Generally, a large body of gas, dust, and stars held together by their mutual gravitational attraction and ranging in mass from about 10^6 to 10^13 M_sun. If a galaxy also contains dark matter its mas
s will be much larger. Galaxies are grouped into three main categories: spiral galaxies, elliptical galaxies, and irregular galaxies.
A spheroidal region at the center of a spiral galaxy which mostly contains old stars. Galactic bulges are generally classified into two types: classical bulges and pseudo-bulges.