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Name: M 70, NGC 6681
Description: Globular Cluster
Position (J2000): RA 18h 43m 12.62s Dec -32° 17' 28.38"
Constellation: Sagittarius
Distance: 30,000 light years
Visual magnitude: 9.1
Angular size: 8.0 arcmin
Field of view: 3.37 x 3.43 arcminutes
Orientation: North is 83.6° left of vertical
Image Credit: ESA/Hubble & NASA
Release date: April 9, 2012
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In this image, Hubble has captured the brilliant, compact center of the globular cluster M70 in both visible and infrared light. Quarters are always tight in globular clusters, where the mutual hold of gravity binds together hundreds of thousands of stars in a small region of space. Having this many shining stars piled on top of one another from our perspective makes globular clusters a popular target for amateur skywatchers and scientists alike. M70 is particularly fascinating because it has undergone what is known as a core collapse. This means that even more stars squeeze into the object's core than in an average globular cluster, such that the brightness of the cluster increases steadily towards its center.

The legions of stars in a globular cluster orbit about a shared center of gravity. Some stars maintain relatively circular orbits, while others loop out into the cluster's fringes. As the stars interact with each other over time, lighter stars tend to pick up speed and migrate out toward the cluster's edges, while the heavier stars are slower and congregate toward the center. This huddling effect produces the characteristically dense center of core-collapsed clusters. About a fifth of the roughly 150 globular clusters in the Milky Way have undergone a core collapse.

Although many globular clusters call the galaxy's edges home, Messier 70 orbits close to the Milky Way's center. It is remarkable that Messier 70 has held together so well, given the strong gravitational pull of the Milky Way's hub.

Messier 70 is only about 68 light-years in diameter and can be seen, albeit very faintly, with binoculars in dark skies in the constellation of Sagittarius (The Archer). M70 was discovered by French astronomer Charles Messier on August 31, 1780. It is located 30,000 light-years away in the constellation Sagittarius. With an apparent magnitude of 9.1, the cluster can be observed through a small telescope best in the month of August.

This picture was obtained with the Wide Field Camera of Hubble's Advanced Camera for Surveys. The field of view is around 3.3 by 3.3 arcminutes.