Omega Centauri: A globular cluster is a group of ancient stars (Omega Centauri is thought to be almost as old as the universe, perhaps 12 - 13 billion years old),
gravitationally bound to each other, orbiting the core of its associated galaxy.
Omega Centauri is about 17,000 light years from Earth (just above the plane of our Milky Way galaxy) and appears to be almost the same size as a full moon when viewed from very dark skies; its diameter varies by source, from 150 light years to 230 light years (I am inclined to believe the latter, given how large it is on my photo). It shines at magnitude 3.9, and is easily visible to the naked eye. It has a mass equal to about 5 million times that of our sun, and contains as many as 10 million stars. It is many times as massive as the next-largest globular cluster in our galaxy, 47 Tucanae; to see a comparison visually (the density should be readily apparent, as well as the size of the densest part), compare this photo with the one I took of 47 Tucanae at the same scale with the same equipment here.
Omega Centauri also is unusual among globular clusters in that it has stars of different ages, and apparently contains a black hole at its center; these facts strongly suggest that Omega Centauri may be the remnant of a dwarf galaxy whose outer stars were stripped off by an earlier collision with our Milky Way. All of the stars in the cluster are close together, by galactic standards; the average distance between any two stars in the cluster's core is only about a third of a light-year, roughly 13 times closer than our Sun's nearest stellar neighbor, Alpha Centauri. According to a NASA website, "if anyone lived in this globular cluster, they would behold a star-saturated sky that is roughly 100 times brighter than Earth's sky."
I had previously taken this beautiful cluster with my little scope, posted here. The fact that a very short focal length scope can take such a detailed, large image of this cluster is further graphic evidence of its size.
Copyright 2016 Mark de Regt