Introductory Astronomy: Galaxies
When you look up at the sky on a dark clear night, you can see thousands of stars shining down on you. These stars are balls of hot gas, like our Sun. The light from these stars has travelled trillions of miles (or more!) to reach your eyes. If you are lucky enough to be observing the stars in the mountains, in the desert, or any place far from city lights, you will also see a large hazy streak across the sky. This hazy streak is a dense collection of faint stars which are so numerous that their light seems to blend together on the sky, forming a streak. This streak is called the Milky Way because of its "milky" appearance.
A dense collection of stars, gas, and dust (like the Milky Way) is called a Galaxy. The Milky Way is our home galaxy; if we could travel outside of the Milky Way and look back at it, we would see that our Sun is just one of the many stars that make up the Milky Way Galaxy.
In 1925, an astronomer named Edwin Hubble made the exciting discovery that there are other galaxies in the universe besides our own. He used Cepheid variables (remember that these are standard candles) discover the distance to the faint, fuzzy "nebulae" astronomers had observed since the late 1700s and found that these objects lie outside our own galaxy. This suggested that the "nebulae" were not really were not really nebulae at all but individual galaxies. Hubble studied the structure of these galaxies and grouped them into three main classes:
Galaxies are interesting to study, not only because they contain massive amounts of stars and dust (and other solar systems!), but also because they interact and collide. Such interactions cause major disruptions in the galaxies themselves. They make galaxies dynamic, exciting places to visit through the eyes of science.
After you review these sections, try a few sample questions to test your understanding. These questions are typical of questions given in introductory astronomy course exams. They are meant only to give you an idea of what kinds of questions MIGHT be on your exam. Just because these questions are here does NOT mean that you will have questions like them on your exam, NOR does it even mean that you will have questions on these topics on your exam. They are just PRACTICE questions!