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Name: Messier 62, NGC 6266
Description: Globular Cluster
Position (J2000): RA 17h 1m 9.44s Dec -30° 6' 37.91"
Constellation: Ophiuchus
Distance: 22,000 light years
Visual Magnitude: 6.5
Angular size: 15 arcmin
Field of view: 2.70 x 2.77 arcminutes
Orientation: North is 2.9° right of vertical
Image Credit: ESA/Hubble & NASA, S. Anderson et al.
Release date: April 15, 2019
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Most globular clusters are almost perfectly spherical collections of stars — but Messier 62 breaks the mould. The 12-billion-year-old cluster is distorted, and stretches out on one side to form a comet-like shape with a bright head and extended tail. As one of the closest globular clusters to the center of our galaxy, Messier 62 is likely affected by strong tidal forces that displace many of its stars, resulting in this unusual shape.

When globular clusters form, they tend to be somewhat denser towards the center. The more massive the cluster, the denser the center is likely to be. With a mass with almost a million times that of the Sun, Messier 62 is one of the densest of them all. With so many stars at the center, interactions and mergers occur regularly. Huge stars form and run out of fuel quickly, exploding violently and their remains collapse to form white dwarfs, neutron stars and even black holes!

For many years, it was believed that any black holes that form in a globular cluster would quickly be kicked out due to the violent interactions taking place there. However, in 2013, a black hole was discovered in Messier 62 — the first ever to be found in a Milky Way globular cluster, giving astronomers a whole new hunting ground for these mysterious objects.

This view comprises ultraviolet and visible light gathered by the NASA/ESA Hubble Space Telescope’s Advanced Camera for Surveys.

From Wikipedia:

Messier 62 or M62, also known as NGC 6266, is a globular cluster of stars in the equatorial constellation of Ophiuchus. It was discovered on June 7, 1771 by Charles Messier, then added to his catalogue in 1779.

M62 is at a distance of about 22.2 kly from Earth and 5.5 kly from the Galactic center. It is among the ten most massive and luminous globular clusters in the Milky Way, showing an integrated absolute magnitude of –9.18. The cluster has an estimated mass of 1.22×106 M and a mass-to-light ratio of 2.05±0.04 in the V band. It has a projected ellipticity of 0.01, meaning is it essentially spherical. The density profile of the cluster members suggests it has not yet undergone core collapse. It has a core radius of 1.3 ly (0.39 pc), a half-mass radius of 9.6 ly (2.95 pc), and a half-light radius of 6.0 ly (1.83 pc). The stellar density at the core is 5.13 M per cubic parsec. It has a tidal radius of 59 ly (18.0 pc).

The cluster shows at least two distinct populations of stars, which most likely represent two separate episodes of star formation. Of the main sequence stars in the cluster, 79%±1% are from the first generation and 21%±1% from the second. The second generation is polluted by materials released by the first. In particular, the abundances of helium, carbon, magnesium, aluminium, and sodium differ between the two populations.

Indications are this is an Oosterhoff type I, or "metal-rich" system. A 2010 study identified 245 variable stars in the cluster's field, of which 209 are RR Lyrae variables, four are Type II Cepheids, 25 are long period variables, and one is an eclipsing binary. The cluster may prove to be the galaxy's richest in terms of RR Lyrae variables. It has six binary millisecond pulsars, including one (COM6266B) that is displaying eclipsing behavior from gas streaming off its companion. There are multiple X-ray sources, including 50 within the half-mass radius. 47 blue straggler candidates have been identified, formed from the merger of two stars in a binary system, and these are preferentially concentrated near the core region.

It is hypothesized that this cluster may be host to an intermediate mass black hole (IMBH), and it is considered particularly suitable for searching for such an object. Examination of the proper motion of stars within 17" of the core does not require an IMBH to explain. However, simulations can not rule out an IMBH with a mass of a few thousand M. Based upon radial velocity measurements within an arcsecond of the core, Kiselev et al. (2008) made the claim that there is an IMBH with a mass in the range (1–9)×103 M.