Black box. The bright x-ray source in the lower left of this Chandra image of the star cluster Westerlund 1 is the newly discovered neutron star.

The Black Hole That Wasn't

The explosion and collapse of a star 40 times as massive as the sun should easily create a black hole. So astronomers are scratching their heads this week as new data from NASA's Chandra X-ray Observatory indicate that such an event formed only a neutron star. "It's a surprising find," says Gertjan Savonije of the University of Amsterdam, the Netherlands.

When a massive star goes supernova, its core collapses into an extremely compact object: either a dense neutron star or a black hole. Theories of stellar evolution predict that stars weighing less than about 25 times the mass of the sun end up as neutron stars, while heftier stars are destined to become black holes. However, there's "a lot of guesswork involved," says Savonije.

Take the case of the newly discovered neutron star in a compact star cluster known as Westerlund 1, 17,000 light years away. All of the stars in this cluster are believed to have been born together about 4 million years ago. But the more massive stars evolve faster than lighter ones do, and so they die sooner. That means the progenitor of the neutron star must have been more massive than the heaviest stars still around in the cluster, which weigh up to 35 solar masses, says astronomer Michael Muno of the University of California, Los Angeles, whose team first identified the object in Chandra observations taken in May and June this year.

So why didn't the death of the massive star produce a black hole? Frank Verbunt of Utrecht University, the Netherlands, believes the riddle may be solved by assuming that the progenitor star was part of a binary system. If that was the case, the massive star would have dumped huge amounts of gas onto its companion, becoming light enough to collapse into a neutron star rather than a black hole.

Muno agrees that this is a possible explanation. Infrared observations of the neutron star, however, reveal no current binary companions that are more massive than the sun. "It's still important to consider other reasons why some extremely massive stars won't collapse into black holes," he says. The team's findings will be published in an upcoming issue of Astrophysical Journal Letters.

Related sites
Abstract of paper, with link to full text
Chandra X-ray Observatory
Muno's home page