galaxy

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Astronomers began to speculate by mid-19c. that some of the spiral nebulae they could see in telescopes were actually immense and immensely distant structures the size and shape of the Milky Way. But the matter was not settled until the 1920s.

1. Any of numerous large-scale collections of stars, gas, and dust that make up the visible universe. Galaxies are held together by the gravitational attraction of the material contained within them, and most are organized around a galactic nucleus into elliptical or spiral shapes, with a small percentage of galaxies classed as irregular in shape. A galaxy may range in diameter from some hundreds of light-years for the smallest dwarfs to hundreds of thousands of light-years for the largest ellipticals, and may contain from a few million to several trillion stars. Many galaxies are grouped into clusters, with the clusters themselves often grouped into larger superclusters. See more at active galaxy. See also elliptical galaxy irregular galaxy lenticular galaxy spiral galaxy.
2. the Galaxy. The Milky Way.

A large, self-contained mass of stars.