Hubble sees Messier 70: Tight and bright

Apr 13, 2012
Credit: ESA/Hubble & NASA

(Phys.org) -- In this image, the NASA/ESA Hubble Space Telescope has captured the brilliance of the compact center of Messier 70, a globular cluster. 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.

Messier 70 offers a special case because it has undergone what is known as a core collapse. 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.

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 huddling effect produces the denser, brighter centers characteristic of core-collapsed clusters. About a fifth of the more than 150 globular clusters in the have undergone a core collapse.

Although many call the galaxy's edges home, Messier 70 orbits close to the Milky Way's center, around 30 000 light-years away from the Solar System. It is remarkable that Messier 70 has held together so well, given the strong 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). French astronomer Charles Messier documented the object in 1780 as the seventieth entry in his famous astronomical catalogue.

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

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Ophelia
5 / 5 (1) Apr 13, 2012
Can someone explain why this is "remarkable"?

"It is remarkable that Messier 70 has held together so well, given the strong gravitational pull of the Milky Way's hub."

Assuming (always dangerous) the "hub" is the black hole at the center of the Milky Way, its gravitational pull is determined by its mass and its distance from the cluster, right? IIRC, the black hole mass is several million solar masses. This cluster has "hundreds of thousands of stars in a small region" and, perhaps, a total mass approaching that of the black hole as a result. But its mass is still concentrated within a volume that "is only about 68 light-years in diameter". Since gravity falls off at the reciprocal of the distance squared, wouldn't the black hole have to be nearly inside the cluster to affect the more than the self-gravity of the cluster itself?

In other words, wouldn't it be more remarkable if the black hole was pulling the cluster to pieces (apart from picking off an outlier, that is)?
Pressure2
not rated yet Apr 13, 2012
I find these clusters remarkable because of their uniformity. They mentioned that it rotates. What I would like to know is how fast it rotates and if it rotates in the same general direction as our galaxy?