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A galaxy is a system of stars, stellar remnants, interstellar gas, dust, and dark matter bound together by gravity. [1] [2] The word is derived from the Greek ...
This is in contrast to extragalactic astronomy, which is the study of everything outside our galaxy, including all other galaxies. Galactic astronomy should not be confused with galaxy formation and evolution, which is the general study of galaxies, their formation, structure, components, dynamics, interactions, and the range of forms they take.
A dwarf galaxy is a small galaxy composed of about 1000 up to several billion stars, as compared to the Milky Way's 200–400 billion stars. [1] The Large Magellanic Cloud , which closely orbits the Milky Way and contains over 30 billion stars, [ 2 ] is sometimes classified as a dwarf galaxy; others consider it a full-fledged galaxy.
The spread of velocities for the individual galaxies is about 150 km/s. However, this definition should be used as a guide only, as larger and more massive galaxy systems are sometimes classified as galaxy groups. [4] Groups are the most common structures of galaxies in the universe, comprising at least 50% of the galaxies in the local universe.
The bulk of the stars in a spiral galaxy are located either close to a single plane (the galactic plane) in more or less conventional circular orbits around the center of the galaxy (the Galactic Center), or in a spheroidal galactic bulge around the galactic core.
The bulgeless galaxy fraction of the Universe has remained roughly constant for at least the last 8 billion years. [5] In contrast, about two thirds of galaxies in dense galaxy clusters (such as the Virgo Cluster ) do possess a classical bulge, demonstrating the disruptive effect of their crowding.
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The reason is that these galaxy formation models predict a large number of mergers. If disk galaxies merge with another galaxy of comparable mass (at least 15 percent of its mass) the merger will likely destroy, or at a minimum greatly disrupt the disk, and the resulting galaxy is not expected to be a disk galaxy (see next section).