Groups of stars are known as clusters. There are two types of clusters in our universe: open and globular. Open clusters can contain as little as a dozen to several hundred stars. Open star clusters are closer to our galaxy than globular clusters. Stars in an open cluster are presumed to have the same relative velocity, which is what keeps them in the formation instead of moving apart or colliding. Because of this, the shape of the open cluster often resembles the shape of the parent nebula. Open clusters are also referred to as galactic clusters. They are thought to be one of the youngest objects in the galaxy. There are over one thousand open clusters in the Milky Way galaxy and are concentrated near the galactic plane. Open clusters are of extreme interest to astrophysicists because of the similar age, chemical composition and distance of the stars. There is one thing the stars in open clusters do not have in common, however, they each have different masses.
The Pleiades are a well known open cluster of stars. “The Seven Sisters,” as it is commonly referred to because of the seven stars that are easily seen with the naked eye, is actually made up of over five hundred stars. The Pleiades are one of the first open clusters to be identified.
Globular clusters are known for their spherical shape and orbit a galactic core. These clusters have significantly more stars than the open clusters. Globular clusters can have thousands or even millions of stars. As a result, these clusters appear densely packed. It is believed that globular clusters are one of the oldest structures in the formation of galaxies and even the universe. Of the globular clusters present in the Milky Way Galaxy, they are believed to be at least eleven billion years old. These clusters are far less dense than their open counterparts. There are an estimated one hundred and fifty globular clusters in the Milky Way alone, most of which are found near the galactic core.
While the actual purpose of globular clusters as they relate to the universe as a whole still remains unknown, they appear to be the birthplace of stars in their respective galaxies. The first globular cluster was at the time believed to be a nebula. The M22 cluster in Sagittarius was likely first discovered by Abraham Ihle in 1665.
Sources – Open Clusters
http://www.peripatus.gen.nz/astronomy/OpeClu.html
http://seds.org/messier/open.html
Sources – Globular Clusters
http://nedwww.ipac.caltech.edu/level5/ESSAYS/Cudworth/cudworth.html
http://www.physics.mcmaster.ca/~harris/mwgc.dat
