May/June 2003 Table of Contents
of NASA, Nolan Walborn and Jesus Maíz-Apellániz
(STScI), and Rodolfo Barbá (La Plata Observatory).
Deidre A. Hunter, Bruce G. Elmegreen, and Philip Massey
struggle to understand why giant star clusters form in the tiniest
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.
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.
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.
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.
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.