Special Report from AAS
Thursday, January 13

Watching the Birth of Globular Clusters

Above: Two views of the dwarf galaxy NGC 5253. The visible-light image at left, taken by the Hubble Space Telescope, shows a collection of young super star clusters. The radio and infrared image at right (at the same scale) shows hot gas heated by the clusters, the densest and brightest one in particular. Check back in a few billion years and this group will probably have become a run-of-the-mill globular star cluster. Courtesy J. Turner, D. Calzetti, Space Telescope Science Institute, and NRAO. Click on image for larger view.

Globular star clusters are among the most familiar and beautiful deep-sky objects for backyard telescope users, and they are among the most enigmatic for cosmologists. Containing up to a million solar masses, they are all extremely old, dating to shortly after the Big Bang. At least in our galaxy.

No one knows why the Milky Way hasn't produced any new globulars in such a long time. Apparently, however, some other galaxies are still at it.

Buried in the dwarf galaxy NGC 5253 is an extremely dense hydrogen cloud blazing in the light of several thousand of the hottest, brightest, and most massive stars known -- all packed into a region no larger than 3 by 6 light-years. Including the lesser stars that must be present, the total mass in this small region should be about a million Suns. This teeming swarm is less than a million years old, Jean Turner (UCLA) and Sara Beck (University of Tel Aviv) told reporters at the AAS meeting. It is exactly the sort of massive blue cluster that ought to evolve into a garden variety globular given a few billion years.

Another globular in the making was described by Kelsey Johnson (University of Colorado, Boulder) and Chip Kobulnicky (University of Wisconsin, Madison). The dwarf galaxy Henize 2-10 contains a tiny, brilliant clump estimated to contain 500 to 1,000 hot, luminous O-type stars in a region less than 10 to 20 light-years across. This group too must be extremely young, with an age of a half million years or less. Compared to familiar globulars, said Kobulnicky, "This is like seeing nothing but old people, and then catching a human in the first day of life."