Download Chapter 5 Galaxies and Star Systems

Chapter 5
Galaxies and Star Systems
Section 5.1
Galaxies
Terms:
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Galaxy
Spiral Galaxy
Elliptical Galaxy
Irregular Galaxy
Milky Way Galaxy
Quasar
Black Hole
Types of Galaxies
A galaxy is a huge group of single stars, star
systems, star clusters, dust, and gas bound together by gravity. There are billions
of galaxies in the universe. The largest galaxies have more than a trillion stars!
Astronomers classify most galaxies into the following types: spiral, elliptical, and
irregular.
Spiral galaxies are those that appear to have a bulge in the middle and arms
that spiral outward, like pinwheels. The spiral arms contain many bright, young
stars as well as gas and dust. Most new stars in the spiral galaxies form in
theses spiral arms. Relatively few new stars form in the central bulge. Some
spiral galaxies, called barred-spiral galaxies, have a huge bar-shaped region of
stars and gas that passes through their center.
Not all galaxies have spiral arms. Elliptical galaxies look like round or flattened
balls. These galaxies contain billions of the stars but have little gas and dust
between the stars. Because there is little gas or dust, stars are no longer forming.
Most elliptical galaxies contain only old stars.
Some galaxies do not have regular shapes, thus they are called irregular
galaxies. These galaxies are typically smaller than other types of galaxies and
generally have many bright, young stars. They contain a lot of gas a dust to from
new stars.
The Milky Way Galaxy
Although it is difficult to know what the shape of
the Milky Way Galaxy is because we are inside of it, astronomers have identified
it as a typical spiral galaxy. From the side, the Milky Way would look like a
narrow disk with a large bulge in the middle. From the top or bottom, the Milky
Way would have a spiral, pinwheel shape. Astronomers estimate that the Milky
34 Way contains 200 to 400 billion stars.
Like other spiral galaxies, our galaxy
has a disk, a central bulge, and spiral
arms. The disk is about 100,000 lightyears across and 3,000 light-years thick.
Most of the Galaxy’s gas, dust, young
stars, and open clusters are in the disk.
The central bulge is about 12,000 to
16,000 light-years wide and 6,000 to
10,000 light-years thick. The central
bulge contains mostly older stars and
globular clusters. Some recent evidence
suggests the bulge might not be
spherical, but is instead shaped like a
bar. The bar might be as long as 27,000 light-years long. The disk and bulge are
surrounded by a faint, spherical halo, which also contains old stars and globular
clusters. Astronomers have discovered that there is a gigantic black hole at the
center of the galaxy.
The Milky Way Galaxy is a big place. If our solar system were the size of your
fist, the Galaxy’s disk would still be wider than the entire United States!
Where We Are
Our solar system, including the Sun, Earth, and
all the other planets, is within one of the spiral arms in the disk of the Milky Way
Galaxy. Most of the stars we see in the sky are relatively nearby stars that are
also in this spiral arm.
Just as Earth orbits the Sun, the Sun and solar system orbit the center of the
35 Galaxy. One orbit of the solar system takes about 225 to 250 million years at a
speed of half a million miles per hour. The solar system has orbited 20 to 25
times since it formed 4.6 billion years ago.
The center of our galaxy is located about 28,000 light-years away, beyond the
constellation Sagittarius (actually just beyond the border of Sagittarius and
Scorpio). So, if you can locate these two constellations in the sky, you'll be
looking toward the center of our galaxy!
The Milky Way is part of a set of galaxies known as the Local Group, which
includes several dozen different galaxies within 3 million light-years. Only one of
these, the Andromeda galaxy, is close to the size of the Milky Way. This Local
Group is part of a supercluster, known as the Virgo supercluster, which has at
least 5,000 member galaxies and is roughly 100 million light-years across.
Beyond this level of organization, not much is known about our position in the
universe.
It's sad that there is so much light pollution in most cities that many people have
never seen the Milky Way. On a clear night away from lights the view is of a
bright white river of stars. You don't need a telescope or even binoculars to see
it. The view of the Milky Way is so bright because you're looking at the stars in
your own galaxy.
Quasars
In the late 1960s, astronomers discovered
objects that are very bright but also very far away. Many of these objects are ten
billion light years or more away, making them among the most distant objects in
the universe. These distant, enormously bright objects looked almost like stars.
Since quasi means “something like” in Latin, these objects were given the name
quasi-stellar objects, or quasars.
Astronomers have concluded that quasars are active young galaxies with giant
black holes in their centers. Black holes are the remains of the most massive
stars when they die. A black hole is an object with gravity so strong that nothing,
not even light, can escape. Each of these black holes has a mass a billion times
more as great as that of the sun. As enormous amounts of gas revolve around
the black hole, the gas heats up and shines brightly.
Summary:
• Astronomers classify most galaxies into the following types: spiral,
elliptical, and irregular.
• A galaxy is a huge group of stars that are held together by gravity. A
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galaxy contains single stars, star systems, star clusters, dust and
gas.
A spiral galaxy has a bulge in the middle and arms that curve outward.
Most new stars are found in its spiral arms.
An elliptical galaxy looks like a round or flattened ball. Most of these
galaxies contain only old stars.
An irregular galaxy does not have a certain shape. They contain many
young stars.
Our solar system is located in a spiral galaxy called the Milky Way. The
Milky Way is part of the Local Group containing several nearby
galaxies. The Local Group is part of the Virgo Supercluster, which
contains thousands of galaxies.
Many galaxies, including our own, have a black hole in the center. A black
hole is an object with very strong gravity that does not give off any
light. Nothing escapes a black hole!
37 Section 5.2
Star Systems
Terms:
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Star Systems
Binary Stars
Eclipsing Binary
Star Clusters
Our sun is the one star in our solar system. As you move past our solar system,
billions of other stars make up our galaxy. Most stars are members of groups of
two or more stars, called star systems.
Binary stars are pairs of stars. Scientists detect binary stars by observing the
brightness and motion of
nearby stars. Often one star
in a binary star is much
brighter and more massive
than the other. Astronomers
can sometimes detect a
binary star even if only one of
the stars can be seen from
Earth. Astronomers can often
tell that there is a dim star in
a binary system by observing
the effects of its gravity. As the dim companion revolves around a bright star, the
dim star’s gravity causes the bright star to wobble back and forth. Imagine
watching a pair of dancers who are twirling each other around. Even if one
dancer were invisible you could tell that the invisible dancer was there from
watching the motion of the visible dancer.
A wobble isn’t the only clue that a star has a dim companion. A dim star in a
binary star may pass in front of a brighter star and eclipse it. From Earth, the
binary star would suddenly look much dimmer. A system in which one star
periodically blocks the light from another is called an eclipsing binary.
In 1995, astronomers first discovered a planet revolving around another ordinary
star. They used a method similar to the one used in studying binary stars. The
astronomers observed that a star was moving slightly toward and away from us.
They knew that the invisible object causing the movement didn’t have enough
mass to be a star. They inferred that it must be a planet. Since then, astronomers
have discovered many more planets and many more stars.
Star clusters are larger grouping of stars. All the stars in a particular cluster
38 formed from the same nebula at about the same time and are about the same
distance from Earth. There are two many types of star clusters: open clusters
and globular clusters. Open clusters are loose, disorganized appearance and
contain no more than a few thousand stars. They often contain many bright
supergiants and a lot of gas and dust. In contrast, globular clusters are larger
grouping of older stars. Globular clusters are round and densely packed with
stars—many contain more than a million.
Summary:
• Most stars are members of groups of two or more stars, called star
systems.
• A star system is a group of two or more stars. A star system that has two
stars is called a double star or a binary star. A star system with three
stars is called a triple star.
• An eclipsing binary is a star system with two stars. In an eclipsing binary,
one of the stars sometimes blocks the light from the other star.
• Many stars belong to larger groups of stars called clusters. An open
cluster is a group of star that is loose and disorganized. A globular
cluster is a group of stars that is round and densely packed.
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