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Ch. 21 Stars, Galaxies, and the Universe
21.4 Star Systems and Galaxies
Identify and describe three types of galaxies.
Galaxies
The Milky Way is the galaxy in which our solar system is located.
Looks milky or hazy because the stars are too close together for your eyes to see them
individually.
There are billions of galaxies in the universe.
Astronomers have classified most galaxies into three main categories: spiral, elliptical
and irregular.
Spiral Galaxies
A spiral galaxy is a galaxy that has the shape of twin spirals. They have arms that spiral
outward, like pinwheels.
The Milky Way is a spiral galaxy.
Elliptical Galaxies
Elliptical galaxies look, like flattened ball.
They contain billions of stars but have little gas and dust. So they cannot form new star.
They contain old stars.
Irregular Galaxies
Some galaxies do not have regular shapes.
The Large Magellanic Cloud is an irregular galaxy about 160,000 light-years away from
our galaxy.
It is one of our closest neighbors.
21.5 History of the Universe
Explain the big bang theory of how the universe was formed.
Describe how the solar system was formed.
Introduction
Andromeda Galaxy is the most distant object you can see with your unaided eye. Light
travels for 2 million years before reaching your eye.
Moving Galaxies
To study how and when the universe formed, astronomers use information about how
galaxies are moving.
In the 1920’s, Edwin Hubble, an American astronomer, discovered that the farther away
a galaxy is from us. The Hubble Space Telescope was named after Hubble in honor of
these discoveries.
Raisin bread analogy
The Big Bang Theory
According to the big bang theory, the universe formed in an enormous explosion about
10 to 15 billion years ago. Since the big bang, the universe is billions of times larger than
it was billions of years ago.
Astronomers have concluded that the galaxies are moving away from each other as a
result of the big bang.
Astronomers estimate that the universe has been expanding for 10 billion to 15 billion
years.
Formation of the Solar System
After the big bang, matter in the universe separated into galaxies. Gas and dust spread
throughout space in our galaxy.
About five billion years ago, a giant cloud of gas and dust, or nebula, collapsed to form
the solar system.
Slowly the nebula shrank to form a spinning disk. As gravity pulled some of the gas
into the center of the disk, the gas became hot and dense enough for nuclear fusion to
begin. The sun was born. Gas and dust formed solid spheres smaller than the sun. The
spheres closest to the sun lost most of their gases and became the inner planets.
The spheres farthest from the sun became the gas giants. Between the inner and outer
planets, asteroids formed. Beyond the outer planets, a huge cloud of ice and other
substance formed which is probably the main source of comets.
The Future of the Universe
One possibility: The universe will continue to expand, as it is doing now. The stars
will run out of fuel, burn out, and the universe will be cold and dark
Second possibility: The force of gravity will begin to pull the galaxies back together.
A reverse “big bang”. The universe will be crushed into an enormous black hole.
It depend on the total mass of the universe which is difficult to estimate because most
of it is in the form of particles that do not give off electromagnetic radiation. The
evidence so far suggests that the total mass of the universe is not great enough to pull the
galaxies back together again. More research has to be done.
Astronomy is one of the oldest sciences, but there are still many discoveries to be made
and puzzles to be solved about this universe of ours.
The End!