English: There are about 150 globular star clusters that roam the Milky Way’s halo, which encases the galaxy like a giant, diffuse eggshell. But among them, Caldwell 25 is unusual. Unlike most globular clusters, which are huge, spherical clouds of old stars believed to be among the oldest in the galaxy, Caldwell 25 may not be from our galaxy at all. Located about 300,000 light-years from Earth, this cluster is sometimes called “the Intergalactic Wanderer” because it is so far away — even farther than the Milky Way’s largest satellite galaxies, the Small and Large Magellanic clouds, which are about 160,000 light-years away. Scientists have surmised that the cluster may be part of the remains of a small galaxy that was captured and disrupted by the Milky Way.

Caldwell 25 seems to be fairly similar to other large globular clusters, though its extreme distance makes it difficult to study and compare its properties with other globular clusters in the Milky Way. Usually all the stars in a globular cluster are very similar to one another in composition, since they typically form in the same general area from the same materials. However, Hubble’s observations of Caldwell 25 hint that it has a different story.

This interesting cluster hosts two distinct sets of red giant stars that have different amounts of certain elements. Red giants are a type of star that results when a medium-mass star expends the fuel that keeps it burning during most of its lifetime. One of Caldwell 25’s red-giant subsets, found mainly in the central region of the cluster, has a surprisingly high amount of helium. The differences between the two groups has led scientists to question whether the two stellar populations formed together or started out as individual groups that later merged.

Caldwell 25 was discovered by astronomer William Herschel on New Year’s Eve of 1788 and is also cataloged as NGC 2419. The cluster is intrinsically bright, but it appears faint because it is so far away. It has a magnitude of 10.4 and is located in the constellation Lynx. In a typical amateur telescope, Caldwell 25 appears as a soft, hazy, circular glow. Because of the cluster’s distance, individual stars are not readily resolved. Observing under clear, winter skies in the Northern Hemisphere will provide the best opportunity to spot Caldwell 25. Southern Hemisphere observers should search for it in the northern sky during the summer.

Credit: ESA/Hubble & NASA, S. Larsen et al.

For more information about Hubble’s observations of Caldwell 25, see:

www.spacetelescope.org/images/potw1908a/

For Hubble's Caldwell catalog site and information on how to find these objects in the night sky, visit:

www.nasa.gov/content/goddard/hubble-s-caldwell-catalog