As gas spirals toward the black hole, it is heated to billions of degrees, so it emits enormous amounts of energy, and the quasar shines brightly. Its gravity is so powerful that nothing can escape from it - not even light. A black hole is an object that is squeezed together so tightly that it has extremely strong gravity. Astronomers believe these objects contain supermassive black holes at their hearts, which are encircled by disks of gas. A quasar may emit more energy than an entire galaxy of stars from a region no bigger than our own solar system. They are among the brightest and most distant objects in the universe. The most powerful active galaxies are quasars. ![]() A particular class is called "active" galaxies, because they produce much more energy than "normal" galaxies. Many small irregular galaxies orbit the Milky Way.Īstronomers also categorize galaxies by how much energy they produce in their cores. They may be like the "building blocks" that came together to form the first large galaxies. These are the smallest galaxies, and may contain as few as one million stars. Their stars, gas, and dust spread randomly. These galaxies have no identifiable form. ![]() The final class of galaxies contains a hodge-podge of shapes - anything that looks neither spiral nor elliptical. Like many large spirals, most of them appear to contain "supermassive" black holes at their hearts - star-gobbling monsters that are as much as three billion times as massive as the Sun. They can contain a trillion stars or more, and span as much as one million light-years - about 10 times the diameter of the Milky Way. The largest galaxies in the universe are giant ellipticals. Instead of spreading out into a thin disk, as they do in spirals, the stars in ellipticals wrap completely around the galaxy's heart in all directions. Like spirals, they are named for their shapes: they look like fat, fuzzy footballs. The Milky Way falls into this class of spirals.Ī second class of galaxy is the ellipticals. The arms of barred galaxies spiral outward from the ends of the bar. a face-on spiral galaxy with tightly wound arms (left), a face-on spiral galaxy with very loose arms (center) and an edge-on spiral galaxy (right) Some spiral galaxies have a bright line, or bar, running through them. In some spirals, a wave organizes the stars in the center into a bar. ![]() The gaps between the arms contain older stars, which are not as bright. These massive stars are blue or white, so the spiral arms look blue-white, too. Some newborn stars are massive, hot, and bright, so they make the spiral arms appear bright. In the case of galaxies, they squeeze clouds of interstellar gas, triggering new star formation. Instead, they influence matter as they pass by. Like the waves on the ocean, these "density waves" don't carry material with them. The spiral galaxy UGC 12158, with its arms, bar, and spurs, as well as its low, quiet rate of star. Spiral arms probably form as the result of waves that sweep through the galactic disk. Many similar galaxies are known, but nobody knows exactly which one is most like our Milky Way. Spirals are flat disks of stars with bright bulges in their centers. The Milky Way is a spiral, and so is the Andromeda galaxy (M31). The most beautiful galaxies are called spirals.
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