What is a Galaxy?

Galaxies are very large groups of stars, dust, gas and dark matter held together by gravity.  They can take many shapes including spiral, elliptical, spherical and irregular. Most galaxies have supermassive black holes at their center.  They can range in size from a few hundred millions stars to over 100 trillion stars in mass.  While some galaxies are merging with others, and most have smaller companion galaxies, galaxies are typically separated by vast distances from one another.  The four galaxies visible under dark skies to the naked eye include the Andromeda Galaxy, the Large Magellanic Cloud, the Small Magellanic Cloud, and the Triangulum galaxy.