A very massive star collapsed to form a neutron star and not a black hole as anticipated, according to new results from NASA's Chandra X-ray Observatory. This discovery shows that nature has a harder time making black holes than previously thought.

The optical image (left) of Westerlund 1 shows a dense cluster of young stars, several with masses of about 40 suns. Some astronomers speculated that repeated collisions between such massive stars in the cluster might have led to formation of an intermediate-mass black hole, more massive than 100 suns. A search of the cluster with Chandra (right) found no evidence for this type of black hole. Instead they found a neutron star (CXO J164710.2-455216), a discovery which may severely limit the range of stellar masses that lead to the formation of stellar black holes. Credit: NASA/CXC/UCLA/M.Muno et al. Download larger image version here

Scientists found this neutron star - a dense whirling ball of neutrons about 12 miles in diameter - in an extremely young star cluster. Astronomers were able to use well-determined properties of other stars in the cluster to deduce that the parent star of this neutron star was at least 40 times the mass of the sun.

"Our discovery shows that some of the most massive stars do not collapse to form black holes as predicted, but instead form neutron stars,” said Michael Muno, a University of California, Los Angeles, postdoctoral Hubble fellow. He is lead author of a paper to be published in an upcoming edition of The Astrophysical Journal Letters.

When very massive stars make neutron stars and not black holes, they will have a greater influence on the composition of future generations of stars. When the star collapses to form the neutron star, more than 95 percent of its mass, much of which is metal-rich material from its core, is returned to the space around it.

"This means that enormous amounts of heavy elements are put back into circulation and can form other stars and planets," said J. Simon Clark of the Open University in the United Kingdom.

Astronomers do not completely understand how massive a star must be to form a black hole rather than a neutron star. The most reliable method for estimating the mass of the parent star is to show that the neutron star or black hole is a member of a cluster of stars, all of which are close to the same age.

Because more massive stars evolve faster than less massive ones, the mass of a star can be estimated if its evolutionary stage is known. Neutron stars and black holes are the end stages in the evolution of a star, so their parent stars must have been among the most massive stars in the cluster.

The work described by Muno was based on two Chandra observations on May 22 and June 18, 2005. NASA's Marshall Space Flight Center, Huntsville, Ala., manages the Chandra program for NASA's Science Mission Directorate. The Smithsonian Astrophysical Observatory controls science and flight operations from the Chandra X-ray Center in Cambridge, Mass.