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Our Cosmic Home Study Guide (page 2)

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Updated on Sep 25, 2011

Galaxies and Stars

It is important to know that as we look out in space, we look back in time. That's because the speed of light, though fast, is finite. Light travels at 186 thousand miles per second (300,000 kilometers per second). The light from stars in our own galaxy, generated hundred of thousands of years ago, or from stars in other galaxies, generated billions of years ago, is just now reaching us. Study the diagrams in Figure 1.4 and see how the term light-year is an astronomical unit of distance.

Figure 1.4 The The Light-Year

The figure shows that light from the sun needs to travel 93 million miles to reach Earth, and this takes the light 8.3 minutes. Thus, we see the sun as it was 8.3 minutes ago. We simply cannot see the sun as it is right now, because it takes time for the light to travel. This has important implications when we look at stars and galaxies. The other portion of the figure shows light reaching the eye of an observer from a star that is 252 trillion miles away. (Many stars that you see are this far, and many are much farther.) Light take 43 years to make the journey from star to eye. Thus, the star is 43 light-years away. If the star were to explode today, we would not know it for 43 years!

A light-year is nearly 6 trillion miles of distance (5.86 trillion miles, which is 9.4 trillion kilometers). When we start to look at galaxies, this "looking back in time" gets really serious. For example, the nearest large galaxy that is similar to our own Milky Way galaxy is Andromeda, and it is 2 million light-years away. Today, we see the light that it emitted 2 million years ago, during a time when the ancestors of humans were just making the first crude stone tools in Africa.

Many galaxies were formed in the first billion years or so after the Big Bang, as clumps of matter floating in space condensed under the attractive power of gravity. Galaxies are cities of stars. Just as gravity made the galaxies, gravity also made the stars within the galaxies. Stars are created when gas clouds in space condense, pulled together by gravity. During the condensation, the gas becomes hotter and hotter. If the density and temperature are high enough, the protostar ignites and is sustained as a glowing star by nuclear fusion. New stars are being formed all the time. Astronomers today have found regions of star births. Stars also die (see the next section). Our own galaxy is called the Milky Way (see Figure 1.5).

Figure 1.5 The Milky Way Galaxy

Our Milky Way galaxy contains about 400 billion stars, an incredible number. It is shaped somewhat like two dinner plates put together face to face, with a bulge in the middle, and within the bulge, a zone exists that is extremely rich with stars. From above, these appear as giant spiral arms. Astronomers have also found evidence for a massive black hole in the center. Black holes occur when matter has contracted to such a high density that even light cannot escape. We know black holes by certain effects they have on radiation in the space around them. Note that our sun is located about three fifths away from the center of the galaxy.

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