M15 a Globular Cluster in our Milky Way

Messier 15 is a globular cluster in our Galaxy, that is to say, it is a bunch of stars that are bound gravitationally and travel along our Milky Way galaxy as a set.   Globular Clusters are different than Open Clusters by the fact that there could be hundred of thousands of stars in them commpared to the tens you find in Open Clusters. So you may ask yourself the following question: Is a Globular Cluster a Mini Galaxy in of itself?   This is a very good question, that has many answers depending on who you ask.   That wasn't very helpful.  Was it?    Well here is one of the defining criteria according to me!   A Globular Cluster is not a Galaxy because there is no current material exchange in the evolution process of their stars.  In English, what that means is that GCs are typically gas less objects.   There is little to no gas in them which inhibits their star formation.  Galaxies in general have gas and star formation process.

So why M15 for today's blog?   Well, other than it is a very pretty and super dense cluster in our Milky Way, it is believed that among its numerous star in its core there is a black hole.  It is also a magnitude 6.2 object in the sky, which means that it is a the limit of naked eye visibility under exceptionally good conditions and easily found with binoculars.   It is also the home of PSR 2127+11 C , a pulsar that is on a binary system with another neutron star.   This binary system is test ground for the theory of General Relativity and gravitational waves. Also, Pease 1 is the first planetary nebula found on a Globular Cluster.   It is home to 119 variable stars.  Its age has been calculated to be around 13.2 billion years old?  Isn't that mind boggling?

So in general M15 is a sandbox of astronomical science.  It has a bit for everyone from observational to theoretical astrophysicists.