April 28, 2013
Messier 70, a globular cluster in Sagittarius
Image Credit: ESA/Hubble & NASA
Messier 70 (also known as NGC 6681) is a 12.8 billion years old globular cluster of some 68 light-years across, located about 29,300 light-years away from Earth in the constellation Sagittarius (the Archer), while it is receding from us at roughly 200 kilometers per second. It is a close neighbor of the rather similar looking globular cluster Messier 69, with only 1,800 light-years separating the two objects.
With a mass of 179,000 times the mass of our Sun, the mutual hold of gravity binds together hundreds of thousands of stars in Messier 70. Although many globular clusters are located at the Milky Way’s edges, Messier 70 orbits close to the galaxy’s center, at about 7,200 light-years distance. So, it is quite remarkable that it has held together so well, given the strong gravitational pull of the Milky Way’s hub.
The core of Messier 70 is of extreme density, as it has undergone a core collapse somewhere in its history, similar to about a fifth of the more than 150 globular clusters in the Milky Way. In these clusters, even more stars squeeze into the object’s core than on average, such that the brightness of the cluster increases steadily towards its center. (Only two variable stars are known within this cluster.)
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 slow and congregate in orbits toward the center. This effect produces the denser, brighter centers characteristic of core-collapsed clusters.
This close-up image of the compact center of Messier 70, was obtained with the Wide Field Camera of the Advanced Camera for Surveys onboard the Hubble Space Telescope, using three different color filters.