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The Crammed Center of Messier 22
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Name: Messier 22, NGC 6656
Description: Globular Star Cluster
Position (J2000): RA 18h 36m 24.21s Dec -23° 54' 9.73"
Constellation: Sagittarius
Distance: 10,000 light years
Visual magnitude: 5.1
Angular Size: 32 arcmins (diameter)
Field of view: 3.42 x 3.49 arcminutes
Orientation: North is 93.7° left of vertical
Image Credit: ESA/Hubble & NASA
Release date: April 6, 2015
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This image shows the center of the globular cluster Messier 22, also known as M22, as observed by the NASA/ESA Hubble Space Telescope. Globular clusters are spherical collections of densely packed stars, relics of the early years of the Universe, with ages of typically 12 to 13 billion years. This is very old considering that the Universe is only 13.8 billion years old.

Messier 22 is one of about 150 globular clusters in the Milky Way and at just 10,000 light-years away it is also one of the closest to Earth. It was discovered in 1665 by Abraham Ihle, making it one of the first globulars ever to be discovered. This is not so surprising as it is one of the brightest globular clusters visible from the northern hemisphere, located in the constellation of Sagittarius, close to the Galactic Bulge - the dense mass of stars at the center of the Milky Way.

The cluster has a diameter of about 70 light-years and, when looking from Earth, appears to take up a patch of sky the size of the full Moon. Despite its relative proximity to us, the light from the stars in the cluster is not as bright as it should be as it is dimmed by dust and gas located between us and the cluster.

As they are leftovers from the early Universe, globular clusters are popular study objects for astronomers. M22 in particular has fascinating additional features: six planet-sized objects that are not orbiting a star have been detected in the cluster, it seems to host two black holes, and the cluster is one of only three ever found to host a planetary nebula - a short-lived gaseous shells ejected by massive stars at the ends of their lives.

From Wikipedia:

Messier 22 (also known as M22 or NGC 6656) is an elliptical globular cluster in the constellation Sagittarius, near the Galactic bulge region. It is one of the brightest globulars that is visible in the night sky. The brightest stars are 11th magnitude, with hundreds of stars bright enough to resolve with an 8" telescope.

M22 was one of the first globulars to be discovered, on August 26, 1665 by Abraham Ihle and it was included in Charles Messier's catalog of comet-like objects on June 5, 1764.

It was one of the first globular clusters to be carefully studied first by Harlow Shapley in 1930. He discovered roughly 70,000 stars and found it had a dense core. Then Halton Arp and William G. Melbourne continued studies in 1959. Because of the large color spread of its red giant branch (RGB) sequence, which is similar to that observed in Omega Centauri, it became the object of intense scrutiny starting in 1977 with James E. Hesser et al. M22 is located just south of the Ecliptic, and northwest of Lambda Sagittarii (Kaus Borealis), the northernmost star of the "Teapot" asterism.

M22 is one of the nearer globular clusters to Earth at a distance of about 10,600 light-years away. It spans 32' on the sky which translates to a spatial diameter of 99 ± 9 light-years. 32 variable stars have been recorded in M22. It is projected in front of the galactic bulge and is therefore useful for its microlensing effect on the background stars in the bulge.

Despite its relative proximity to us, this metal-poor cluster's light is limited by dust extinction, giving it an apparent magnitude of 5.5 making it the brightest globular cluster visible from mid-northern latitudes (e.g. Europe and most of North America). However, due to its southerly declination, M22 never rises high in the sky and so appears less impressive to northern hemisphere observers than other summer sky globulars such as M13 and M5.