Neutron Star Discovered Where a Black Hole Was Expected (Forwarded)

Steve Roy
Marshall Space Flight Center, Huntsville, Ala. November 02, 2005
(Phone: 256/544-6535)

Megan Watzke
Chandra X-ray Center, Cambridge, Mass.
(Phone: 617/496-7998)

RELEASE: 05-08

Neutron Star Discovered Where a Black Hole Was Expected

A very massive star collapsed to form a neutron star and not a black
hole as expected, 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.

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 progenitor 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 UCLA postdoctoral Hubble Fellow and lead author of a
paper to be published in 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% 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 progenitor 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 from if its evolutionary stage is known.
Neutron stars and black holes are the end stages in the evolution of a
star, so their progenitors must have been among the most massive stars
in the cluster.

Muno and colleagues discovered a pulsing neutron star in a cluster of
stars known as Westerlund 1. This cluster contains a hundred thousand or
more stars in a region only 30 light years across, which suggests that
all the stars were born in a single episode of star formation. Based on
optical properties such as brightness and color some of the normal stars
in the cluster are known to have masses of about 40 suns. Since the
progenitor of the neutron star has already exploded as a supernova, its
mass must have been more than 40 solar masses.

Introductory astronomy courses sometimes teach that stars with more than
25 solar masses become black holes -- a concept that until recently had
no observational evidence to test it. However, some theories allow such
massive stars to avoid becoming black holes. For example, theoretical
calculations by Alexander Heger of the University of Chicago and
colleagues indicate that extremely massive stars blow off mass so
effectively during their lives that they leave neutron stars when they
go supernovae. Assuming that the neutron star in Westerlund 1 is one of
these, it raises the question of where the black holes observed in the
Milky Way and other galaxies come from.

Other factors, such as the chemical composition of the star, how rapidly
it is rotating, or the strength of its magnetic field might dictate
whether a massive star leaves behind a neutron star or a black hole. The
theory for stars of normal chemical composition leaves a small window of
initial masses -- between about 25 and somewhat less than 40 solar
masses -- for the formation of black holes from the evolution of single
massive stars. The identification of additional neutron stars or the
discovery of black holes in young star clusters should further constrain
the masses and properties of neutron star and black hole progenitors.

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 the agency's Science Mission
Directorate. The Smithsonian Astrophysical Observatory controls science
and flight operations from the Chandra X-ray Center in Cambridge, Mass.

Additional information and images are available at:

http://chandra.harvard.edu/photo/2005/wd1/
and

http://www.nasa.gov/centers/marshall...05/05-171.html