Globular Cluster A collection of stars that orbits a galaxy’s core. GCs are very tightly bound by gravity which gives them their spherical shapes and high central density. The number of stars in a globular cluster varies, but normally hundreds of thousands. “Globs” contain considerably more stars and are much older (often > 13 billion years old) than open clusters. There are about 150 currently known globular clusters in the Milky Way, with perhaps more still undiscovered.

Open Cluster An open cluster is a group of up to a few thousand stars that were formed from the same gas cloud and are roughly the same age. More than 1,100 open clusters have been discovered within the Milky Way Galaxy. Open clusters only survive for a few hundred million years unlike the more massive globular clusters that can survive for many billions of years.

Galaxy A galaxy is a massive system of stars, gas, dust, and dark matter held together by gravity. The word galaxy is derived from the Greek galaxias, literally "milky", a reference to the Milky Way. Galaxies range from dwarfs with as few as ten million stars to giants with a hundred trillion, each orbiting their galaxy's own center of mass. Supermassive black holes reside at the center of most galaxies.

Planetary Nebula A planetary nebula consists of an expanding glowing shell of ionized gas ejected from old red giant stars late in their lives. The word 'nebula' is Latin for mist or cloud. They last only a few tens of thousands of years. At the end of the star's life, during the red giant phase, the outer layers of the star are expelled by strong stellar winds. Our sun’s evolution will probably result in the formation of a planetary nebula when it has exhausted all its fuel in a few billion years.