On a cosmic scale, galaxies are localized concentrations of huge numbers of stars. Across the known universe, stars almost exclusively live in these giant star cities, drawn together by the collective force of gravity from the mass of the galaxy. Large galaxies like the Milky Way grow in size over time by absorbing other smaller galaxies.
Galaxies mostly collect together in large groups called galaxy groups or clusters. Some galaxy groups contain only 2 or 3 individual galaxies, some clusters contain hundreds or even thousands of members. Our Milky Way is a part of a group of galaxies known as the Local Group, which itself is a part of a much larger galaxy cluster known as the Virgo Supercluster. The Local Group has at least 80 known members, most of which are small dwarf galaxies.
Galaxies come in all shapes and sizes, and little is known about how or why galaxies have formed in the way they do. Scientists have devised a classification system for the shapes of galaxies, in an attempt to begin to understand how they form and change over time.
The two main large galaxy types are the spiral disk galaxies and the spherical elliptical galaxies. There are other types of galaxies, such as the small dwarf galaxies, but most galaxies in the universe are either spiral galaxies or elliptical galaxies.
Spiral galaxies like our own Milky Way have a central core, often with a large spherical bulge of stars, and a disk of stars orbiting the core in a flat galactic plane. True spiral galaxies have two or more spiral arms that stretch far out into space and wrap around the core to create the classical spiral shape. Another group of disk galaxies are known as lenticular galaxies. They have a large flat disk of stars that encircle the central core of the galaxy, but in lenticular galaxies the stars are spread out almost uniformly across the disk and are not concentrated into spiral arms.
Elliptical galaxies are gigantic spheres of stars, often nearly spherical like a ball, but sometimes distorted into the shape of an oval like a football. The largest galaxies in the known universe are all elliptical galaxies, many made up of trillions of stars. Scientists believe that these giant elliptical galaxies are formed by the mergers of smaller galaxies. Over time these mergers scramble the original shapes of the merging galaxies, and they take on the shape of a giant ball of stars. The largest known galaxy, IC 1101 is estimated to contain well over 100 trillion stars. By contrast, the Milky Way is estimated to contain 100 to 200 billion stars. By those estimates, the galaxy IC 1101 could contain as many as 1,000 times the number of stars as our home galaxy.
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