Chapter 28 Stars and the Universe

Sun
100,000 light-years
is an additional cyclic motion of Earth in space. Our planet
rotates on its axis in a 24-hour cycle. It also revolves around
the sun each year. The solar system revolves around the center
of the Milky Way Galaxy in about 220 million years. Although
this is a long time, the Milky Way Galaxy is so large
that this motion is actually about 10 times faster than
Earth’s revolution in its orbit around the sun.
Clusters and Superclusters
WHAT IS THE HISTORY OF THE UNIVERSE? 725
10,000 light-years
Figure 28-8 Earth and the solar system are located about two-thirds of the way
from the galactic center to the outer edge of the Milky Way.
The structure of the universe does not stop at galaxies. The
Milky Way and Andromeda galaxies are two of about 30
galaxies known as the local group. Astronomers are now
mapping superclusters of galaxies and even larger structures
of matter. Why the matter of the universe is so unevenly distributed
is one of the most important questions that astronomers
are investigating today.
WHAT IS THE HISTORY OF THE UNIVERSE?
When you look at very distant objects in the universe, you
are looking back in time. This is because light has a limited
speed. You learned in an earlier chapter that you could estimate
the distance to a lightning strike by counting the seconds
between seeing the flash and hearing the thunder. In
this procedure, you see the flash at essentially the same time
it occurred. Light travels so fast that it could circle Earth
about seven times in a single second.