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 d