Messier 9 by HST 6. Image credit: NASA/ESA
The Messier Catalog is full of globular clusters. Truth be told, some are very interesting, and others may seem not so much. Messier 9 may fall into this second category.
A globular cluster is a type of star cluster consisting of a cluster of 105 to 106 old (ie, Population II) stars, gravitationally bound, approximately spherically distributed, and orbiting a galaxy in a manner similar to a satelite. It is these old stars that give globular clusters their typical golden color, only visible through color photography.
Messier 9 is a globular cluster one of the closest to the nucleus of the Milky Way. It is located about 25,800 light-years from the Solar System, and it moves away from us at 224 km/s (that is, more than 800,000 km/ h). It is so close to the galactic center that its gravitational influence slightly deforms it. It is possible that you see, that it has a slightly crushed appearance, instead of being perfectly spherical.
In addition, like many globular clusters, it contains some of the oldest stars in the galaxy (they are about twice as old as the Sun) and have a composition very different from our star: there are not so many heavy elements and that is important.
In the center of the cluster, thanks to the power of the Hubble telescope we see thousands of stars. It is estimated that there are about 250,000 (and of all of these, we know that at least 13 are variables). There are many colors and shapes. Many of them are red giants, but there are also yellows and blues.