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October 22, 2011

There’s a lively debate over what lies at the heart of the star cluster M15. One side says it’s a medium-sized black hole, the other says it’s a clump of dead stars. So far, there’s no winner.

M15 is a globular cluster — a tightly bound collection of old stars. It’s high in the southern sky at nightfall. Through binoculars, it looks like a fuzzy star.

The stars in M15’s core are packed more tightly than the cores of most other globulars. Half of its stars — perhaps a quarter-million or more — fall into a region that’s only about 20 light-years across. By comparison, a similar volume of space in our region of the galaxy contains only a handful of stars.

The collapsed core suggests that something heavy but dark lurks there, but the nature of that dark mass is unclear.

Some astronomers have calculated that stars in the core are orbiting so quickly that they must be going around a black hole that’s more than a thousand times as massive as the Sun. That would make it one of only a few possible “intermediate”-mass black holes discovered to date — all of them in the centers of star clusters.

But other researchers say that a tight clump of dead stars — white dwarfs and neutron stars — would provide the same result. They don’t rule out the possibility of a black hole, they just don’t see any evidence for it.

It may take a better view of stars even closer to the cluster’s center to provide an answer. Until then, the debate continues.


Script by Damond Benningfield, Copyright 2011


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