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Mercury, May/June 2003 Table of Contents

Courtesy of NASA, Nolan Walborn and Jesus Maíz-Apellániz (STScI), and Rodolfo Barbá (La Plata Observatory).

by Deidre A. Hunter, Bruce G. Elmegreen, and Philip Massey

Astronomers struggle to understand why giant star clusters form in the tiniest of galaxies

Our Sun is a solitary star living in the countryside of local space. It has no close companions, and any siblings from birth have long since moved away. But many other stars live in the equivalent of downtown New York City. These are the inhabitants of compact clusters, stars bound together by gravity and all about the same age.

The most massive star clusters in the Milky Way Galaxy are globular clusters. Each globular cluster contains several hundred thousand stars packed into a sphere no bigger than 150 light-years across. All of the Milky Way's globular clusters are more than 10 billion years old. They formed when our Galaxy formed, or soon after, and none have formed since.

Star clusters forming in our galaxy today are much smaller, with at most 10,000 members. But several nearby dwarf galaxies have somehow produced dense, massive star clusters in the recent past. These clusters are so big that if they were at the distance of the Orion Nebula (1,500 light-years), they would cover three times as much sky as the full Moon and would be as bright as Venus. These clusters are probably young versions of globular clusters, but they are commonly called super star clusters. Seven dwarf galaxies within 16 million light-years have formed super star clusters, with one of these galaxies, NGC 1569, possessing two young clusters. Another three possible super star clusters — still shrouded in their natal cocoons — exist in other nearby dwarfs.

These dwarf galaxies are tiny compared with the Milky Way. NGC 1569, for example, contains only 1% the mass of our Milky Way. As a result, super star clusters constitute a substantial portion of the stars and light of such small galaxies. The two clusters in NGC 1569 contribute a whopping 6% of the total light of that galaxy.

Super star clusters have a lot to tell us about star formation. Some theories predicted that stars forming in dense environments would have few low-mass stars and many high-mass stars resulting from collisions and mergers. But the relative abundance of stars with various masses is about the same in dense clusters as in lower density regions, indicating that the star formation process ends before collisions occur. In addition, the surprising abundance of super star clusters in dwarf galaxies, in comparison to their relative absence in large galaxies, should tell us about the special conditions required for their formation. We might even learn what caused them to form so readily in the early universe.