Details: As with many of the objects in Charles Messier’s Catalog, M15 was discovered by another astronomer Giovanni Domenico Maraldi, who did not get the globular cluster named after him, but did posthumously receive the honor of the named lunar crater Maraldi. Since of all astronomical objects, globular clusters most resemble comets approaching earth, Messier and others charted them as fixed celestial objects so as not to confuse then with newly discovered comets.

Globular vs. Open Clusters vs. Galaxies: Globular clusters both compare with open clusters, in that they both form out of giant molecular clouds, however, open clusters are less dense and usually do not stay together as long as globular clusters. Many if not most open clusters formed recently and may be a few million up to a few billion years old. The age of globular clusters, however, can approach the age of our universe. Some have suggested that globular clusters are early galaxies that have not merged with others to form an actual galaxy, however, the fact that most large galaxies have many globular clusters orbiting about them may give doubt to this theory. In contrast, globular clusters have tens of thousands to tens of millions of stars, while galaxies have billions to even a trillion or more stars. Why so many globular clusters exist around the outskirts of galaxies and are not swallowed up (yet) by them remains a mystery.

The largest, most massive globular cluster orbiting our own galaxy is Omega Centauri or ω Cen. Greek letters order stars within a constellation from its brightest, alpha, then second brightest, beta, and so on. Therefore the omega (ω) designation in ω Cen makes it the 24th brightest “star” in the constellation Centaurus with Alpha Centauri (α Cen) its  brightest. Of course ω Cen is not a star, but at 160-180 light-years in diameter, it contains 10 millions stars. At magnitude 3.9, it is easily bright enough to be seen with the unaided eye both today and by the ancients who thought it was a star. Although much less massive than ω Cen, M15 is far denser at its core, making it the most dense globular cluster core known. At its age of over 12 billion years, or nearly 90% the age of the universe, this core density is caused by a core collapse, common in older globular clusters. Furthermore, since the average time for a globular cluster’s core to collapse is less than the age of our galaxy, it has been suggested that cores, after collapsing, may even expand and become less dense over time.

Abundancy of Objects: There are visible here an enormous number of stars surrounding what may indeed be a massive black hole. M15 is home to around 100,000 stars and contains 112 variable stars, 8 pulsars, 2 X-ray sources, 1 double neutron star, and even a planetary nebula, called Pease 1 (visible in the image above). Only three other globular clusters have been found to contain planetary nebula.

X-Ray Sources: The X-Ray sources in M15 include Low-Mass X-ray Binaries or LMXBs produced when matter from a common star (the donor) falls into its component star usually a neutron star, a white dwarf, or a black hole. Of the 200 or so LMXBs discovered in our own galaxy, 13 are in globular clusters such as M15!

More Details: While globular clusters are relatively dense, their stars are on average half a trillion miles apart or more. At the core of this cluster, however, you might see up to 1000 stars as bright as Venus in the night sky, but unlikely any planets since the relatively close proximity to other stars makes protoplanetary disk formation unlikely to ever be stable enough to form planets.

At only 35,700 light years from Earth, M15 is between 11.5 and 13.5 billion years old making it one of the oldest known globular clusters. It is also one of our galaxy’s most densely populated globular clusters. At apparent magnitude 6.2, M15 would be a very challenging object to view with the unaided eye even at a very dark site, however, it is easily visible through binoculars or small telescopes. The brightest stars in M15 are around 12.5 magnitude or 400 times fainter than the faintest star visible to the naked eye.

Annotations: Using a mouse, hover over the image above for annotations, insets, and enlargements. At its center, M15 can be seen to be extremely dense compared to other globular clusters. Just up and left of center is a planetary nebula visible here with a 10X enlarged inset. Just to the upper left of the cluster is a much closer foreground star, yellowish and similar spectral class of G0 as is our own sun at G2V. Surrounding M15 are dozens of faint, very distant galaxies of which four are highlighted with enlarged insets.

Below images: The below image shows a zoomed in view of M15. The second image below compares the latest image processing to that of a few years ago.