Messier 15, Globular Cluster
Optics:   Ritchey–Chrétien 20" F/8.2 (4166mm FL) Processing:   PixInsight, Photoshop
Camera:   SBIG STXL-11000 with Adaptive Optics Date:   August 2023
11 Megapixel (4008 x 2672 16-bit sensor) Location:   Columbus, Texas
Exposure:   LRGB = 64:24:24:24 minutes Imager:   Kent E. Biggs
Messier 15, or just M15, is a globular cluster located in the direction of the constellation Pegasus, the winged horse with magical powers born of Medusa’s headless blood. 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.

Globular clusters are spherical collections of stars bound together by gravity with a higher concentration of stars at their centers. Globular clusters are in contrast to open clusters, which are formed out of the same giant molecular cloud, less dense, and usually do not stay together as long as globular clusters. 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 to hundreds of thousands of stars, galaxies have millions, billions, and even a trillion or more stars. Why so many globular clusters exist around galaxies and are not swallowed up (yet?) by them remains a mystery.

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 like Venus since their relatively close proximity to other stars makes protoplanetary disks unlikely to maintain stable enough to form planets.

At only 35,700 light years from Earth, M15 is around 175 light years in diameter. M15 is 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. Its center is especially dense as its core has collapsed (see center of image above) with 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.

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.

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