What is a Galaxy?

 

Definition of a Galaxy: A large aggregation of gas, dust, and typically billions of stars. It is held together by the gravitational attraction between its parts, and its rotational motion prevents it from collapsing on itself. A typical spiral galaxy is shaped like a flat disk, about 100,000 light-years in diameter, with a central bulge, or nucleus, containing old stars; winding through the disk are the characteristic spiral arms of dust, gas, and young stars. An elliptical galaxy, lacking spiral arms entirely and containing little or no gas and dust, resembles the nucleus of a spiral galaxy. Space Science Solar System