Page 40 - Curriculum Visions Dynamic Book

When you look up at the night sky, you might think that there are stars everywhere. But stars are not scattered evenly. Instead, they are clumped into certain regions with very little matter between them. These concentrated regions of matter and stars are galaxies.
Galaxies are also the places in the Universe where matter is transformed into energy. They are, then, both the source and location of everything. They are the factories of the Universe.
Galaxies contain matter that will emit light if it gets hot enough or will reflect light from other bodies. However,
it is certain that what we see is not all there is. Calculations show that a galaxy has more mass in it than we can see, so the rest must be made up of matter that is invisible to us—that is, it gives out no light. This is called dark matter. It, too, can be a source of energy, although as yet we know little about it.
The nature of galaxies
Our galaxy, which we call the Milky Way, is also known by astronomers as the galaxy. All galaxies contain a huge mass of gas, dust, and stars spinning slowly around a center. Many have huge trailing spiral arms. At the center of such a galaxy is a bulging region made of stars. Some galaxies also have a flat disk made of stars that are less concentrated than in the bulge, but that form spiral arms and a halo beyond the spiral arms or main disk made of old stars.
Space gas and dust—interstellar (between the stars) matter—are found everywhere in a galaxy, although they make up only about 1% of the mass of a galaxy. Ninety-nine percent of the mass of a galaxy is concentrated in the stars.
 The Andromeda Galaxy (catalogues NGC 224 and M31) is a typical spiral galaxy in the constellation Andromeda. It is the separate galaxy nearest the Milky Way Galaxy. It can be seen in the night sky without a telescope. It is 2,000,000 light-years from the Earth. It is 200,000 light-years across.
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