Galaxies
Galaxies are collections of stars, gas, and dust, combined with some unknown form of dark matter, all bound together by gravity. The visible parts come in a variety of sizes, ranging from a few thousand light years with a billion stars, to 100,000 light-years with a trillion stars. Our own Milky Way galaxy contains about 200 billion stars.
Types of Galaxies
The invisible parts of galaxies are known to exist only because of their influence on the motions of the visible parts. Stars and gas rotate around galaxy centers too fast to be gravitationally bound by their own mass, so dark matter has to be present to hold it together. Scientists do not yet know the size of the dark matter halos of galaxies; they might extend over ten times the extent of the visible galaxy. What we see in our telescopes as a giant galaxy of stars may be likened to the glowing hearth in the center of a big dark house.
Imagine viewing a galaxy through a small telescope, as pioneering astronomers William and Caroline Herschel and Charles Messier did in the late eighteenth century. You would see mostly a dull yellow color from countless stars similar to the Sun, all blurred together by the shimmering Earth atmosphere.
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