec Life on land Tertiary First anthropoids mammals

Nov Cambrian period First horses Age of
reptiles
100 Cretaceous First flowering (dinosaurs)
plants
Age of
Oct 1 amphibians
Sept
Jurassic First birds Age of
Aug Precambrian 200 fishes
period First primitive
2 Triassic mammals
July Last
Permian trilobites

Billion years ago 300 Pennsylvanian Coal-forming
Million years ago Mississippian forests

June

May 3 Devonian First forests
April 400 First life on land

Silurian

Origin of life Ordovician Age of
marine
March 500 invertebrates
Cambrian
Feb 4 Cambrian
Pre- explosion
cambrian
600 Ocean life only

Jan
4.6 Formation of Earth

■ Figure 20-9

Complex life has developed on Earth only recently. If the entire history of Earth were represented in a time line (left), you would have
to magnify the end of the line to see details such as life leaving the oceans and dinosaurs appearing. The age of humans would still
be only a thin line at the top of your diagram. If the history of Earth were a yearlong videotape, humans would not appear until the
last hours of December 31.

sphere, but it is too hot for liquid water to survive on the surface. powerful downdraft currents in the gas giants’ atmospheres would
The Jovian planets have deep atmospheres, and at a certain level it quickly carry any reproducing molecules that did form there into
is likely that water condenses into liquid droplets. However, it inhospitably hot regions of the atmosphere.
seems unlikely that life could have originated there. The Jovian
planets do not have solid surfaces (see Chapter 9), so isolated water As you learned in Chapter 9, at least one of the Jovian satel-
droplets cannot mingle to mimic the rich primordial oceans of lites could potentially support life. Jupiter’s moon Europa appears
Earth, where organic molecules grew and interacted. Additionally, to have a liquid-water ocean below its icy crust, and minerals dis-
solved in the water could provide a rich source of raw material for

438 P A R T 5 | L I F E

chemical evolution. Europa’s ocean is kept warm and liquid now ■ Figure 20-10
by tidal heating. There also may be liquid water layers under the
surfaces of Ganymede and Callisto. That can change as the orbits Meteorite ALH84001 is one of a dozen meteorites known to have originated on
of the moons interact; Europa, Ganymede, and Callisto may have Mars. A research group claimed that the meteorite contained chemical and
been frozen solid at other times in their histories. physical traces of ancient life on Mars, including what appear to be fossils of
microscopic organisms. That evidence has not been confirmed, and the claim
Saturn’s moon Titan is rich in organic molecules. Chapter continues to be tested and debated. (NASA)
19 described how sunlight converts the methane in Titan’s atmo-
sphere into organic smog particles that settle to the surface. The ing this question, you can try to identify the kinds of stars that
chemistry of life that could have evolved from those molecules seem most likely to have stable planetary systems where life could
and survived in Titan’s lakes of methane is unknown. It is fasci- evolve.
nating to consider possibilities, but Titan’s extremely low tem-
perature of Ϫ180°C (Ϫ290°F) would make chemical reactions If a planet is to be a suitable home for living things, it must
slow to the point where life processes seem unlikely. be in a stable orbit around its sun. That is easy in a planetary
system like our own, but planet orbits in binary star systems
Water containing organic molecules has been observed venting would be unstable unless the component stars are very close to-
from the south polar region of Saturn’s moon Enceladus. (See gether or very far apart. Astronomers can calculate that, in binary
Chapter 9.) It is possible that life could exist in that water under systems with stars separated by intermediate distances of a few
Enceladus’s crust, but the moon is very small and its tidal heating AU, the planets should eventually be swallowed up by one of the
may operate only occasionally. Enceladus may not have had plenti- stars or ejected from the system. Half the stars in the galaxy are
ful liquid water for the extended time necessary for the rise of life. members of binary systems, and many of them are unlikely to
support life on planets.
Mars is the most likely place for life to exist in the solar
system because, as you learned in Chapter 8, there is a great deal Moreover, just because a star is single does not necessarily
of evidence that liquid water once flowed on its surface. Even so, make it a good candidate for sustaining life. Earth required per-
results from searches for signs of life on Mars are not encourag- haps as much as 1 billion years to produce the first cells and
ing. The robotic spacecraft Viking 1 and Viking 2 landed on 4.6 billion years for intelligence to emerge. Massive stars that live
Mars in 1976 and tested soil samples for living organisms. Some only a few million years do not meet this criterion. If the history
of the tests had puzzling semi-positive results that scientists hy- of life on Earth is representative, then stars more massive and
pothesize were caused by nonbiological chemical reactions in the luminous than about spectral type F5 are too short lived for
soil. No evidence clearly indicates the presence of life or even of complex life to develop. Main-sequence stars of types G and K,
organic molecules in the Martian soil. and possibly some of the faint M stars, are the best candidates.

There was a splash of news stories in the 1990s regarding The temperature of a planet is also important, and that de-
supposed chemical and physical traces of life on Mars discovered pends on the type of star it orbits and its distance from the star.
inside a Martian meteorite found in Antarctica (■ Figure 20-10). Astronomers have defined a habitable zone around a star as a
Scientists were excited by the announcement, but they employed region within which planets have temperatures that permit the
professional skepticism and immediately began testing the evi- existence of liquid water. The sun’s habitable zone extends from
dence. Their results suggest that the unusual chemical signatures around the orbit of Venus to the orbit of Mars, with Earth right
in the rock may have formed by processes that did not involve life.
Tiny features in the rock that were originally thought to be fossils
of ancient Martian microorganisms could possibly be nonbio-
logical mineral formations instead. This is the only direct evi-
dence yet found regarding potential life on Mars, but it remains
highly controversial. Conclusive evidence of life on Mars may
have to wait until a geologist from Earth can scramble down dry
Martian streambeds and crack open rocks looking for fossils.

There is no strong evidence for the existence of life elsewhere
in the solar system. Now your search will take you to distant
planetary systems.

Life in Other Planetary Systems

Could life exist in other planetary systems? You already know
that there are many different kinds of stars and that many of
these stars have planetary systems. As a first step toward answer-

C H A P T E R 2 0 | L I F E O N O T H E R W O R L D S 439

in the middle. A low-luminosity star has a small habitable zone, with the mass of a yacht to the nearest star, 4 light-years away,
and a high-luminosity star has a large one. and you wanted to travel at half the speed of light so as to arrive
in 8 years, the trip would require 400 times as much energy as
Scientists on Earth are finding life in places previously the entire United States consumes in a year.
judged inhospitable, such as the bottoms of ice-covered lakes in
Antarctica and far underground inside solid rock. Life has also These limitations not only make it difficult for humans to
been found in boiling hot springs with highly acidic water. As a leave the solar system, but they would also make it difficult for
result, it is difficult to pin down a range of environments and be aliens to visit Earth. Reputable scientists have studied “unidenti-
sure that life cannot exist outside those conditions. You should fied flying objects” (UFOs) and have never found any evidence
also note that three of the environments listed as possible havens that Earth is being visited or has ever been visited by aliens
for life, Europa, Titan, and Enceladus, are in the outer solar sys- (■ How Do We Know? 20-2). Humans are unlikely ever to meet
tem, far outside the sun’s conventionally defined habitable zone. aliens face to face. However, communication by radio across in-
Stable planets inside the habitable zones of long-lived stars are terstellar distances takes relatively little energy.
the places where life seems most likely, but, given the tenacity
and resilience of Earth’s life forms, there might be other, seem- Radio Communication
ingly inhospitable, places in the universe where life exists.
Nature puts restrictions on travel through space, and it also re-
̇ SCIENTIFIC ARGUMENT ̈ stricts astronomers’ ability to communicate with distant civiliza-
tions by radio. One restriction is based on simple physics. Radio
What evidence indicates that life is possible on other worlds? signals are electromagnetic waves and travel at the speed of light.
A good scientific argument involves careful analysis of evidence. Fossils on Due to the distances between the stars, the speed of radio waves
Earth show that life originated in the oceans at least 3.4 billion years ago, would severely limit astronomers’ ability to carry on normal
and biologists have proposed likely chemical processes that could have conversations with distant civilizations. Decades could elapse
eventually yielded reproducing molecules inside membranes, the first between asking a question and getting an answer.
simple life forms. Fossils show that life developed slowly at first. The pace
of evolution quickened about half a billion years ago, when life took on So, rather than try to begin a conversation, one group of
complex forms. Later, when life emerged onto the land, it evolved rapidly astronomers decided in 1974 to broadcast a message of greeting
into diverse forms. Intelligence is a relatively recent development: It is only toward the globular cluster M13, 26,000 light-years away, using
a few million years old. the Arecibo radio telescope (see Figure 5-20). When the signal
arrives 26,000 years in the future, alien astronomers may be able
If this evolutionary process occurred on Earth, it seems reasonable that it to understand it because the message is anticoded, meaning that
could have occurred on other worlds as well. Earth-like worlds could be it is intended to be decoded by beings about whom we know
plentiful in the universe. Life may begin and eventually evolve to intelligence nothing except that they build radio telescopes. The message is a
on any world where conditions are right. Now make a related argument. What string of 1679 pulses and gaps. Pulses represent 1s, and gaps
are the conditions you should expect on other worlds that host life? represent 0s. The string can be arranged in only two possible
ways: as 23 rows of 73 or as 73 rows of 23. The second arrange-
̇̈ ment forms a picture containing information about life on Earth
(■ Figure 20-11).
20-3 Communication with
Although the 1974 Arecibo beacon was the only powerful
Distant Civilizations signal sent purposely from Earth to other solar systems, Earth is
sending out many other signals. Short-wave radio signals, such as
Visiting extrasolar planets is, for now, impossible. Neverthe- TV and FM, have been leaking into space for the last 50 years or
less, if other civilizations exist, it is possible humans can com- so. Any civilization within 50 light-years could already have de-
municate with them. Nature puts restrictions on the pace of such tected Earth’s civilization. That works both ways: Alien signals,
conversations, but the main problem lies in the unknown life whether intentional messages of friendship or the blather of their
expectancy of civilizations. equivalent to daytime TV, could be arriving at Earth now. As-
tronomers all over the world are pointing radio telescopes at the
Travel Between the Stars most likely stars and listening for alien civilizations.

The distances between stars are almost beyond comprehension. Which channels should astronomers monitor? Wavelengths
The space shuttle would take about 150,000 years to reach the longer than 30 cm would get lost in the background noise of the
nearest star. The obvious way to overcome these huge distances Milky Way Galaxy, while wavelengths shorter than about 1 cm
is with tremendously fast spaceships, but even the closest stars are are mostly absorbed in Earth’s atmosphere. Between those wave-
many light-years away. lengths is a radio window that is open for communication. Even
this restricted window contains millions of possible radio-
Nothing can exceed the speed of light, and accelerating a
spaceship close to the speed of light takes huge amounts of en-
ergy. Even if you travel more slowly, your rocket would still re-
quire massive amounts of fuel. If you were piloting a spaceship

440 P A R T 5 | L I F E

20-2

UFOs and Space Aliens

Has Earth been visited by aliens? If you con- Most are mistakes and unintentional misinter-
clude that there is likely to be life on other pretations, committed by honest people, of nat-
worlds, then you might be tempted to use UFO ural events or human-made objects. Over many
sightings as evidence to test your hypothesis. decades, experts have studied these incidents
Scientists don’t do this for two reasons. and found none that are convincing. In short,
despite false claims to the contrary on TV shows,
First, the reputation of UFO sightings and there is no dependable evidence that Earth has
alien encounters does not inspire confidence ever been visited by aliens.
that these data are reliable. Most people hear of
such events in grocery store tabloids, daytime In a way, that’s too bad. A confirmed visit by
talk shows, or sensational “specials” on viewer- intelligent creatures from beyond our solar sys-
hungry cable networks. You should take note of tem would answer many questions. It would be
the low reputation of the media that report UFOs exciting, enlightening, and, like any real adven-
and space aliens. Most of these reports, like the ture, a bit scary. But scientists must profession-
reports that Elvis is alive and well, are simply ally pay attention to what is supported by evi-
made up for the sake of sensation, and you can- dence rather than what might be thrilling. There
not use them as reliable evidence. is not yet any direct evidence of life on other
worlds.
Second, the few UFO sightings that are not
made up do not survive careful examination.

Flying saucers from space are fun to think about, but
there is no evidence that they are real.

frequency bands and is too wide to monitor easily, but astrono- searches and is currently building a new radio telescope array in
mers may have thought of a way to narrow the search. Within northern California, collaborating with the University of Califor-
this window lie the 21-cm spectral line of neutral hydrogen and nia, Berkeley, and partly funded by Paul Allen of Microsoft.
the 18-cm line of OH (■ Figure 20-12). The interval between
those lines has low background interference and is named the There is even a way for you to help with searches. The
water hole because H plus OH yields water. Any civilizations Berkeley SETI team (Note: they are separate from the SETI In-
sophisticated enough to do radio astronomy research must know stitute), with the support of the Planetary Society, has recruited
of these lines and might appreciate their significance in the same about 4 million owners of personal computers that are connected
way as do Earthlings. to the Internet. You can download a screen saver that searches
data files from the Arecibo radio telescope for signals whenever
A number of searches for extraterrestrial radio signals have you are not using the computer. For information, locate the
been made, and some are now under way. This field of study is seti@home project at http://setiathome.ssl.berkeley.edu/.
known as SETI, Search for Extra-Terrestrial Intelligence, and it
has generated heated debate among astronomers, philosophers, The search continues, but radio astronomers struggle to hear
theologians, and politicians. Congress funded a NASA search for anything against the worsening babble of radio noise from human
a short time but ended support in the early 1990s because it feared civilization. Wider and wider sections of the electromagnetic spec-
public reaction. In fact, the annual cost of a major search is only trum are being used for Earthly communication, and this, com-
about as much as a single Air Force attack helicopter, but much of bined with stray electromagnetic radiation from electronic devices
the reluctance to fund searches probably stems from issues other including everything from computers to refrigerators, makes hear-
than cost. One point to keep in mind is that the discovery of real ing faint radio signals difficult. It would be ironic if humans fail to
alien intelligence would cause a huge change in humans’ world- detect faint signals from another world because our own world has
view, akin to Galileo’s discovery that the moons of Jupiter do not become too noisy. Ultimately, the chance of success depends on
go around the Earth. Some turmoil would likely result. the number of inhabited worlds in the galaxy.

In spite of the controversy, the search continues. The NASA How Many Inhabited Worlds?
SETI project canceled by Congress was renamed Project Phoenix
and completed using private funds. The SETI Institute, founded Given enough time, the searches will find other worlds with
in 1984, managed Project Phoenix plus several other important civilizations, assuming that there are at least a few out there. If
intelligence is common, scientists should find signals relatively

C H A P T E R 2 0 | L I F E O N O T H E R W O R L D S 441

An anticoded message ■ Figure 20-11

1010011111 (a) An anticoded message is designed for easy decoding. Here a string of 35 radio pulses,
0010100100 represented as 1s and 0s, can be arranged in only two ways, as 5 rows of 7 or 7 rows of 5.
0101001010 The second way produces a friendly message. (b) The Arecibo message describes life on
Earth (color added for clarity). Binary numbers give the height of the human figure (1110)
and the diameter of the telescope dish (100101111110) in units of the wavelength of the
signal, 12.3 cm. (NASA) Animated!

01010

5 rows of 7 The Arecibo message
1010011
1110010 Start of number markers Binary numbers 1 to 0
1001000
1010010 Formulas for sugars Atomic numbers of
1001010 and bases in DNA hydrogen, carbon,
nitrogen, oxygen,
and phosphorus

7 rows of 5 Start of number markers DNA double helix
10100 Human figure
11111 Number of units
00101 Population of Earth in DNA
00100
01010 Arecibo radio dish Start of number marker
01010 transmitting signal
01010 Height of human
Start of number marker in wavelengths
a
Sun and planets with
Earth offset

Diameter of dish
in wavelengths

b

soon — within the next few decades — but if intelligence is rare, Signal strength Noise from galaxy The Noise fromQEuaamrnthetu’csmhaatmnicoaspl nhoeirsee
it may take much longer. water
hole
Simple arithmetic can give you an estimate of the number of
technological civilizations in the Milky Way Galaxy with which H OH
you might communicate, Nc. The formula proposed for discus-
sions about Nc is named the Drake equation after the radio as- Total noise
tronomer Frank Drake, a pioneer in the search for extraterrestrial
intelligence. The version of the Drake equation presented here is Background
modified slightly from its original form: radiation

Nc ϭ N* и fP и nHZ и fL и fI и fS 300 30 3 0.3 0.03
N* is the number of stars in our galaxy, and fP represents the Wavelength (cm)
fraction of stars that have planets. If all single stars have planets,
fP is about 0.5. The factor nHZ is the average number of planets ■ Figure 20-12
in each planetary system suitably located in the habitable
zone — meaning for the sake of the present discussion, the num- Radio noise from various astronomical sources makes it difficult to detect dis-
ber of planets per planetary system possessing liquid water. Eu- tant signals at wavelengths longer than 30 cm or shorter than 1 cm. In this
ropa and Enceladus in our solar system show that liquid water range, wavelengths of radio emission lines from H atoms and from OH molecules
can exist due to tidal heating outside the conventional habitable mark a small wavelength range named the water hole that may be a likely chan-
zone that in our system contains Earth’s orbit. Thus, nHZ may be nel for communication.

442 P A R T 5 | L I F E

larger than had been originally thought. The factor fL is the frac- Earth may be the only planet that is capable of communication
tion of suitable planets on which life begins, and fI is the fraction within thousands of the nearest galaxies.
of those planets where life evolves to intelligence.
̇ SCIENTIFIC ARGUMENT ̈
The six factors on the right-hand side of the Drake equation
can be roughly estimated, with decreasing certainty as you pro- Why does the number of civilizations that could be detected depend on
ceed from left to right, The final factor is extremely uncertain. how long civilizations survive at a technological level?
That factor fS is the fraction of a star’s life during which an intel- This scientific argument depends on the timing of events. If you turned a
ligent species is communicative. If a society survives at a techno- radio telescope to the sky and scanned millions of frequency bands for
logical level for only 100 years, the chances of communicating many stars, you would be taking a snapshot of the universe at a particular
with it are small. On the other hand, a society that stabilizes and time. Broadcasts from other civilizations must be arriving at that same time
remains technological for a long time is much more likely to be if they are to be detected. If most civilizations survive for a long time, there
detected. For a star with a life span of 10 billion years, fS might is a much greater chance that you will detect one of them in your snapshot
conceivably range from 10Ϫ8 for extremely short-lived societies than if civilizations tend to disappear quickly due to nuclear war or
to 10Ϫ4 for societies that survive for a million years. ■ Table 20-1 environmental collapse. If most civilizations last only a short time, there
summarizes what many scientists consider a reasonable range of may be none capable of transmitting during the short interval when
values for fS and the other factors. Earthlings are capable of building radio telescopes to listen for them.

If the optimistic estimates are true, there could be a com- The speed at which astronomers can search for signals is limited
municative civilization within a few tens of light-years from because computers must search many frequency intervals, but not all
Earth. On the other hand, if the pessimistic estimates are true, frequencies inside Earth’s radio window are subject to intensive search.
Build a new argument to explain: Why is the water hole an especially
good frequency band in which to listen?

̇̈

■ Table 20-1 ❙ The Number of Technological Civilizations per Galaxy

Estimates Variables Pessimistic Optimistic

N* Number of stars per galaxy 2 ϫ 1011 2 ϫ 1011
fP Fraction of stars with planets 0.1 0.5
nHZ Number of planets per star that lie in habitable zone for longer than 4 billion years 0.01 1
fL Fraction of suitable planets on which life begins 0.01 1
fI Fraction of planets with life where life forms evolve to intelligence 0.01 1
fS Fraction of star’s existence during which a technological society survives 10Ϫ8 10Ϫ4
Nc Number of communicative civilizations per galaxy 2 ϫ 10-4 1 ϫ 107

What Are We? Matter and Spirit

There are over 4000 religions around the world, fish, bacteria, grass, birds, worms, and other the universe. Among all of the galaxies, stars,
and nearly all hold that humans have a dual living things. You are using your atoms now, but planets, planetesimals, and bits of matter, hu-
nature. We are physical objects made of atoms, when you are done with them, they will go back mans are objects that can think, and that
but we are also spiritual beings. Science is un- to Earth and be used again and again. means we can understand what we are.
able to examine the spiritual side of existence,
but it can tell us about our physical nature. When the sun swells into a red giant star Is the human race the only thinking species?
and dies in a few billion years, Earth’s atmo- If so, we bear the sole responsibility to under-
The matter you are made of appeared in the sphere and oceans will be driven away, and at stand and admire the universe. The detection of
big bang and was cooked into a wide range of least the outer few kilometers of Earth’s crust signals from another civilization would demon-
elements inside stars. Your atoms may have will be vaporized and blown outward to become strate that we are not alone, and such commu-
been inside at least two or three generations of part of the nebula around the white-dwarf re- nication would end the self-centered isolation
stars. Eventually, your atoms became part of a mains of the sun. Your atoms are destined to of humanity and stimulate a reevaluation of the
nebula that contracted to form our sun and the return to the interstellar medium and will be- meaning of human existence. We may never re-
planets of the solar system come part of future generations of stars and alize our full potential as humans until we com-
planets. municate with nonhuman intelligent life.
Your atoms have been part of Earth for the
last 4.6 billion years. They have been recycled The message of astronomy is that humans
many times through dinosaurs, stromatolites, are not just observers: We are participants in

C H A P T E R 2 0 | L I F E O N O T H E R W O R L D S 443

Summary ̈ Because the origin of life and its evolution into intelligent creatures took
so long on Earth, scientists do not consider short-lived middle- and up-
̈ The process of life extracts energy from the surroundings, maintains an per-main-sequence stars as likely homes for life.
organism, and modifies the surroundings to promote the organism’s
survival. ̈ Main-sequence G and K stars are thought to be likely candidates to host
planets with life. Scientists are not sure whether the fainter M stars are
̈ Living things have a physical basis — the arrangement of matter and en- also good candidates or not.
ergy that makes life possible. Life on Earth is based on carbon chemistry.
̈ The habitable zone (p. 439) around a star, within which planets can
̈ Living things must also have a controlling unit of information that can be have liquid water on their surfaces, may be larger than scientists had ex-
passed to successive generations. pected, given the wide variety of living things now found in extreme envi-
ronments on Earth and the possibility of tidal heating of moons orbiting
̈ Genetic information for life on Earth is stored in long carbon-chain mole- large planets.
cules such as DNA (deoxyribonucleic acid) (p. 432).
̈ Because of distance, speed, and fuel, travel between the stars seems al-
̈ The DNA molecule stores information in the form of chemical bases linked most impossible for humans, or for aliens who might visit Earth.
together like the rungs of a ladder. Copied by the RNA (ribonucleic acid)
(p. 433) molecule, the patterns of bases act as recipes for connecting to- ̈ Radio communication between planetary systems may be possible, but a
gether amino acid (p. 432) subunits to construct proteins (p. 432), in- real conversation would be difficult because of very long travel times for
cluding enzymes (p. 432), that are respectively the main structural and radio signals.
control components of the life process.
̈ Broadcasting a radio beacon of pulses would distinguish the signal from
̈ The unit of heredity is a gene (p. 433), a piece or several pieces of DNA naturally occurring radio emission and identify the source as a technologi-
that in most cases specifies the construction of just one particular type of cal civilization. The signal can be anticoded (p. 440) in the hope it
protein molecule. Genes are connected together in structures called chro- would be easy for another civilization to decode.
mosomes (p. 433), which are essentially single long DNA molecules.
̈ One good part of the radio spectrum for communication is called the wa-
̈ When a cell divides, the chromosomes split lengthwise and duplicate ter hole (p. 441), the wavelength range from the 21-cm spectral line of
themselves so that each of the new cells can receive a copy of the genetic hydrogen to the 18-cm line of OH. Even so, millions of radio wavelengths
information. need to be tested to fully survey the water hole for a given target star.

̈ Biological evolution (p. 430) begins when molecules begin reproducing. ̈ Sophisticated searches are now underway to detect radio transmissions
from civilizations on other worlds, but such SETI (Search for Extra-
̈ Errors in duplication or damage to the DNA molecule can produce mu- Terrestrial Intelligence) (p. 441) programs are hampered by limited
tants (p. 431), organisms that contain new DNA information and there- computer power and radio noise pollution from human civilization.
fore have new properties. Variation in genetic codes can become wide-
spread among individuals in a species. Natural selection (p. 431) ̈ The number of civilizations in our galaxy that are at a technological level
determines which of these variations are best suited to survive, and the and able to communicate while humans are listening can be estimated by
species evolves to fit its environment. the Drake equation (p. 442); this number is limited primarily by the life-
times of their and our civilizations.
̈ The oldest definitely identified fossils on Earth, structures called stromat-
olites (p. 431) that are composed of stacks of bacterial mats and sedi- Review Questions
ments, are at least 3.4 billion years old. Those fossils provide evidence
that life began in the oceans. To assess your understanding of this chapter’s topics with additional quizzing
and animations, go to academic.cengage.com/astronomy/seeds
̈ Fossil evidence indicates that life began on Earth as simple single-celled
organisms like bacteria and much later evolved into more complex, multi- 1. If life is based on information, what is that information?
cellular (p. 435) creatures.
2. What would happen to a life form if the information handed down to off-
̈ The Miller experiment (p. 431) shows that the chemical building blocks spring was always the same? How would that endanger the future of the
of life form naturally under a wide range of circumstances. life form?

̈ Biologists hypothesize that chemical evolution (p. 434) concentrated 3. How does the DNA molecule produce a copy of itself?
simple molecules into a diversity of larger stable organic molecules dis-
solved in the young Earth’s oceans, but those molecules did not repro- 4. Give an example of natural selection acting on new DNA patterns to select
duce copies of themselves. The hypothetical organic-rich water is some- the most advantageous characteristics.
times referred to as the primordial soup (p. 434).
5. What evidence do scientists have that life on Earth began in the sea?
̈ Life forms did not become large and complex until about 0.5 billion years
ago, during what is called the Cambrian explosion (p. 436). 6. Why do scientists think that liquid water is necessary for the origin of
life?
̈ Life emerged from the oceans only about 0.4 billion years ago, and hu-
man intelligence developed over the last 4 million years. 7. What is the difference between chemical evolution and biological
evolution?
̈ Life as it is known on Earth requires liquid water and thus a specific range
of temperatures. 8. What is the significance of the Miller experiment?

̈ No other planet in our solar system appears to harbor life at present. 9. How does intelligence make a creature more likely to survive?
Most are too hot or too cold, although life might have begun on Mars be-
fore it became too cold and dry. 10. Why are upper-main-sequence (high-luminosity) stars unlikely sites for
intelligent civilizations?
̈ Liquid water exists, and therefore Earth-like life is at least possible, under
the surfaces of Jupiter’s moon Europa and Saturn’s moon Enceladus. 11. Why is it reasonable to suspect that travel between stars is nearly
Saturn’s moon Titan has abundant organic compounds but does not have impossible?
liquid water.
12. How does the stability of technological civilizations affect the probability
that Earth can communicate with them?

444 P A R T 5 | L I F E

13. What is the water hole, and why would it be a good “place” to look for 6. Mathematician Karl Gauss suggested planting forests and fields in a gi- ESO NASA
other civilizations? gantic geometric proof to signal to possible Martians that intelligent life
exists on Earth. If Martians had telescopes that could resolve details no
14. How Do We Know? How do science and religion have complimentary ex- smaller than 1 arc second, how large would the smallest element of
planations of the world? Gauss’s signal have to be for it to be visible at Mars’s closest approach to
Earth? (Hint: See Reasoning with Numbers 3-1 and Appendix A.)
15. How Do We Know? Why are scientists sure Earth has never been visited
by aliens? 7. If you detected radio signals with an average wavelength of 20 cm and
suspected that they came from a civilization on a distant planet, roughly
Discussion Questions how much of a change in wavelength should you expect to see because of
the orbital motion of the distant planet? (Hint: See Reasoning with Num-
1. Do you expect hypothetical alien recipients of the Arecibo message will be bers 6-2.)
able to decode it? Why or why not?
8. Calculate the number of communicative civilizations per galaxy using your
2. What do you think it would mean if decades of careful searches for radio own estimates of the factors in Table 20-1.
signals for extraterrestrial intelligence turn up nothing?
Learning to Look
Problems
1. The star cluster shown in the image to the right, con-
1. A single human cell encloses about 1.5 m of DNA, containing 4.5 billion tains cool red giants and main-sequence stars from
base pairs. What is the spacing between these base pairs in nanometers? hot blue stars all the way down to red dwarfs. Discuss
That is, how far apart are the rungs on the DNA ladder? the likelihood that planets orbiting any of these stars
might be home to life. (Hint: Estimate the age of the
2. If you represent the history of the Earth by a line 1 m long, how long a cluster.)
segment would represent the 400 million years since life moved onto the
land? How long a segment would represent the 4-million-year history of 2. If you could search for life in the galaxy shown in the
human life? image to the right, would you look among disk stars
or halo stars? Discuss the factors that influence your
3. If a human generation, the average time from birth to childbearing, has decision.
been 20 years long, how many generations have passed in the last 1 mil-
lion years? Visual

4. If a star must remain on the main sequence for at least 5 billion years for
life to evolve to intelligence, what is the most massive a star can be and
still possibly harbor intelligent life on one of its planets? (Hint: See Rea-
soning with Numbers 13-1.)

5. If there are about 1.4 ϫ 10Ϫ4 stars like the sun per cubic light-year, how
many lie within 100 light-years of Earth? (Hint: The volume of a sphere is
-43 ␲r3.)

C H A P T E R 2 0 | L I F E O N O T H E R W O R L D S 445

The aggregate of all our joys and sufferings, thousands of confident religions,
ideologies and economic doctrines, every hunter and forager, every hero and coward,

every creator and destroyer of civilizations, every king and peasant, every young
couple in love, every hopeful child, every mother and father, every inventor and
explorer, every teacher of morals, every corrupt politician, every superstar, every
supreme leader, every saint and sinner in the history of our species, lived there on a

mote of dust, suspended in a sunbeam.

CARL SAGAN (1934–1996)

Earth photographed by Voyager 1 from
the edge of the solar system. NASA

446

Our journey together is over, but before we part company, Earth is the only inhabited planet, our responsibility is over-
let’s ponder one final time the primary theme of this book — hu- whelming. We are the only creatures who can take action to
manity’s place in the physical universe. Astronomy gives us some preserve the existence of life on Earth, and, ironically, our own
comprehension of the workings of stars, galaxies, and planets, actions are the most serious hazards.
but its greatest value lies in what it teaches us about ourselves.
Now that you have surveyed astronomical knowledge, you can The future of humanity is not secure. We are trapped on a
better understand your own position in nature. tiny planet with limited resources and a population growing
faster than our ability to produce food. We have already driven
To some, the word nature conjures up visions of furry rabbits some creatures to extinction and now threaten others. We are
hopping about in a forest glade. To others, nature is the blue- changing the climate of our planet in ways we do not fully un-
green ocean depths, and still others think of nature as windswept derstand. Even if we reshape our civilization to preserve our
mountaintops. As diverse as these images are, they are all Earth- world, the sun’s evolution will eventually destroy Earth.
bound. Having studied astronomy, you can see nature as a beau-
tiful mechanism composed of matter and energy, interacting ac- This may be a sad prospect, but a few factors are comforting.
cording to simple rules, forming galaxies, stars, planets, First, everything in the universe is temporary. Stars die, galaxies
mountaintops, ocean depths, forest glades, and people. die; perhaps the entire universe will someday end. Our distant
future is limited, and this assures us that we are a part of a much
Perhaps the most important astronomical lesson is that hu- larger whole. Second, we have a few billion years to prepare, and
manity is a small but important part of the universe. Most of the a billion years is a very long time. Only a few million years ago,
universe is probably lifeless. The vast reaches between the galaxies our ancestors were starting to walk upright and communicate. A
appear to be empty of all but the thinnest gas, and stars are much billion years ago, our ancestors were microscopic organisms liv-
too hot to preserve the chemical bonds that seem necessary for ing in the oceans. To suppose that a billion years hence there will
life to survive and develop. It seems that only on the surfaces of be beings resembling today’s humans, or that humans will still be
a few planets, where temperatures are moderate, can atoms link the dominant intelligence on Earth, or that humans will even
together in special ways to form living matter. exist, are ultimately conceits.

If life is special, then intelligence is precious. The universe Our responsibility is not to save our race for all eternity but
must contain many planets devoid of life, planets where sunlight to behave as dependable custodians of our planet, preserving it,
has shined unfelt for billions of years. There may also exist plan- admiring it, and trying to understand it. That calls for drastic
ets on which life has developed but has not become complex, changes in our behavior toward other living things and a revolu-
planets where the wind stirs wide plains of grass and rustles tion in our attitude toward our planet’s resources. Whether we
through dark forests. On some planets, creatures resembling can change our ways is debatable — humanity is far from perfect
Earth’s insects, fish, birds, and animals may watch the passing in its understanding, abilities, or intentions. However, you must
days only dimly aware of their own existence. It is intelligence, not imagine that we, and our civilization, are less than precious.
human or alien, that gives meaning to the landscape. We have the gift of intelligence, and that is the finest thing this
planet has ever produced.
Science is the process by which Earth’s intelligence has tried
to understand the physical universe. Science is not the invention Text not available due to copyright restrictions
of new devices or processes. It does not create home computers,
cure the mumps, or manufacture plastic spoons — those are en-
gineering and technology, the adaptation of scientific under-
standing for practical purposes. Science is the understanding of
nature, and astronomy is that understanding on the grandest
scale. Astronomy is the science by which the universe, through
its intelligent lumps of matter, tries to understand its own
existence.

As the primary intelligent species on this planet, we are the
custodians of a priceless gift — a planet filled with living things.
This is especially true if life is rare in the universe. In fact, if

AFTERWORD 447

Appendix A
Units and Astronomical Data

Introduction the meter, kilogram, and second. These three fundamental units
define the rest of the units, as given in ■ Table A-2.
The metric system is used worldwide as the system of units, not
only in science but also in engineering, business, sports, and daily The SI unit of force is the newton (N), named after Isaac
life. Developed in 18th-century France, the metric system has Newton. It is the force needed to accelerate a 1 kg mass by
gained acceptance in almost every country in the world because 1 m/s2, or the force roughly equivalent to the weight of an apple
it simplifies computations. at Earth’s surface. The SI unit of energy is the joule (J), the en-
ergy produced by a force of 1 N acting through a distance of
A system of units is based on the three fundamental units for 1 m. A joule is roughly the energy in the impact of an apple fall-
length, mass, and time. Other quantities, such as density and ing off a table.
force, are derived from these fundamental units. In the English
(or British) system of units (commonly used only in the United Exceptions
States, Tonga, and Southern Yemen, but, ironically, not in Great
Britain) the fundamental unit of length is the foot, composed of Units can help you in two ways. They make it possible to make
12 inches. The metric system is based on the decimal system of calculations, and they can help you to conceive of certain quanti-
numbers, and the fundamental unit of length is the meter, com- ties. For calculations, the metric system is far superior, and it is
posed of 100 centimeters. used for calculations throughout this book.

Because the metric system is a decimal system, it is easy to Americans commonly use the English system of units, so for
express quantities in larger or smaller units as is convenient. You conceptual purposes this book also expresses quantities in English
can give distances in centimeters, meters, kilometers, and so on. units. Instead of saying the average person would weigh 133 N on
The prefixes specify the relation of the unit to the meter. Just as the moon, it might be more helpful to some readers for that weight
a cent is 1/100 of a dollar, so a centimeter is 1/100 of a meter. A to be expressed as 30 lb. Consequently, this text commonly gives
kilometer is 1000 m, and a kilogram is 1000 g. The meanings of quantities in metric form followed by the English form in paren-
the commonly used prefixes are given in ■ Table A-1. theses: The radius of the moon is 1738 km (1080 mi).

The SI Units In SI units, density should be expressed as kilograms per
cubic meter, but no human hand can enclose a cubic meter, so
Any system of units based on the decimal system would be easy that unit does not help you grasp the significance of a given den-
to use, but by international agreement, the preferred set of units, sity. This book refers to density in grams per cubic centimeter. A
known as the Système International d’Unités (SI units) is based on gram is roughly the mass of a paperclip, and a cubic centimeter
is the size of a small sugar cube, so you can easily conceive of a
density of 1 g/cm3, roughly the density of water. This is not a

■ Table A-1 ❙ Metric Prefixes ■ Table A-2 ❙ SI (Système International)
Metric Units

Prefix Symbol Factor Quantity SI Unit

Mega M 106 Length Meter (m)
Kilo k 103 Mass Kilogram (kg)
Centi c 10Ϫ2 Time Second (s)
Milli m 10Ϫ3 Force Newton (N)
Micro ␮ 10Ϫ6 Energy Joule (J)
Nano n 10Ϫ9

448 A P P E N D I X A

bothersome departure from SI units because you will not have to The Fahrenheit scale fixes the freezing point of water at 32°F
make complex calculations using density. and the boiling point at 212°F. Named after the German physi-
cist Gabriel Daniel Fahrenheit (1686–1736), who made the first
Conversions successful mercury thermometer in 1720, the Fahrenheit scale is
used routinely only in the United States.
To convert from one metric unit to another (from meters to
kilometers, for example), you have only to look at the prefix. It is easy to convert temperatures from one scale to another
However, converting from metric to English or English to metric using the information given in ■ Table A-4.
is more complicated. The conversion factors are given in ■ Table
A-3. Powers of 10 Notation

Example: The radius of the moon is 1738 km. What is this Powers of 10 make writing very large numbers much simpler.
in miles? Table A-3 indicates that 1.000 mile equals 1.609 km, For example, the nearest star is about 43,000,000,000,000 km
so from the sun. Writing this number as 4.3 x 1013 km is much
easier.
1.000 mi
1738 km ϫ 1.609 km ϭ 1080 mi Very small numbers can also be written with powers of 10. For
example, the wavelength of visible light is about 0.0000005 m. In
Temperature Scales powers of 10 this becomes 5 ϫ 10Ϫ7 m.

In astronomy, as in most other sciences, temperatures are ex- The powers of 10 used in this notation appear below. The
pressed on the Kelvin scale, although the centigrade (or Celsius) exponent tells you how to move the decimal point. If the expo-
scale is also used. The Fahrenheit scale commonly used in the nent is positive, move the decimal point to the right. If the expo-
United States is not used in scientific work. nent is negative, move the decimal point to the left. For example,
2.0000 ϫ 103 equals 2000, and 2 ϫ 10Ϫ3 equals 0.002.
Temperatures on the Kelvin scale are measured from abso-
lute zero, the temperature of an object that contains no extract- 105 ϭ 100,000
able heat. In practice, no object can be as cold as absolute zero, 104 ϭ 10,000
although laboratory apparatuses have reached temperatures lower 103 ϭ 1,000
than 10Ϫ6 K. The Kelvin scale is named after the Scottish math- 102 ϭ 100
ematical physicist William Thomson, Lord Kelvin (1824– 101 ϭ 10
1907). 100 ϭ 1
10Ϫ1 ϭ 0.1
The centigrade scale refers temperatures to the freezing 10Ϫ2 ϭ 0.01
point of water (0°C) and to the boiling point of water (100°C). 10Ϫ3 ϭ 0.001
One degree Centigrade is 1/100, the temperature difference be- 10Ϫ4 ϭ 0.0001
tween the freezing and boiling points of water, thus the prefix
centi. The centigrade scale is also called the Celsius scale after its If you use scientific notation in calculations, be sure you
inventor, the Swedish astronomer Anders Celsius (1701–1744). correctly enter the numbers into your calculator. Not all calcula-
tors accept scientific notation, but those that can have a key la-
beled EXP, EEX, or perhaps EE that allows you to enter the ex-

■ Table A-3 ❙ Conversion Factors Between ■ Table A-4 ❙ Temperature Scales
British and Metric Units and Conversion Formulas

1 inch ϭ 2.54 centimeters 1 centimeter ϭ 0.394 inch Absolute zero Kelvin Centigrade Fahrenheit
1 foot ϭ 0.3048 meter 1 meter ϭ 39.36 inches ϭ 3.28 feet Freezing point of water (K) (°C) (°F)
1 mile ϭ 1.609 kilometers 1 kilometer ϭ 0.6214 mile Boiling point of water
1 slug ϭ 14.59 kilograms 1 kilogram ϭ 0.0685 slug Conversions: 0K Ϫ273°C Ϫ459°F
1 pound ϭ 4.448 Newtons 1 Newton ϭ 0.2248 pound 273 K 0°C 32°F
1 foot-pound ϭ 1.356 Joules 1 Joule ϭ 0.7376 foot-pound K ϭ °C ϩ 273 373 K 212°F
1 horsepower ϭ 745.7 Joules/s 1 Joule/s ϭ 0.001341 horsepower °C ϭ 5/9 (°F Ϫ 32) 100°C
1 Joule/s ϭ 1 Watt °F ϭ 9/5 (°C) ϩ 32

A P P E N D I X A 449

ponent of ten. To enter a number such as 3 ϫ 108, press the keys ■ Table A-5 ❙ Astronomical Constants
3 EXP 8. To enter a number with a negative exponent, you must
use the change-sign key, usually labeled ϩ/Ϫ or CHS. To enter Velocity of light (c) ϭ 3.00 ϫ 108 m/s
the number 5.2 ϫ 10Ϫ3, press the keys 5.2 EXP ϩ/Ϫ 3. Try a Gravitational constant (G) ϭ 6.67 ϫ 10Ϫ11 m3/s2kg
few examples. Mass of H atom ϭ 1.67 ϫ 10Ϫ27 kg
Mass of Earth (M ) ϭ 5.98 ϫ 1024 kg
To read a number in scientific notation from a calculator Earth equatorial radius (R ) ϭ 6.38 ϫ 103 km
you must read the exponent separately. The number 3.1 ϫ 1025 Mass of sun (M᭪) ϭ 1.99 ϫ 1030 kg
may appear in a calculator display as 3.1 25 or on some calcula- Radius of sun (R᭪) ϭ 6.96 ϫ 108 m
tors as 3.1 1025. Examine your calculator to determine how such Solar luminosity (L᭪) ϭ 3.83 ϫ 1026 J/s
numbers are displayed. Mass of moon ϭ 7.35 ϫ 1022 kg
Radius of moon ϭ 1.74 ϫ 103 km
Astronomy Units
■ Table A-6 ❙ Units Used in Astronomy
and Constants
1 Angstrom (Å) ϭ 10Ϫ8 cm
Astronomy, and science in general, is a way of learning about ϭ 10Ϫ10 m
nature and understanding the universe. To test hypotheses about 1 astronomical unit (AU) ϭ 10 nm
how nature works, scientists use observations of nature. The ta- 1 light-year (ly) ϭ 1.50 ϫ 1011 m
bles that follow contain some of the basic observations that sup- ϭ 93.0 ϫ 106 mi
port science’s best understanding of the astronomical universe. 1 parsec (pc) ϭ 6.32 ϫ 104 AU
Of course, these data are expressed in the form of numbers, not ϭ 9.46 ϫ 1015 m
because science reduces all understanding to mere numbers, but 1 kiloparsec (kpc) ϭ 5.88 ϫ 1012 mi
because the struggle to understand nature is so demanding that 1 megaparsec (Mpc) ϭ 2.06 ϫ 105 AU
science must use every valid means available. Quantitative ϭ 3.09 ϫ 1016 m
thinking — reasoning mathematically — is one of the most pow- ϭ 3.26 ly
erful techniques ever invented by the human brain. Thus, these ϭ 1000 pc
tables are not nature reduced to mere numbers but numbers sup- ϭ 1,000,000 pc
porting humanity’s growing understanding of the natural world
around us.

450 A P P E N D I X A

■ Table A-7 ❙ Properties of Main-Sequence Stars

Spectral Absolute Visual Luminosity* Temp. ␭max Mass* Radius* Average
Type Magnitude (K) (nm) Density
(Mv) 40 17.8 (g/cm3)
72.4 18 7.4
05 Ϫ5.8 501,000 40,000 100 6.4 3.8 0.01
190 3.2 2.5 0.1
B0 Ϫ4.1 20,000 28,000 290 2.1 1.7 0.2
340 1.7 1.4 0.3
B5 Ϫ1.1 790 15,000 390 1.3 1.2 0.6
440 1.1 1.0 1.0
A0 ϩ0.7 79 9900 480 0.9 0.9 1.1
520 0.8 0.8 1.4
A5 ϩ2.0 20 8500 590 0.7 0.7 1.6
700 0.5 0.6 1.8
F0 ϩ2.6 6.3 7400 830 0.2 0.3 2.4
1000 0.1 0.1 2.5
F5 ϩ3.4 2.5 6600 1200 10.0
63
G0 ϩ4.4 1.3 6000

G5 ϩ5.1 0.8 5500

K0 ϩ5.9 0.4 4900

K5 ϩ7.3 0.2 4100

M0 ϩ9.0 0.1 3500

M5 ϩ11.8 0.01 2800

M8 ϩ16 0.001 2400

*Luminosity, mass, and radius are given in terms of the sun’s luminosity, mass, and radius.

■ Table A-8 ❙ The 15 Brightest Stars

Star Name Apparent Visual Spectral Absolute Visual Distance
Magnitude Type Magnitude (ly)
␣ CMa A Sirius (mv) (Mv)
␣ Car Canopus A1 8.7
␣ Cen Rigil Kentaurus Ϫ1.47 F0 1.4 98
␣ Boo Arcturus Ϫ0.72 G2 Ϫ3.1 4.3
␣ Lyr Vega Ϫ0.01 K2 36
␣ Aur Capella Ϫ0.06 A0 4.4 26.5
␤ Ori A Rigel G8 Ϫ0.3 45
␣ CMi A Procyon 0.04 B8 900
␣ Ori Betelgeuse 0.05 F5 0.5 11.3
␣ Eri Achernar 0.14 M2 Ϫ0.6 520
␤ Cen AB Hadar 0.37 B3 Ϫ7.1 118
␣ Aql Altair 0.41 B1 490
␣ Tau A Aldebaran 0.51 A7 2.7 16.5
␣ Cru Acrux 0.63 K5 Ϫ5.6 68
␣ Vir Spica 0.77 B2 Ϫ2.3 260
0.86 B1 Ϫ5.2 220
0.90
0.91 2.2
Ϫ0.7

3.5
Ϫ3.3

A P P E N D I X A 451

■ Table A-9 ❙ The 15 Nearest Stars

Name Absolute Distance Spectral Apparent Visual
Magnitude (ly) Type Magnitude
Sun (mv)
Proxima Cen (Mv) 4.28 G2
␣ Cen A 4.3 M5 Ϫ26.8
␣ Cen B 4.83 4.3 G2 11.05
Barnard’s Star 15.45 5.9 K5 0.1
Wolf 359 4.38 7.6 M5 1.5
Lalande 21185 5.76 8.1 M6 9.5
Sirius A 13.21 8.6 M2 13.5
Sirius B 16.80 8.6 A1 7.5
Luyten 726-8A 10.42 8.9 white dwarf Ϫ1.5
Luyten 726-8B (UV Cet) 1.41 8.9 M5 7.2
Ross 154 11.54 9.4 M6 12.5
Ross 248 15.27 10.3 M5 13.0
⑀ Eri 15.8 10.7 M6 10.6
Luyten 789-6 13.3 10.8 K2 12.2
14.8 M7 3.7
6.13 12.2
14.6

■ Table A-10 ❙ Properties of the Planets

PHYSICAL PROPERTIES (Earth ϭ )

Equatorial Radius Mass Average Surface Escape Sidereal Inclination
( ϭ 1) Density Gravity Velocity Period of of Equator
Planet (km) ( ϭ 1) (g/cm3) ( ϭ 1) (km/s) Rotation
0.056 to Orbit
Mercury 2439 0.38 0.815 5.44 0.38 4.3 58.65d
Venus 6052 0.95 1.000 5.24 0.90 10.3 243.02d 0°
Earth 6378 1.00 0.108 5.50 1.00 11.2 23.93h 177°
Mars 3396 0.53 317.8 3.94 0.38 5.0 24.62h 23°.5
Jupiter 71,494 11.20 95.2 1.34 2.54 61 25°.3
Saturn 60,330 9.42 14.5 0.69 1.16 35.6 9.92h 3°.1
Uranus 25,559 4.01 17.2 1.19 0.92 22 10.23h 26°.4
Neptune 24,750 3.93 1.66 1.19 25 17.23h 97°.9
16.05h 28°.8

ORBITAL PROPERTIES Inclination
to Ecliptic
Semimajor Axis (a) Orbital Period (P) Average Orbital
(y) (days) Orbital Eccentricity 7°.0
Planet (AU) (106 km) Velocity 3°.4
(km/s) 0.206 0° *
Mercury 0.39 57.9 0.24 87.97 0.007 1°.9
Venus 0.72 108.2 0.62 224.68 47.89 0.017 1°.3
Earth 1.00 149.6 1.00 365.26 35.03 0.093 2°.5
Mars 1.52 227.9 1.88 686.95 29.79 0.048 0°.8
Jupiter 5.20 778.3 11.87 4334.3 24.13 0.056 1°.8
Saturn 9.54 1427.0 29.46 10,760 13.06 0.046
Uranus 19.18 2869.0 84.01 30,685 9.64 0.010
Neptune 30.06 4497.1 164.79 60,189 6.81
* By definition. 5.43

452 A P P E N D I X A

■ Table A-11 ❙ Principal Satellites of the Solar System

Planet Satellite Radius Distance Orbital Orbital Orbital
Earth (km) from Planet Period Eccentricity Inclination
Mars Moon (days)
Jupiter Phobos 1738 (103 km) 0.055 5°.1
Deimos 14ϫ12ϫ10 27.32 0.018 1°.0
Saturn Amalthea 384.4 0.32 0.002 2°.8
Io 8ϫ6ϫ5 9.4 1.26 0.003 0°.4
Uranus Europa 135ϫ100ϫ78 23.5 0.50 0.000 0°.3
Ganymede 1.77 0.000 0°.5
Neptune Callisto 1820 182 3.55 0.002 0°.2
Himalia 1565 422 7.16 0.008 0°.2
Janus 2640 671 16.69 0.158 27°.6
Mimas 2420 1071 250.6 0.007 0°.1
Enceladus ϳ85 1884 0.70 0.020 1°.5
Tethys 110ϫ80ϫ100 11,470 0.94 0.004 0°.0
Dione 196 151.5 1.37 0.000 1°.1
Rhea 250 185.5 1.89 0.002 0°.0
Titan 530 238.0 2.74 0.001 0°.4
Hyperion 560 294.7 4.52 0.029 0°.3
Iapetus 765 377 15.94 0.104 ϳ0°.5
Phoebe 2575 527 21.28 0.028 14°.7
Miranda 205ϫ130ϫ110 1222 79.33 0.163 150°
Ariel 720 1484 550.4 0.017 3°.4
Umbriel 110 3562 1.41 0.003 0°
Titania 242 12,930 2.52 0.003 0°
Oberon 580 129.9 4.14 0.002 0°
Proteus 595 190.9 8.71 0.001 0°
Triton 805 266.0 13.46 ϳ0 ϳ0°
Nereid 775 436.3 1.12 0.00 160°
205 583.4 5.88 0.76 27.7°
1352 117.6 360.12
170 354.59
5588.6

■ Table A-12 ❙ Meteor Showers

Shower Dates Hourly Radiant of shower Associated
Rate R.A. Dec. Comet
Quadrantids Jan. 2–4
Lyrids April 20–22 30 15h24m 50° 1861 I
␩ Aquarids May 2–7 8 18h4m 33° Halley?
␦ Aquarids July 26–31 10 22h24m 0°
Perseids Aug. 10–14 15 22h36m Ϫ10° 1982 III
Orionids Oct. 18–23 40 3h4m 58° Halley?
Taurids Nov. 1–7 15 6h20m 15° Encke
Leonids Nov. 14–19 8 3h40m 17° 1866 I Temp
Geminids Dec. 10–13 6 10h12m 22°
50 7h28m 32°

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