In the core collapse scenario, the stars swarmed like insects towards the core of the cluster for a few million years. The laws of physics finally stopped the collapsing once the stars rushed too close to each other. Gravity pulls the stars together, but if they come too close they will snap apart, shooting far out into space. Since then, the stars would have remained in a situation where they are both attracted and repelled by each other.

Alternatively, the stars might be spiraling inward to a black hole. In 1975, John Bahcall and Jeremiah Ostriker of Princeton University proposed the black hole theory. Such a black hole would have formed early in the cluster's history, and the stars would have remained orbiting around its massive gravitational force ever since. However, says Puragra Guhathakurta of the University of California at Santa Cruz, "... a black hole is not necessarily the best explanation for what we see."

how crowded is M15? Brian Yanny of the Fermi National Accelerator Laboratory says most theorists lean towards core collapse. He explains the problem with the black hole theory lies in cluster formation. Since all of the stars in a globular cluster are the same age, its a mystery how one would have all of a sudden become older than the others. The star would have died and collapsed into a black hole, yet would be surrounded by stars supposedly at the same point in stellar evolution.

Researchers can use computers to create models of the two theories. It turns out that core collapse has been observed in computer simulations, but modeling a black hole has been more difficult, says Yanny. The drawback to computer modeling is that exact simulations of a cluster core would require faster computers than those that exist today. For now, Yanny says, people creating computer models have to simplify some parts of the equations with regard to the interactions of stars and gas in the earlier stages of cluster evolution.

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