January 20, 2013
NGC 6642, a globular cluster in Sagittarius
Image Credit: ESA/Hubble & NASA
NGC 6642 is a very old, highly evolved globular star cluster with a mass of roughly 110,000 solar masses, located 26,400 light-years away from Earth in the constellation of Sagittarius, and just 5.500 light-years away from the galactic center. It is moving toward us at approximately 57.2 kilometers per second.
The compact nature of globular clusters is a double-edged sword. On the one hand, having so many stars of a similar age in one bundle gives astronomers insights into the chemical makeup of our galaxy in its early history. But, at the same time, the high density of stars in the cores of globulars also makes it difficult for astronomers to resolve individual stars.
The core of NGC 6642, shown here, is particularly dense, making this cluster a difficult observational target for most telescopes. Furthermore, it lies very close to the center of the Milky Way galaxy, which means that images inadvertently capture many stars that don’t belong to the cluster — these “field stars” just get in the way.
However, in this image we can see the cluster’s dense core in unprecedented detail, and astronomers have already made many interesting finds about NGC 6642. For example, many blue stragglers (stars which seemingly lag behind in their rate of aging) have been spotted in this cluster, and it is known to be lacking in low-mass stars.
NGC 6642 is a good candidate as one of the few genuine metal-poor clusters, what means that the cluster has almost no other elements than hydrogen or helium, and is very primitive. It might be one of the oldest fossils in our galaxy.
This image was created from visible and infrared images with the Wide Field Channel of the Advanced Camera for Surveys on the Hubble Space Telescope.