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Unusual Star Clusters Fill the Gap

By Stephen Luntz

A new type of star cluster has filled a gap in charts of cluster mass and size.

The Milky Way galaxy is surrounded by globular clusters, the origins of which are much debated (AS, Jan/Feb 2011, p.12). Other galaxies are surrounded by similar objects, but sometimes with different characteristics. Charts mapping mass against size for these extra-galactic clusters have a void – an absence of clusters of large area and middling mass.

Now a study of three elliptical galaxies 50 million light years away has revealed a number of objects that appear to fill this space. “We now know that star clusters have a rather continuous range of size and mass without any gaps in their properties,” says Prof Duncan Forbes of Swinburne University’s Centre for Astrophysics and Supercomputing.

Like globular clusters, the stars in these new objects appear to be very old, but otherwise little is known about them and what makes them different from those around other galaxies. However, Forbes says: “It seems these extend the class of large-size, lower-mass objects towards higher masses, rather than being from the lower end of the high-mass class.

Colour measurements of star clusters produce a pattern where low-mass objects are generally red, while those of higher mass are blue. However, Forbes says it is not safe to leap to conclusions about what this reveals. “Blue can indicate young stars, but it can also indicate stars that are metal-poor while red stars are richer in metals.” At such distances, identifying the differences can be a challenge.

While it is hoped that the discovery will provide insights into star cluster formation, for the moment Forbes says: “There are two different proposed formation mechanisms, with smaller objects merging, or with these being the core of a galaxy that has been stripped.” The new discovery does not settle this question.