Chapter 28 Stars and the Universe
Chapter 28 Stars and the Universe
Chapter 28 Stars and the Universe
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<strong>Chapter</strong> <strong>28</strong><br />
<strong>Stars</strong> <strong>and</strong> <strong>the</strong> <strong>Universe</strong><br />
THE SEARCH FOR EXTRATERRESTRIAL LIFE<br />
In 1996, a team of scientists announced that a meteorite recovered<br />
from <strong>the</strong> ice of Antarctica might contain evidence of<br />
life outside Earth. Microscopic studies revealed something<br />
that could be fossils of bacteria. Organic compounds thought<br />
to be <strong>the</strong> result of biological processes were identified near<br />
this “fossil.”<br />
Most scientists agree that <strong>the</strong> meteorite is from Mars.<br />
However, no evidence of life on Mars has been detected by<br />
remote observations or by spacecraft sent to Mars to photograph<br />
surface features. If life exists <strong>the</strong>re, it is probably in<br />
<strong>the</strong> form of primitive microscopic organisms that live under<br />
<strong>the</strong> planet’s surface. Some scientists suggest that both <strong>the</strong><br />
fossil-like shape <strong>and</strong> <strong>the</strong> compounds thought to be of biological<br />
origin could be <strong>the</strong> result of inorganic processes. There is<br />
still discussion among scientists about whe<strong>the</strong>r <strong>the</strong> features<br />
in this meteorite are or are not <strong>the</strong> first direct evidence of life<br />
outside Earth.<br />
709
710 CHAPTER <strong>28</strong>: STARS AND THE UNIVERSE<br />
ET, Phone Earth<br />
Ano<strong>the</strong>r attempt to discover extraterrestrial life is project<br />
SETI: <strong>the</strong> Search for Extra Terrestrial Intelligence. This program<br />
seeks to identify radio transmissions coming from outside<br />
<strong>the</strong> solar system. Considering <strong>the</strong> billions of stars in <strong>the</strong><br />
universe, it seems possible that planets with conditions similar<br />
to those on Earth could be orbiting some stars. It also<br />
seems possible that on some of <strong>the</strong>se planets <strong>the</strong>re may be<br />
technological civilizations like our own.<br />
The huge distances in space may prevent anyone from<br />
traveling from one star system to ano<strong>the</strong>r. However, radio<br />
waves, which are relatively easy to generate, travel at <strong>the</strong><br />
speed of light. Large radio receivers, also known as radio telescopes,<br />
can pick up very faint radio transmissions. Large<br />
radio telescopes were built to explore <strong>the</strong> universe with long<br />
wavelengths of electromagnetic radiation that are not visible<br />
to our eyes. Radio waves can penetrate dust <strong>and</strong> clouds of gas<br />
that prevent visual observations.<br />
How would scientists know if <strong>the</strong>y were receiving signals<br />
from ano<strong>the</strong>r civilization? It seems likely that <strong>the</strong> signals<br />
would not be in a familiar language. If astronomers detect<br />
patterns in radio transmissions that have no known sources<br />
in natural sources, <strong>the</strong>y may be listening to communications<br />
from ano<strong>the</strong>r civilization. Some of <strong>the</strong> largest radio telescopes<br />
in <strong>the</strong> world have been used over <strong>the</strong> past several decades to<br />
listen for intelligent transmissions.<br />
At first, <strong>the</strong> job of analyzing <strong>the</strong>se signals was a severe<br />
limitation. How could scientists separate intelligent communications<br />
from <strong>the</strong> great amount of radio noise generated by<br />
stars? This is where computers came to <strong>the</strong> aid of scientists.<br />
Computers can quickly analyze radio signals, looking for patterns.<br />
The development of faster <strong>and</strong> more powerful computers<br />
has enabled astronomers to scan far more observations<br />
than humans could ever analyze. However, since SETI began<br />
in about 1960 <strong>the</strong>re have been no signals identified as likely<br />
forms of intelligent communication.
What Would We Answer?<br />
If scientists did detect intelligent communications, how could<br />
it affect Earth? Perhaps <strong>the</strong> information in <strong>the</strong> data would<br />
provide new insights into ma<strong>the</strong>matics, science, or technology.<br />
Perhaps people could learn more about <strong>the</strong> promise <strong>and</strong><br />
<strong>the</strong> dangers of a developing civilization. On <strong>the</strong> o<strong>the</strong>r h<strong>and</strong>,<br />
<strong>the</strong>re is always <strong>the</strong> danger of conquest by a distant power.<br />
The future has always brought people into <strong>the</strong> unknown. Fortunately,<br />
people have learned to apply discoveries to improve<br />
<strong>the</strong>ir lives. Experience has taught that, in <strong>the</strong> long run, <strong>the</strong><br />
developments of science seem to benefit humans.<br />
WHAT IS A STAR?<br />
A star is a massive object in space that creates energy <strong>and</strong> radiates<br />
it as electromagnetic radiation. The sun is a star. If you<br />
compare <strong>the</strong> sun with <strong>the</strong> thous<strong>and</strong>s of stars known to astronomers,<br />
<strong>the</strong> sun appears to be a typical star. Actually, most<br />
of <strong>the</strong> stars visible in <strong>the</strong> night sky are larger <strong>and</strong> brighter than<br />
<strong>the</strong> sun. At <strong>the</strong> same distances as <strong>the</strong> visible stars, <strong>the</strong>re are<br />
more stars too dim to be visible from Earth. Observations of <strong>the</strong><br />
sun give astronomers insights into most o<strong>the</strong>r stars <strong>and</strong> <strong>the</strong>ir<br />
observations of o<strong>the</strong>r stars help <strong>the</strong>m underst<strong>and</strong> <strong>the</strong> sun.<br />
ACTIVITY <strong>28</strong>-1 LIGHT INTENSITY AND DISTANCE<br />
WHAT IS ASTAR? 711<br />
Using a light meter that measures <strong>the</strong> intensity of light, you can<br />
measure <strong>the</strong> change in <strong>the</strong> intensity with distance. Place a lightbulb<br />
in a dark room. Measure <strong>the</strong> intensity of <strong>the</strong> light at different distances<br />
from <strong>the</strong> lightbulb. Make a data table of <strong>the</strong> intensity of illumination<br />
at various distances from <strong>the</strong> lightbulb. Graph your data.<br />
What o<strong>the</strong>r factor shows a similar change of intensity with distance?
712 CHAPTER <strong>28</strong>: STARS AND THE UNIVERSE<br />
Starlight<br />
For centuries, astronomers have wondered how <strong>the</strong> sun could<br />
produce <strong>the</strong> great quantities of energy it radiates into space.<br />
They knew that light sources on Earth produced light by<br />
chemical changes such as combustion. Fuels such as wood <strong>and</strong><br />
coal rapidly combine with oxygen in <strong>the</strong> atmosphere to produce<br />
heat <strong>and</strong> light. If <strong>the</strong> sun burned coal or wood to produce<br />
energy, it would run out of fuel very quickly. As scientists became<br />
aware of <strong>the</strong> age <strong>the</strong> solar system, it became clear that a<br />
very different process was taking place in <strong>the</strong> sun.<br />
Advances in science in <strong>the</strong> early twentieth century showed<br />
that matter could be changed into energy.You may have heard<br />
of Albert Einstein’s famous formula E mc 2, in which E is energy,<br />
m is mass, <strong>and</strong> c is <strong>the</strong> speed of light. The speed of light<br />
is a very large number. Therefore, <strong>the</strong> square of that number<br />
is enormous. The point of this formula is that great quantities<br />
of energy can be created by <strong>the</strong> loss of a small amount of mass.<br />
In <strong>the</strong> nearly 5 billion years since <strong>the</strong> solar system originated,<br />
it is estimated that <strong>the</strong> sun has only lost about one-third of 1<br />
percent of its total mass.<br />
Nuclear Fusion in <strong>Stars</strong><br />
Most of <strong>the</strong> mass of <strong>the</strong> sun is hydrogen, <strong>the</strong> lightest element.<br />
When four hydrogen nuclei join to make a helium nucleus,<br />
<strong>the</strong>y lose about 1 percent of <strong>the</strong>ir mass. The process by which<br />
light elements join to make heavier elements is called nuclear<br />
fusion. (See Figure <strong>28</strong>-1.) While 1 percent may seem<br />
like a small loss of mass, it is enough to create a great amount<br />
Figure <strong>28</strong>-1 The sun generates<br />
most of its energy by fusing<br />
hydrogen to make helium<br />
deep within <strong>the</strong> sun. The loss<br />
of about 1 percent in mass<br />
during this process creates<br />
vast quantities of energy.
of energy. However, nuclear fusion can occur only under extreme<br />
conditions of heat <strong>and</strong> pressure. In <strong>the</strong> last chapter,<br />
you learned that Jupiter, <strong>the</strong> largest planet in our solar system,<br />
is too small to have enough internal pressure to support<br />
fusion.<br />
ACTIVITY <strong>28</strong>-2 MAKING LIGHT<br />
Make a list of <strong>the</strong> methods you can use to create light energy in<br />
an Earth science lab setting. This can be a competitive activity<br />
among lab groups with one point awarded for each method<br />
to create light energy <strong>and</strong> two points if you can safely demonstrate<br />
it. As in any o<strong>the</strong>r laboratory procedures, your teacher must<br />
approve all materials <strong>and</strong> methods you plan to use before you<br />
try <strong>the</strong>m. Duplication such as burning two different substances<br />
counts as a single idea. Remember that you are looking for ways<br />
to create light energy <strong>and</strong> not methods to bring in light from ano<strong>the</strong>r<br />
source like <strong>the</strong> sun.<br />
Energy Escapes from <strong>Stars</strong><br />
WHAT IS ASTAR? 713<br />
Once <strong>the</strong> energy is created deep in <strong>the</strong> sun, it moves to <strong>the</strong><br />
sun’s visible surface by radiation <strong>and</strong> convection. Convection<br />
is <strong>the</strong> same process of heat flow by density currents that distributes<br />
energy through Earth’s atmosphere <strong>and</strong> oceans.<br />
Slow convection currents within Earth also carry heat energy<br />
from Earth’s interior to <strong>the</strong> surface. From <strong>the</strong> solar surface,<br />
<strong>the</strong> energy escapes as electromagnetic radiation. The surface<br />
temperature of <strong>the</strong> star determines <strong>the</strong> kind of electromagnetic<br />
energy it radiates into space. The sun is a yellow star<br />
because its roughly 6000°C surface radiates most intensely<br />
as yellow light in <strong>the</strong> visible part of <strong>the</strong> spectrum.<br />
Based on observations of o<strong>the</strong>r stars, astronomers predict<br />
that <strong>the</strong> sun will continue to radiate energy as it now does for<br />
approximately ano<strong>the</strong>r 5 billion years. The next section will<br />
tell you about <strong>the</strong> evolution of stars such as <strong>the</strong> sun.
714 CHAPTER <strong>28</strong>: STARS AND THE UNIVERSE<br />
HOW ARE STARS CLASSIFIED?<br />
In <strong>the</strong> early twentieth century, astronomers in Denmark <strong>and</strong><br />
<strong>the</strong> United States discovered that <strong>the</strong>y could classify stars on<br />
<strong>the</strong> basis of <strong>the</strong> amount of electromagnetic energy <strong>the</strong>y generate<br />
<strong>and</strong> <strong>the</strong>ir temperature. The total energy output of a star<br />
is called its luminosity, or absolute brightness. Apparent<br />
brightness, or stellar magnitude, is how bright <strong>the</strong> star looks<br />
as seen from Earth. The closer a star is, <strong>the</strong> brighter it appears<br />
to us.<br />
A good example is <strong>the</strong> sun. The sun is actually a smaller<br />
star, <strong>and</strong> gives off less light than most of <strong>the</strong> stars you see in<br />
<strong>the</strong> night sky. However, <strong>the</strong> sun is so close to Earth that during<br />
<strong>the</strong> day its light drowns out <strong>the</strong> light of <strong>the</strong> o<strong>the</strong>r stars. If<br />
we could see <strong>the</strong> sun at <strong>the</strong> same distance as <strong>the</strong> nighttime<br />
stars, it would be dimmer than most of <strong>the</strong>m. Therefore, <strong>the</strong><br />
brightness of a star depends on its absolute magnitude, or luminosity,<br />
<strong>and</strong> its distance from <strong>the</strong> observer.<br />
You may have noticed that when you turn off an inc<strong>and</strong>escent<br />
lightbulb <strong>the</strong> color of <strong>the</strong> hot wire briefly changes to<br />
red before it goes dark. Red is <strong>the</strong> coolest color of light visible<br />
to our eyes. If a material is heated beyond red-hot, it becomes<br />
white <strong>and</strong> <strong>the</strong>n blue. Continued heating would push <strong>the</strong> radiation<br />
into <strong>the</strong> ultraviolet part of <strong>the</strong> spectrum <strong>and</strong> beyond.<br />
These forms of electromagnetic energy are not visible to us.<br />
However, <strong>the</strong>y can affect us in o<strong>the</strong>r ways. Sunburn is caused<br />
primarily by ultraviolet light, which is a part of <strong>the</strong> spectrum<br />
of sunlight. Figure <strong>28</strong>-2 compares <strong>the</strong> sun’s spectrum with<br />
<strong>the</strong> spectrum of light radiated by hotter (blue) <strong>and</strong> cooler<br />
(red) stars.<br />
Hertzsprung-Russell Diagram<br />
The graph used to classify stars is often called <strong>the</strong> Hertzsprung-Russell,<br />
or H-R, diagram in honor of <strong>the</strong> two men who<br />
developed it. This graph is printed below from <strong>the</strong> Earth Sci-
Figure <strong>28</strong>-3 When stars are<br />
plotted on a graph according<br />
to <strong>the</strong>ir energy output (luminosity)<br />
<strong>and</strong> surface temperature<br />
(which determines <strong>the</strong><br />
star’s color), most stars fall<br />
into groups. Nine stars of<br />
special significance () are<br />
labeled by name.<br />
Figure <strong>28</strong>-2 The solar spectrum<br />
illustrates why <strong>the</strong> sun is<br />
classified as a yellow star. It<br />
gives off its most intense radiation<br />
in <strong>the</strong> middle of <strong>the</strong> visible<br />
part of <strong>the</strong> spectrum. Blue<br />
stars are hotter <strong>and</strong> stronger<br />
in short-wave radiation. Red<br />
stars are cooler, <strong>and</strong> <strong>the</strong>y radiate<br />
less energy per square<br />
meter of surface area.<br />
Intensity of Radiation<br />
HOW ARE STARS CLASSIFIED? 715<br />
Visible light<br />
ence Reference Tables, where it is labeled “Luminosity <strong>and</strong><br />
Temperature of <strong>Stars</strong>.” (See Figure <strong>28</strong>-3.) The graph is usually<br />
plotted with <strong>the</strong> temperatures decreasing to <strong>the</strong> right<br />
along <strong>the</strong> bottom axis. This is contrary to <strong>the</strong> way most<br />
graphs are made. (Usually, values increase to <strong>the</strong> right as<br />
well as upward on <strong>the</strong> vertical axis.) This graph is different<br />
because it is usually shown <strong>the</strong> way astronomers originally<br />
developed it.<br />
Luminosity (Relative to <strong>the</strong> Sun)<br />
1,000,000<br />
Massive<br />
<strong>Stars</strong><br />
10,000<br />
100<br />
1<br />
Blue<br />
Supergiants<br />
Blue<br />
star<br />
BLUE LIGHT<br />
YELLOW<br />
Sun<br />
RED LIGHT<br />
Red<br />
star<br />
0 5<br />
Wavelength (× 10<br />
10<br />
–5 cm)<br />
Supergiants<br />
Rigel<br />
Betelgeuse<br />
+ +<br />
Main Sequence<br />
+ Sirius<br />
Polaris +<br />
Red Giants<br />
+ Aldebaran<br />
+ Alpha Centauri<br />
+<br />
Sun<br />
0.01<br />
White Dwarfs<br />
+ Procyon B<br />
Red<br />
Small<br />
<strong>Stars</strong><br />
0.0001<br />
20,000 10,000<br />
Temperature (°C)<br />
Dwarfs<br />
Barnard's<br />
Star +<br />
5,000 2,500<br />
Blue <strong>Stars</strong> White <strong>Stars</strong><br />
Color<br />
Yellow <strong>Stars</strong> Red <strong>Stars</strong>
716 CHAPTER <strong>28</strong>: STARS AND THE UNIVERSE<br />
Main Sequence <strong>Stars</strong><br />
When plotted on this graph, most stars fall into distinct<br />
groups. The greatest number of stars fall into an elongated<br />
group that runs across <strong>the</strong> luminosity <strong>and</strong> temperature diagram<br />
from <strong>the</strong> upper left to <strong>the</strong> lower right. This region of <strong>the</strong><br />
graph is known as <strong>the</strong> main sequence. The position of a star<br />
along <strong>the</strong> main sequence is primarily a function of <strong>the</strong> mass<br />
of <strong>the</strong> star.<br />
RED DWARF STARS The smallest stars, such as Barnard’s Star,<br />
are red dwarf stars, which are barely large enough to support<br />
nuclear fusion. They are red in color because <strong>the</strong>y are relatively<br />
cool. These stars are so dim that even <strong>the</strong> relatively<br />
close red dwarfs are difficult to see without a telescope. In<br />
fact, about 80 percent of <strong>the</strong> night stars closest to Earth are<br />
too dim to be visible to <strong>the</strong> unaided eye. This leads astronomers<br />
to infer that red dwarf stars are more numerous<br />
than all o<strong>the</strong>r groups of stars. However, we do not see <strong>the</strong>m because<br />
<strong>the</strong>y are so dim.<br />
Small stars last longer than larger stars. The lower temperature<br />
<strong>and</strong> pressure in <strong>the</strong>se stars allow <strong>the</strong>m to conserve<br />
hydrogen fuel <strong>and</strong> continue nuclear fusion much longer than<br />
larger stars. The combination of small size <strong>and</strong> slow production<br />
of energy makes <strong>the</strong>m very dim.<br />
BLUE SUPERGIANT STARS At <strong>the</strong> o<strong>the</strong>r end of <strong>the</strong> end of <strong>the</strong><br />
main sequence are <strong>the</strong> blue supergiants. These massive stars<br />
do not last as long as <strong>the</strong> smaller stars. The extreme conditions<br />
of temperature <strong>and</strong> pressure at <strong>the</strong> center of <strong>the</strong>se stars<br />
cause rapid depletion of <strong>the</strong>ir large quantities of hydrogen.<br />
Some of <strong>the</strong>m are a million times brighter than <strong>the</strong> sun. They<br />
are also much hotter than <strong>the</strong> sun, giving <strong>the</strong>m a blue color.<br />
These largest stars are not nearly as common as <strong>the</strong> smaller<br />
stars, in part because <strong>the</strong>y burn out quickly. The most massive<br />
stars last less than one-thous<strong>and</strong>th of <strong>the</strong> life of <strong>the</strong> sun.<br />
Rigel, a bright star in <strong>the</strong> winter constellation Orion, is<br />
10,000 times as luminous as <strong>the</strong> sun. The blue color of Rigel
is apparent if you compare it with Betelgeuse, ano<strong>the</strong>r bright<br />
star in Orion. Betelgeuse is a red giant star on <strong>the</strong> opposite<br />
side of <strong>the</strong> same constellation.<br />
Most o<strong>the</strong>r stars fall into one of <strong>the</strong> three groups on <strong>the</strong><br />
temperature-luminosity chart. White dwarfs, red giants, <strong>and</strong><br />
<strong>the</strong> supergiants are <strong>the</strong> most common star groups outside <strong>the</strong><br />
main sequence.<br />
HOW DO STARS EVOLVE?<br />
Different sizes of stars have different life cycles. However,<br />
<strong>the</strong> evolution of stars can be illustrated by considering a star<br />
about <strong>the</strong> size of <strong>the</strong> sun.<br />
Birth of a Star<br />
HOW DO STARS EVOLVE? 717<br />
Star formation begins when a cloud of gas <strong>and</strong> dust (mostly<br />
hydrogen) begins to draw toge<strong>the</strong>r under <strong>the</strong> influence of<br />
gravity. There are two sources of this material. Some of it is<br />
hydrogen <strong>and</strong> helium left over from <strong>the</strong> formation of <strong>the</strong> universe<br />
about 14 billion years ago. The rest is <strong>the</strong> debris from<br />
<strong>the</strong> explosions of massive stars that formed earlier in <strong>the</strong> history<br />
of <strong>the</strong> universe. This initial phase takes place over a period<br />
on <strong>the</strong> order of 50 million years. (The process is faster for<br />
larger stars <strong>and</strong> slower for smaller stars.)<br />
As <strong>the</strong> material draws toge<strong>the</strong>r, heat from <strong>the</strong> collapse of<br />
<strong>the</strong> matter <strong>and</strong> from friction causes <strong>the</strong> temperature to increase<br />
until <strong>the</strong>re is enough heat <strong>and</strong> pressure to support nuclear<br />
fusion. At this time, <strong>the</strong> star becomes easily visible since<br />
it produces <strong>and</strong> radiates great quantities of energy. The condensation<br />
process can be observed with binoculars or a small<br />
telescope in <strong>the</strong> constellation Orion. Several young stars<br />
below <strong>the</strong> belt of Orion can be seen shining through a giant<br />
cloud of gas that surrounds <strong>the</strong>m.
718 CHAPTER <strong>28</strong>: STARS AND THE UNIVERSE<br />
Middle Age<br />
The star becomes less luminous after it fully condenses, <strong>and</strong><br />
it spends most of its life on <strong>the</strong> main sequence region of <strong>the</strong><br />
luminosity <strong>and</strong> temperature chart. (See Figure <strong>28</strong>-3 on page<br />
715.) Gravitational pressure balanced by heat from nuclear<br />
fusion prevents <strong>the</strong> star from fur<strong>the</strong>r shrinkage. This is <strong>the</strong><br />
longest <strong>and</strong> most stable phase of stellar evolution.<br />
Death of an Average Star<br />
After about 10 billion years, a star <strong>the</strong> size of <strong>the</strong> sun runs<br />
low on hydrogen. Fusion slows, <strong>and</strong> <strong>the</strong> core of helium collapses,<br />
causing <strong>the</strong> outer part of <strong>the</strong> star to exp<strong>and</strong> quickly,<br />
becoming a red giant. Fusion of helium <strong>and</strong> o<strong>the</strong>r heavier elements<br />
replaces <strong>the</strong> hydrogen fusion process. The outer shell<br />
of gases exp<strong>and</strong>s <strong>and</strong> cools in <strong>the</strong> red giant stage, leaving behind<br />
a dense, hot core, which is a white dwarf star.<br />
Death of a Massive Star<br />
<strong>Stars</strong> with more than about 10 times <strong>the</strong> mass of <strong>the</strong> sun end<br />
<strong>the</strong>ir period in <strong>the</strong> main sequence more violently. These stars<br />
create a variety of heavier elements before <strong>the</strong>y collapse. The<br />
collapse process of larger stars generates so much energy<br />
that <strong>the</strong>se stars end <strong>the</strong>ir life in an explosion known as a supernova.<br />
They briefly generate more energy than <strong>the</strong> billions<br />
of stars that make up <strong>the</strong> whole galaxy. Most of <strong>the</strong> mass of<br />
<strong>the</strong> star is blown into space. The core of <strong>the</strong> star may form an<br />
extremely dense object called a neutron star. Some stars are<br />
so massive that <strong>the</strong>y form an object with gravity so strong<br />
that not even light can escape. This is called a black hole.<br />
Black holes cannot radiate energy, but <strong>the</strong>y can be detected<br />
because energy is given off by matter that falls into <strong>the</strong> black<br />
hole. They can also be located by <strong>the</strong>ir gravitational effects on<br />
o<strong>the</strong>r objects.
HOW DO ASTRONOMERS STUDY STARS?<br />
<strong>Stars</strong> are extremely hot <strong>and</strong> have no solid surface. Scientists<br />
can send instruments <strong>and</strong> cameras to l<strong>and</strong> on Mars or<br />
o<strong>the</strong>r solid objects. However, <strong>the</strong>se methods cannot be used to<br />
investigate stars. Any devices scientists build would melt <strong>and</strong><br />
probably vaporize long before reaching <strong>the</strong> visible surface of<br />
a star. Fur<strong>the</strong>rmore, <strong>the</strong> night stars are too distant to reach<br />
with spacecraft. With our present technology, it would take<br />
tens or even hundreds of thous<strong>and</strong>s of years for a spacecraft<br />
to reach even <strong>the</strong> nearest star beyond <strong>the</strong> sun. Therefore,<br />
most of <strong>the</strong> information astronomers have about stars comes<br />
from light <strong>and</strong> o<strong>the</strong>r electromagnetic energy <strong>the</strong>y radiate into<br />
space.<br />
Optical Telescopes<br />
HOW DO ASTRONOMERS STUDY STARS? 719<br />
Astronomers use telescopes to concentrate <strong>the</strong> light of stars.<br />
Telescopes allow <strong>the</strong>m to observe objects that are too dim to<br />
be visible to unaided eyes. Some people think that <strong>the</strong> most<br />
important feature of a telescope is how much it magnifies.<br />
However, <strong>the</strong> stars are so distant that even <strong>the</strong> most powerful<br />
telescopes show nearly all of <strong>the</strong>m as points of light. When<br />
an image is magnified, it will become dim, unclear, or fuzzy if<br />
<strong>the</strong> object is too far away.<br />
O<strong>the</strong>r factors are more important than magnification in<br />
telescope construction <strong>and</strong> use. The size, or diameter, of <strong>the</strong><br />
front lens (or light-ga<strong>the</strong>ring mirror) of <strong>the</strong> telescope determines<br />
<strong>the</strong> dimmest object that can be observed. The far<strong>the</strong>r<br />
astronomers look into space, <strong>the</strong> dimmer <strong>the</strong> objects become.<br />
The second factor is <strong>the</strong> quality of optics of <strong>the</strong> telescope.<br />
If <strong>the</strong> lenses or mirrors that ga<strong>the</strong>r <strong>the</strong> light are not made<br />
with great precision, magnified images will not be sharp.<br />
Earth’s atmosphere is also a limiting factor. This is why<br />
major observatories are built on high mountains, where <strong>the</strong><br />
atmosphere is thin <strong>and</strong> has less effect on <strong>the</strong> light. Figure<br />
<strong>28</strong>-4 on page 720 shows several buildings containing large
720 CHAPTER <strong>28</strong>: STARS AND THE UNIVERSE<br />
telescopes on a mountaintop in Arizona. The Hubble Space<br />
Telescope, which orbits Earth above <strong>the</strong> distorting effects of<br />
Earth’s atmosphere, was a major step forward in observational<br />
astronomy.<br />
ACTIVITY <strong>28</strong>-3 MAKING A TELESCOPE<br />
You can construct a simple telescope using two convex lenses. A<br />
tube to hold <strong>the</strong> lenses at <strong>the</strong> proper distance <strong>and</strong> alignment is<br />
helpful but not essential. By using lenses with more or less curvature,<br />
you can change magnification. Moving <strong>the</strong> lens that is<br />
closest to your eye adjusts <strong>the</strong> focus.<br />
Radio Telescopes<br />
Figure <strong>28</strong>-4 The telescopes<br />
of <strong>the</strong> Kitt Peak Observatory<br />
are located on a mountaintop<br />
to reduce problems<br />
associated with light passing<br />
through <strong>the</strong> atmosphere.<br />
The higher <strong>the</strong> observatory<br />
<strong>and</strong> <strong>the</strong> far<strong>the</strong>r it is from<br />
atmospheric pollution <strong>and</strong><br />
artificial lights, <strong>the</strong> better <strong>the</strong><br />
quality of <strong>the</strong> observations<br />
<strong>and</strong> images.<br />
Some telescopes ga<strong>the</strong>r long-wavelength radio energy ra<strong>the</strong>r<br />
than visible light. Radio telescopes, like those in Figure <strong>28</strong>-5,<br />
are not blocked by clouds of dust <strong>and</strong> gas in space that block<br />
visible light. They are also useful in detecting objects that do<br />
not produce radiation in <strong>the</strong> visible part of <strong>the</strong> electromagnetic<br />
spectrum. Radio telescopes do not make sharp images,<br />
<strong>and</strong> it is difficult to tell <strong>the</strong> exact positions of a radio source.<br />
However, radio telescopes allow astronomers to make observations<br />
that would not be possible with telescopes that work
Figure <strong>28</strong>-5 Objects that do<br />
not give off visible light can<br />
be investigated with radio<br />
telescopes. Radio signals<br />
penetrate clouds of dust<br />
<strong>and</strong> gas that block visible<br />
light. They have been especially<br />
useful in mapping <strong>the</strong><br />
Milky Way Galaxy.<br />
in <strong>the</strong> visible part of <strong>the</strong> spectrum.<br />
O<strong>the</strong>r Telescopes<br />
HOW DO ASTRONOMERS STUDY STARS? 721<br />
O<strong>the</strong>r kinds of telescopes allow astronomers to use electromagnetic<br />
wavelengths shorter than visible light, such as X<br />
rays <strong>and</strong> gamma rays. These instruments must be located in<br />
orbit above Earth’s atmosphere, which filters out <strong>the</strong>se forms<br />
of radiation.<br />
Technology has changed <strong>the</strong> ways astronomers use telescopes.<br />
The first telescopes were used for direct observations.<br />
If astronomers wanted a permanent record of <strong>the</strong>ir observations,<br />
<strong>the</strong>y had to draw by h<strong>and</strong> what <strong>the</strong>y observed through<br />
<strong>the</strong> telescope. Chemical photography enabled astronomers to<br />
take pictures through <strong>the</strong>ir telescopes. Today, <strong>the</strong> more advanced<br />
telescopes use electronic sensors like those in digital<br />
cameras along with computers to create better quality images<br />
than ever before possible.
722 CHAPTER <strong>28</strong>: STARS AND THE UNIVERSE<br />
Source of<br />
White Light<br />
Spectroscope<br />
Glass<br />
Prism<br />
The spectroscope is one of <strong>the</strong> most important tools that astronomers<br />
use. This instrument separates light into its component<br />
colors (wavelengths), like <strong>the</strong> glass prism shown in<br />
Figure <strong>28</strong>-6. When starlight is passed through a spectroscope,<br />
dark lines appear in certain parts of <strong>the</strong> spectrum. These<br />
dark lines are produced when certain wavelengths of light<br />
are absorbed by gaseous elements within <strong>the</strong> outer parts of<br />
<strong>the</strong> star.<br />
Each element has its own characteristic absorption lines.<br />
Since stars are composed primarily of hydrogen <strong>and</strong> helium,<br />
white light that passes through <strong>the</strong>se elements shows dark<br />
lines in <strong>the</strong> orange, yellow, green, <strong>and</strong> blue colors that characterize<br />
hydrogen <strong>and</strong> helium. These spectral lines correspond<br />
to <strong>the</strong> energy that electrons absorb when <strong>the</strong>y move to<br />
higher energy levels within <strong>the</strong> atoms. The atoms give off <strong>the</strong><br />
same colors when <strong>the</strong> electrons fall to lower or inner energy<br />
levels. Each element has a unique set of energy levels. Therefore,<br />
<strong>the</strong>se “spectral fingerprints” allow astronomers to identify<br />
<strong>the</strong> composition of distant stars.<br />
ACTIVITY <strong>28</strong>-4 MAKING A SPECTRUM<br />
Long Wavelengths<br />
Red<br />
Orange<br />
Yellow<br />
Green<br />
Blue<br />
Violet<br />
Short Wavelengths<br />
Figure <strong>28</strong>-6 When white<br />
light passes through a glass<br />
prism, <strong>the</strong> light separates into<br />
<strong>the</strong> spectrum of colors, or<br />
wavelengths, of which it is<br />
composed.<br />
You can separate sunlight into its spectrum with a glass prism. This<br />
works best in a darkened room where windows face <strong>the</strong> sun.<br />
Close <strong>the</strong> shades so that a narrow slit of direct sunlight enters <strong>the</strong><br />
room. Place <strong>the</strong> prism near <strong>the</strong> narrow opening that admits sunlight.<br />
The prism will bend <strong>the</strong> light beam <strong>and</strong> separate it into its
colors. You may need to rotate <strong>the</strong> glass prism to project a visible<br />
spectrum. The spectrum can be projected onto a sheet of white<br />
paper. The stronger <strong>the</strong> light <strong>and</strong> <strong>the</strong> closer <strong>the</strong> paper is held to <strong>the</strong><br />
prism, <strong>the</strong> brighter <strong>the</strong> spectrum will be. To increase <strong>the</strong> size of<br />
<strong>the</strong> spectrum, move <strong>the</strong> paper screen away from <strong>the</strong> prism. What<br />
two changes in <strong>the</strong> spectrum do you observe as <strong>the</strong> paper screen<br />
is moved away from <strong>the</strong> prism?<br />
WHAT IS THE STRUCTURE OF THE UNIVERSE?<br />
Early astronomers noticed fuzzy objects in <strong>the</strong> night sky.<br />
They called <strong>the</strong>se objects nebulae (singular nebula). The<br />
word nebula comes from <strong>the</strong> Latin word for cloud. Unlike <strong>the</strong><br />
stars, <strong>the</strong>se objects looked like dim fuzzy patches of light.<br />
Nebulae <strong>and</strong> Galaxies<br />
Telescopes revealed that some nebulae are regions of gas <strong>and</strong><br />
dust where stars are forming. In addition, some nebulae were<br />
at greater distances than any known stars. Astronomers<br />
eventually realized that some nebulae are huge groups of<br />
stars held toge<strong>the</strong>r by gravity. These objects are called galaxies.<br />
The whole Andromeda galaxy is visible as a small, faint<br />
patch of light high in <strong>the</strong> autumn sky. Powerful telescopes revealed<br />
that <strong>the</strong> Andromeda galaxy, like thous<strong>and</strong>s of o<strong>the</strong>r<br />
galaxies, is a gigantic group of billions of stars. Galaxies are<br />
separated by vast distances that contain relatively few stars.<br />
Figure <strong>28</strong>-7 on page 724 is a typical spiral galaxy.<br />
The Milky Way<br />
WHAT IS THE STRUCTURE OF THE UNIVERSE? 723<br />
Astronomers realized that all <strong>the</strong> individual stars visible to<br />
us in <strong>the</strong> night sky are a part of <strong>the</strong> group called <strong>the</strong> Milky<br />
Way Galaxy. The sun <strong>and</strong> solar system are part of <strong>the</strong> Milky
724 CHAPTER <strong>28</strong>: STARS AND THE UNIVERSE<br />
Figure <strong>28</strong>-7 Galaxy NGC 4414 is a typical spiral galaxy composed of billions of<br />
stars. Both <strong>the</strong> Milky Way Galaxy <strong>and</strong> its relatively nearby twin, <strong>the</strong> Andromeda<br />
Galaxy, are spiral galaxies.<br />
Way Galaxy. This name came from observations of a faint,<br />
white b<strong>and</strong> of light that can be seen stretching across <strong>the</strong> sky<br />
on very dark, moonless nights. (The Milky Way is not visible<br />
in urban areas where light pollution prevents <strong>the</strong> night sky<br />
from being dark enough to make it visible.) This broad b<strong>and</strong><br />
is actually made of thous<strong>and</strong>s of stars.<br />
Radio telescopes enabled astronomers to map <strong>the</strong> Milky<br />
Way Galaxy <strong>and</strong> estimate that it is composed of roughly 100<br />
billion stars. Clouds of dust <strong>and</strong> gas that are also a part of our<br />
galaxy obscure most of <strong>the</strong>m. The center of our galaxy is located<br />
in <strong>the</strong> direction of <strong>the</strong> summer constellation Sagittarius.<br />
The shape of <strong>the</strong> Milky Way Galaxy, like <strong>the</strong> Andromeda<br />
Galaxy, is a flattened spiral. The sun <strong>and</strong> solar system are located<br />
about two-thirds of <strong>the</strong> way from <strong>the</strong> center to <strong>the</strong> outer<br />
edge, as shown in Figure <strong>28</strong>-8.<br />
As stars orbit <strong>the</strong> core of <strong>the</strong> galaxy, inertia keeps gravity<br />
from drawing <strong>the</strong>m toge<strong>the</strong>r. Orbiting <strong>the</strong> core of <strong>the</strong> galaxy
Sun<br />
100,000 light-years<br />
is an additional cyclic motion of Earth in space. Our planet<br />
rotates on its axis in a 24-hour cycle. It also revolves around<br />
<strong>the</strong> sun each year. The solar system revolves around <strong>the</strong> center<br />
of <strong>the</strong> Milky Way Galaxy in about 220 million years. Although<br />
this is a long time, <strong>the</strong> Milky Way Galaxy is so large<br />
that this motion is actually about 10 times faster than<br />
Earth’s revolution in its orbit around <strong>the</strong> sun.<br />
Clusters <strong>and</strong> Superclusters<br />
WHAT IS THE HISTORY OF THE UNIVERSE? 725<br />
10,000 light-years<br />
Figure <strong>28</strong>-8 Earth <strong>and</strong> <strong>the</strong> solar system are located about two-thirds of <strong>the</strong> way<br />
from <strong>the</strong> galactic center to <strong>the</strong> outer edge of <strong>the</strong> Milky Way.<br />
The structure of <strong>the</strong> universe does not stop at galaxies. The<br />
Milky Way <strong>and</strong> Andromeda galaxies are two of about 30<br />
galaxies known as <strong>the</strong> local group. Astronomers are now<br />
mapping superclusters of galaxies <strong>and</strong> even larger structures<br />
of matter. Why <strong>the</strong> matter of <strong>the</strong> universe is so unevenly distributed<br />
is one of <strong>the</strong> most important questions that astronomers<br />
are investigating today.<br />
WHAT IS THE HISTORY OF THE UNIVERSE?<br />
When you look at very distant objects in <strong>the</strong> universe, you<br />
are looking back in time. This is because light has a limited<br />
speed. You learned in an earlier chapter that you could estimate<br />
<strong>the</strong> distance to a lightning strike by counting <strong>the</strong> seconds<br />
between seeing <strong>the</strong> flash <strong>and</strong> hearing <strong>the</strong> thunder. In<br />
this procedure, you see <strong>the</strong> flash at essentially <strong>the</strong> same time<br />
it occurred. Light travels so fast that it could circle Earth<br />
about seven times in a single second.
726 CHAPTER <strong>28</strong>: STARS AND THE UNIVERSE<br />
Using Light as a Yardstick<br />
Redshift<br />
Distances in space are so vast that light cannot reach Earth<br />
instantaneously. For example, to reach Earth, electromagnetic<br />
energy takes about 3 seconds to travel from <strong>the</strong> moon<br />
<strong>and</strong> 8 minutes from <strong>the</strong> sun. Light takes more than 4 years<br />
to arrive from <strong>the</strong> nearest night star, Proxima Centauri. The<br />
most distant object visible to <strong>the</strong> unaided eye is <strong>the</strong> Andromeda<br />
Galaxy. Light from <strong>the</strong> Andromeda galaxy takes<br />
about 2 million years to reach us.<br />
In fact, light provides a good method to measure distances<br />
in <strong>the</strong> universe. A light-year is <strong>the</strong> distance that<br />
any form of electromagnetic energy can travel in 1 year:<br />
about 6 trillion miles, or 10 trillion km. Although <strong>the</strong> light<br />
year may sound like a measure of time, it is a measure of distance.<br />
When astronomers look at distant objects in space, <strong>the</strong>y<br />
see <strong>the</strong>m as <strong>the</strong> objects were when <strong>the</strong> light started its long<br />
journey toward Earth. The far<strong>the</strong>r away astronomers look<br />
into space, <strong>the</strong> far<strong>the</strong>r back in time <strong>the</strong>y see. At present, <strong>the</strong><br />
most distance objects visible to astronomers are estimated<br />
to be about 13 billion light-years away. Astronomers can now<br />
look at <strong>the</strong> universe about a billion years after its origin,<br />
which is estimated to be 14 billion years ago.<br />
Astronomer Edwin Hubble examined <strong>the</strong> spectra of distant<br />
galaxies in <strong>the</strong> early 1900s. He compared <strong>the</strong> dark absorption<br />
lines, or spectral lines, of light from <strong>the</strong>se far away galaxies<br />
to <strong>the</strong> absorption lines of nearby stars. Nearby stars had<br />
spectral lines similar to those produced in <strong>the</strong> laboratory. The<br />
light from <strong>the</strong> distant galaxies did not show <strong>the</strong> dark lines in<br />
<strong>the</strong> same colors as <strong>the</strong> light from <strong>the</strong> nearby stars. However,<br />
<strong>the</strong> dark lines in <strong>the</strong> spectra of distant galaxies were shifted<br />
toward <strong>the</strong> red end of <strong>the</strong> spectrum. Hubble reasoned that <strong>the</strong><br />
motion of distant galaxies away from Earth causes <strong>the</strong> redshift<br />
of spectral lines. The redshift of spectral lines is illustrated<br />
in Figure <strong>28</strong>-9.
Figure <strong>28</strong>-9 The sun <strong>and</strong><br />
nearby galaxies show spectral<br />
lines similar to those produced<br />
in a laboratory. However,<br />
distant galaxies show<br />
<strong>the</strong>se characteristic lines<br />
shifted toward <strong>the</strong> red <strong>and</strong> of<br />
<strong>the</strong> spectrum. Astronomers<br />
interpret this as evidence that<br />
<strong>the</strong> universe is exp<strong>and</strong>ing.<br />
Blue<br />
WHAT IS THE HISTORY OF THE UNIVERSE? 727<br />
Short Waves Long Waves<br />
Blue<br />
If <strong>the</strong> galaxies were moving toward Earth, <strong>the</strong> spectral<br />
lines would shift toward <strong>the</strong> blue end of <strong>the</strong> spectrum. The<br />
shift toward <strong>the</strong> red end of <strong>the</strong> spectrum indicates that <strong>the</strong><br />
galaxies are moving away from Earth.<br />
You can observe a similar change with sound. If you st<strong>and</strong><br />
next to a racetrack, <strong>the</strong> high-pitched sound of <strong>the</strong> approaching<br />
car changes to a lower pitch as <strong>the</strong> car speeds past you.<br />
This apparent change in frequency <strong>and</strong> wavelength of energy<br />
that occurs when <strong>the</strong> source of a wave is moving relative to<br />
an observer is called <strong>the</strong> Doppler effect. It was named for<br />
Christian Johann Doppler, <strong>the</strong> scientist who explained it in<br />
1842. The change in <strong>the</strong> frequency <strong>and</strong> wavelength of sound<br />
waves is similar to <strong>the</strong> changes that Hubble observed with<br />
light. The greater <strong>the</strong> redshift, <strong>the</strong> faster <strong>the</strong> object is moving<br />
away. Astronomers have found that <strong>the</strong> most distant galaxies<br />
are moving away <strong>the</strong> fastest.<br />
ACTIVITY <strong>28</strong>-5 DEMONSTRATING THE DOPPLER EFFECT<br />
This procedure should be done only under teacher or adult supervision.<br />
This activity requires a noisemaker that can be tied to a<br />
strong cord, or string, <strong>and</strong> swung around your head. A noisemaker<br />
such as an alarm clock or a small battery-operated device from a<br />
Red<br />
Short Waves Long Waves<br />
Blue<br />
Reference<br />
Wavelength<br />
Red<br />
Red<br />
Short Waves Long Waves<br />
Sun<br />
Nearby<br />
Galaxy<br />
Distant<br />
Galaxy
7<strong>28</strong> CHAPTER <strong>28</strong>: STARS AND THE UNIVERSE<br />
science supplier works well. Swing <strong>the</strong> noisemaker in a circle<br />
around your head as people at a safe distance listen for changes<br />
in <strong>the</strong> pitch of <strong>the</strong> sound. Can <strong>the</strong> person swinging <strong>the</strong> device also<br />
hear <strong>the</strong> pitch of <strong>the</strong> sound change? For observers outside <strong>the</strong> circle,<br />
what part of <strong>the</strong> swing best represents redshift, <strong>and</strong> what part<br />
of <strong>the</strong> swing represents a “blueshift.”<br />
Two o<strong>the</strong>r factors supported Hubble’s hypo<strong>the</strong>sis of an exp<strong>and</strong>ing<br />
universe. The redshift of light of distant galaxies<br />
could be observed in all directions. In addition, <strong>the</strong> dimmer<br />
galaxies, which are thought to be dim because <strong>the</strong>y are far<strong>the</strong>r<br />
from Earth, showed greater redshift. Hubble explained<br />
that <strong>the</strong> redshift is caused by motion of distant galaxies away<br />
from Earth. He also reasoned that this kind of motion is a<br />
characteristic of an explosion.<br />
You might think that <strong>the</strong> motion of distant galaxies away<br />
from Earth in all directions means that we are at <strong>the</strong> center<br />
of <strong>the</strong> expansion. However, from any position within <strong>the</strong> exp<strong>and</strong>ing<br />
matter of an explosion, matter is moving away in all<br />
directions.<br />
In <strong>the</strong> 1960s, Arno Penzias <strong>and</strong> Robert Wilson were working<br />
on long-distance radio communications for <strong>the</strong> Bell Telephone<br />
Company. They constructed a special outdoor receiver<br />
to detect weak radio signals. However, <strong>the</strong> device picked up<br />
annoying radio noise that <strong>the</strong>y were not able to eliminate. As<br />
<strong>the</strong>y investigated <strong>the</strong> source of <strong>the</strong>se radio waves, <strong>the</strong>y realized<br />
that <strong>the</strong> energy <strong>the</strong>y were picking up was billions of<br />
years old. They were actually listening to <strong>the</strong> origin of <strong>the</strong><br />
universe. These radio signals, known as cosmic background<br />
radiation, are weak electromagnetic radiation left<br />
over from <strong>the</strong> formation of <strong>the</strong> universe.<br />
The Big Bang<br />
The outward motions of distant galaxies <strong>and</strong> <strong>the</strong> cosmic background<br />
radiation are evidence that <strong>the</strong> universe began as an<br />
event now called <strong>the</strong> big bang. The name was first proposed<br />
as a joke to make fun of <strong>the</strong> <strong>the</strong>ory, but <strong>the</strong> name stuck. This
<strong>the</strong>ory proposes that at <strong>the</strong> time of its origin, <strong>the</strong> universe<br />
was a concentration of matter so dense that <strong>the</strong> laws of nature<br />
as we know <strong>the</strong>m today did not apply. This matter exp<strong>and</strong>ed<br />
explosively, forming <strong>the</strong> universe. Even <strong>the</strong> most<br />
extreme conditions that exist within <strong>the</strong> largest stars could<br />
not compare with <strong>the</strong> beginning of <strong>the</strong> universe.<br />
Experiments <strong>and</strong> <strong>the</strong> <strong>the</strong>ories of physics show that <strong>the</strong><br />
greatest possible velocity for matter or energy is <strong>the</strong> speed of<br />
light: about 300 million meters per second. Like <strong>the</strong> temperature<br />
of absolute zero (0 K), this is one of <strong>the</strong> absolute limits<br />
known to science. Astronomers reason that <strong>the</strong> universe is<br />
exp<strong>and</strong>ing at this rate.<br />
By working backward, astronomers estimate that <strong>the</strong> universe<br />
began in <strong>the</strong> big bang 14 billion years ago. Exp<strong>and</strong>ing<br />
outward in all directions, <strong>the</strong> universe today could be as much<br />
as <strong>28</strong> billion light years across. This is so gigantic that it is<br />
nearly impossible for humans to comprehend how large this is.<br />
Earth, <strong>the</strong> solar system, <strong>and</strong> even <strong>the</strong> huge Milky Way Galaxy<br />
are incredibly small compared with <strong>the</strong> size of <strong>the</strong> universe.<br />
ACTIVITY <strong>28</strong>-6 A MODEL OF THE BIG BANG<br />
WHAT IS THE FUTURE OF THE UNIVERSE? 729<br />
Inflate a round balloon into a small ball. Draw several small dots<br />
on <strong>the</strong> balloon’s surface. Notice that as <strong>the</strong> balloon is inflated<br />
more, <strong>the</strong> dots always move apart. If observers were located anywhere<br />
on <strong>the</strong> surface of <strong>the</strong> balloon or even inside <strong>the</strong> balloon, as<br />
<strong>the</strong> balloon is inflated, <strong>the</strong>y would see <strong>the</strong> dots moving away in<br />
all directions. No matter what location is chosen, it would appear<br />
that <strong>the</strong> observer is at <strong>the</strong> center of <strong>the</strong> expansion. Therefore, <strong>the</strong>re<br />
is no way that astronomers can find <strong>the</strong> center of <strong>the</strong> universe.<br />
WHAT IS THE FUTURE OF THE UNIVERSE?<br />
From an astronomical point of view, Earth is in a reasonably<br />
unchanging state. Earth’s orbit around <strong>the</strong> sun is stable <strong>and</strong><br />
scientists expect <strong>the</strong> sun to continue its energy production on
730 CHAPTER <strong>28</strong>: STARS AND THE UNIVERSE<br />
<strong>the</strong> main sequence for billions of years. Collisions with large<br />
objects from space, such as those thought to mark <strong>the</strong> ends of<br />
past geologic eras, are possible. Fortunately, <strong>the</strong>se events are<br />
becoming less likely as <strong>the</strong> age of <strong>the</strong> solar system increases.<br />
However, <strong>the</strong> very-long-term future of <strong>the</strong> universe is not<br />
clear.<br />
Three Possible Futures<br />
The universe seems to have three possible futures. Some scientists<br />
propose that <strong>the</strong> expansion of <strong>the</strong> universe may be<br />
slowing due to gravity. However, it is possible that <strong>the</strong>re is<br />
not enough gravity to stop <strong>the</strong> expansion. In this case, <strong>the</strong><br />
universe will continue to exp<strong>and</strong> without limit. O<strong>the</strong>r scientists<br />
propose that <strong>the</strong>re may be enough gravity to just stop<br />
<strong>the</strong> expansion, leading to a steady state. If <strong>the</strong> universe has<br />
enough gravity to reverse its exp<strong>and</strong>ing phase, it could fall<br />
back toge<strong>the</strong>r in a very distant event that some astronomers<br />
call <strong>the</strong> “big crunch.”<br />
A good way to underst<strong>and</strong> this is to consider a baseball<br />
thrown straight up. Gravity brings <strong>the</strong> ball back to <strong>the</strong><br />
ground. However, if you could propel <strong>the</strong> ball fast enough, it<br />
would continue upward into space <strong>and</strong> never return to Earth.<br />
In recent years, astronomers have found evidence that <strong>the</strong><br />
universe is not only exp<strong>and</strong>ing, but that it is exp<strong>and</strong>ing at an<br />
increasing rate. What force could work against <strong>the</strong> force of<br />
gravity to cause this? It is as if you threw a ball up into <strong>the</strong><br />
air <strong>and</strong> it did not fall back to Earth. In fact, it is as if <strong>the</strong> ball<br />
flew upward faster <strong>and</strong> faster with time. This would be surprising,<br />
indeed. Astronomers find <strong>the</strong>se observations just as<br />
surprising.<br />
Astronomers have named <strong>the</strong> mysterious cause of this accelerating<br />
expansion “dark energy.” However, <strong>the</strong>y cannot explain<br />
it. Nor can <strong>the</strong>y explain <strong>the</strong> source of gravitational force<br />
that holds <strong>the</strong> rapidly spinning galaxies from breaking apart.<br />
This force is attributed to <strong>the</strong> gravitational attraction of<br />
“dark matter,” which astronomers think makes up about 90<br />
percent of <strong>the</strong> matter in <strong>the</strong> universe. Dark matter <strong>and</strong> dark<br />
energy, <strong>the</strong> mysteries of science just keep coming.
TERMS TO KNOW<br />
Therefore, <strong>the</strong> ultimate future of <strong>the</strong> universe depends<br />
upon <strong>the</strong> balance between <strong>the</strong> expansion of <strong>the</strong> big bang,<br />
gravity, <strong>and</strong> dark energy. To date, astronomers have not been<br />
able to determine which process will dominate. This remains<br />
one of many questions that guide scientific investigation.<br />
big bang luminosity<br />
cosmic background radiation Milky Way Galaxy<br />
Doppler effect nuclear fusion<br />
galaxy redshift<br />
light-year star<br />
CHAPTER REVIEW QUESTIONS<br />
Base your answers to questions 1–4 on <strong>the</strong> Earth Science Reference Tables or<br />
Figure <strong>28</strong>-3.<br />
1. Which star has about <strong>the</strong> same surface temperature as <strong>the</strong> sun?<br />
(1) Betelgeuse (3) Sirius<br />
(2) Polaris (4) Procyon B<br />
2. Which star is cooler, yet many times brighter than Earth’s sun?<br />
(1) Barnard’s Star (3) Rigel<br />
(2) Betelgeuse (4) Sirius<br />
3. According to <strong>the</strong> “Luminosity <strong>and</strong> Temperature of <strong>Stars</strong>” graph in <strong>the</strong><br />
Earth Science Reference Tables, <strong>the</strong> sun is classified as<br />
(1) a main sequence star. (3) a blue supergiant.<br />
(2) a white dwarf. (4) a red giant.<br />
4. What is <strong>the</strong> color of a main sequence star that gives off about 100 times as<br />
much light as <strong>the</strong> sun?<br />
(1) blue (3) yellow<br />
(2) white (4) red<br />
CHAPTER REVIEW QUESTIONS 731
732 CHAPTER <strong>28</strong>: STARS AND THE UNIVERSE<br />
5. How do stars like <strong>the</strong> sun create energy that is later radiated away into<br />
space?<br />
(1) nuclear fusion changing hydrogen into helium<br />
(2) burning of carbon fuels<br />
(3) changes in state such as melting <strong>and</strong> evaporation<br />
(4) absorbing electromagnetic radiation from space<br />
6. What instrument uses long-wave electromagnetic radiation to help astronomers<br />
make celestial observations?<br />
(1) radio telescopes (3) X-ray telescopes<br />
(2) optical telescopes (4) binoculars<br />
7. According to <strong>the</strong> Earth Science Reference Tables, in what property do ultraviolet,<br />
visible, <strong>and</strong> infrared radiation differ?<br />
(1) half-life (3) wavelength<br />
(2) atomic mass (4) wave velocity<br />
8. The Milky Way Galaxy is best described as<br />
(1) a type of solar system.<br />
(2) a constellation visible to everyone on Earth.<br />
(3) a region of space between <strong>the</strong> orbits of Mars <strong>and</strong> Jupiter.<br />
(4) a spiral-shaped formation composed of billions of stars.<br />
9. In which list are celestial features correctly shown in order of increasing<br />
size?<br />
(1) galaxy→solar system→universe→planet<br />
(2) solar system→galaxy→planet→universe<br />
(3) planet→solar system→galaxy→universe<br />
(4) universe→galaxy→solar system→planet<br />
10. What causes <strong>the</strong> spectral lines of light from distant galaxies to be shifted<br />
toward <strong>the</strong> red end of <strong>the</strong> spectrum?<br />
(1) <strong>the</strong> gravitational field of Earth<br />
(2) <strong>the</strong> gravitational field of <strong>the</strong> sun<br />
(3) motion of <strong>the</strong> galaxies toward us<br />
(4) motion of <strong>the</strong> galaxies away from us
11. The diagram below illustrates three stages of a current <strong>the</strong>ory of <strong>the</strong> formation<br />
of <strong>the</strong> universe.<br />
Stage 1 Stage 2 Stage 3 (present)<br />
A ball of hydrogen<br />
exploded<br />
A huge hydrogen<br />
cloud moved cutward<br />
with cloud parts condensing<br />
to form galaxies<br />
A major piece of scientific evidence supporting this <strong>the</strong>ory is <strong>the</strong> fact that<br />
wavelengths of light from galaxies moving away from Earth in stage 3 are<br />
observed to be<br />
(1) shorter than normal (a redshift).<br />
(2) shorter than normal (a blueshift).<br />
(3) longer than normal (a redshift).<br />
(4) longer than normal (a blueshift).<br />
12. In <strong>the</strong> diagram below, <strong>the</strong> spectral lines of hydrogen gas from three galaxies,<br />
A, B, <strong>and</strong> C, are compared to <strong>the</strong> spectral lines of hydrogen gas observed<br />
in a laboratory.<br />
Blue Red<br />
Laboratory<br />
Hydrogen<br />
Spectral Lines<br />
Earth<br />
The galaxies continue<br />
to move outward<br />
Galaxy A<br />
Spectral Lines<br />
Galaxy B<br />
Spectral Lines<br />
Galaxy C<br />
Spectral Lines<br />
CHAPTER REVIEW QUESTIONS 733<br />
Blue<br />
Blue<br />
Blue<br />
Red<br />
Red<br />
Red
734 CHAPTER <strong>28</strong>: STARS AND THE UNIVERSE<br />
What is <strong>the</strong> best inference that can be made concerning <strong>the</strong> movement of<br />
galaxies A, B, <strong>and</strong> C?<br />
(1) Galaxy A is moving away from Earth, but galaxies B <strong>and</strong> C are moving<br />
toward Earth.<br />
(2) Galaxy B is moving away from Earth, but galaxies A <strong>and</strong> C are moving<br />
toward Earth.<br />
(3) Galaxies A, B, <strong>and</strong> C are all moving toward Earth.<br />
(4) Galaxies A, B, <strong>and</strong> C are all moving away from Earth<br />
13. Because of <strong>the</strong> Doppler redshift, <strong>the</strong> observed wavelengths of light from<br />
distant celestial objects appear closer to <strong>the</strong> red end of <strong>the</strong> spectrum than<br />
light from nearby celestial objects. The explanation for <strong>the</strong> redshift is that<br />
<strong>the</strong> universe is presently<br />
(1) contracting, only.<br />
(2) exp<strong>and</strong>ing, only.<br />
(3) remaining constant in size.<br />
(4) alternating between contracting <strong>and</strong> exp<strong>and</strong>ing.<br />
14. How can we best describe <strong>the</strong> general pattern of motion that we observe<br />
for distant galaxies in <strong>the</strong> universe?<br />
(1) Most galaxies are moving toward <strong>the</strong> Milky Way Galaxy, <strong>and</strong> <strong>the</strong> closer<br />
galaxies are generally approaching faster.<br />
(2) Most galaxies are moving toward <strong>the</strong> Milky Way Galaxy, <strong>and</strong> <strong>the</strong> more<br />
distant galaxies are generally approaching faster.<br />
(3) Most galaxies are moving away from <strong>the</strong> Milky Way Galaxy, <strong>and</strong> <strong>the</strong><br />
closer galaxies are generally moving faster.<br />
(4) Most galaxies are moving away from <strong>the</strong> Milky Way Galaxy, <strong>and</strong> <strong>the</strong><br />
more distant galaxies are generally moving faster.<br />
15. What could cause <strong>the</strong> expansion of <strong>the</strong> universe to slow?<br />
(1) energy production by nuclear fusion<br />
(2) energy production by nuclear fission<br />
(3) gravitational force<br />
(4) electromagnetic radiation
Open-Ended Questions<br />
The graph below shows <strong>the</strong> inferred stages of development of <strong>the</strong> sun. Use this<br />
graph to answer questions 16 <strong>and</strong> 17.<br />
1,000,000<br />
Luminosity<br />
10,000<br />
100<br />
1<br />
0.01<br />
0.0001<br />
Inferred Stages of Development<br />
Sun<br />
White Dwarf<br />
stage<br />
Dust<br />
<strong>and</strong><br />
gases<br />
20,000 10,000 5,000 2,500<br />
Surface Temperature (°C)<br />
CHAPTER REVIEW QUESTIONS 735<br />
16. Describe <strong>the</strong> change in luminosity of <strong>the</strong> sun that will occur from its current<br />
Main Sequence stage to its final White Dwarf stage.<br />
17. Which star shown on <strong>the</strong> “Luminosity <strong>and</strong> Temperature of <strong>Stars</strong>” graph in<br />
<strong>the</strong> Earth Science Reference Tables is currently in <strong>the</strong> sun’s final predicted<br />
stage of development?<br />
18. According to <strong>the</strong> “Luminosity <strong>and</strong> Temperature of <strong>Stars</strong>” graph in <strong>the</strong><br />
Earth Science Reference Tables, what is <strong>the</strong> surface temperature of <strong>the</strong><br />
sun?<br />
19. According to <strong>the</strong> Earth Science Reference Tables, what kind of electromagnetic<br />
radiation has a wavelength of about 1 meter (100 cm)?<br />
20. Name one characteristic that X rays, visible light, <strong>and</strong> radio waves have in<br />
common.