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A galaxy contains stars, gas, and dust which are held together as a group by gravity. There are millions, or even billions, of stars in one galaxy. There are billions of galaxies in the Universe. Galaxies cover a huge range of size and mass as well as exhibiting a variety of structures and properties. The smallest galaxies known are relatively nearby dwarf galaxies containing only 100,000 stars, fewer than in a typical globular cluster. At the other end of the scale, the most massive galaxy known, the giant elliptical M87, contains 3,000 billion solar masses, about 15 times more than our own Galaxy. Most galaxies can be categorized into a number of broad morphological types. Spiral galaxies are disc-shaped, with a central bulge, from which spiral arms appear to wind outwards. In barred spirals, a bar of stars extends out from the bulge and the arms appear to be attached to the ends of the bar. Spiral galaxies contain very luminous young stars and significant amounts of interstellar material concentrated in the arms. Most of the conspicuous galaxies in the sky are spirals, but the most numerous type is the elliptical galaxy. Both the smallest and largest galaxies are of this kind. They are thought to consist entirely of old stars with relatively little interstellar material. The three-dimensional shape of galaxies in the elliptical category can be spheroidal or virtually spherical. The third main group is that of irregular galaxies, which are neither spiral nor elliptical. These account for up to a quarter of all known galaxies. At visible wavelengths, irregular galaxies show no particular circular symmetry and look chaotic. A very small number of galaxies have unusual structure, often attributable to a gravitational interaction with another galaxy. Others emit exceptionally large amounts of energy and exhibit other evidence, such as variability, suggesting that unusual and violent processes are at work. Such active galaxies include Seyfert galaxies and radio galaxies.
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