Download Arrangement of the Universe

s8pe-41201-ca
11/2/05
8:22 PM
MAZER
Page 376
We still have much to learn about the universe. Within the last few
hundred years, however, new tools and scientific theories have greatly
increased our knowledge. In this chapter you will learn about the
arrangement of planets and stars. You will also learn about the ways
in which astronomers explore and study space.
Arrangement of the Universe
If you look up at the sky on a clear night, you will see only a tiny
fraction of the planets and stars that exist. The number of objects in
the universe and the distances between them are greater than most
people can imagine. Yet these objects are not spread around randomly.
Gravity causes objects in space to be grouped together in different ways.
The images on page 377 show some basic structures in the
universe. Like a camera lens zooming out, the images provide views
of space at different levels of size.
1
Our planet’s diameter is about 13,000 kilometers (8000 mi).
This is almost four times the diameter of the Moon, which orbits
Earth. An orbit is the path of an object in space as it moves
around another object because of gravity.
2
Earth and eight other major planets orbit the Sun.
The Sun, the planets, and various smaller bodies make up the
solar system. The Sun is about 100 times greater in diameter
than Earth. You could fit more than 4000 bodies the size of the
Sun between the Sun and the solar system’s outermost planet at its
average distance from the Sun. The Sun is one of countless stars in
space. Astronomers have detected planets orbiting some of these
other stars.
3
Our solar system and the stars you can see with
your bare eyes are part of a galaxy called the Milky Way. A galaxy
is a group of millions or billions of stars held together by their own
gravity. Most stars in the Milky Way are so far away that our galaxy
appears to us as a hazy band of light.
4
The Universe
reading tip
The word orbit can be
a noun or a verb.
reading tip
It is often hard to imagine
what distances in space are
like. If the solar system were
the size of a penny, the
length of the Milky Way
would be roughly the
length of California.
Earth
Solar System
The Milky Way
The universe is everything—space and all the matter and energy in it. The Milky Way is just one of many billions of
galaxies in the universe. These galaxies extend in all directions.
Astronomers study space at each of these different levels. Some
focus on planets in the solar system. Other astronomers study distant
galaxies. To learn how the universe formed, astronomers even study
the smallest particles that make up all matter.
check your reading
376 Unit 4: Space Science
What is the relationship between the solar system and the
Milky Way?
PDF