Several hundred million of them may be found in our
galaxy, but the world’s most powerful telescope has
captured the one thought to be closest to Earth. NASA’s
Hubble Space Telescope has caught up with a runaway
neutron star believed to be 200 light years away.

The object known as RX J185635-3754 is expected to
swing by the planet at a safe distance in about 300,000
years. A neutron star is the remnants left behind after a
supernova explosion, as the material at the core
collapses into a dense mass of neutrons. The star has the
mass of the sun packed into an area about 12 miles in

Precise observations made with the Hubble telescope
confirm the isolated interstellar traveler is now located
in the southern constellation Corona Australis. Since the
object has no companion star that would affect its
appearance, this discovery will allow future astronomers
to more easily confirm stellar theories against a variety
of its physical properties such as size, inherent
brightness and true age.

"Because this is the closest and brightest of the
few known isolated neutron stars, it is the easiest to
study and is an excellent test bed for nuclear
astrophysical theories, " said Frederick M. Walter of
the State University of New York (SUNY), in Stony Brook,

"The scientific importance of this object lies in
the fact that the neutron star is isolated," added
Walter. "It appears to be hot, not because it is
accreting hydrogen gas as it moves through space, but
because it is still young and cooling off. Since we know
its approximate age, we can test how fast neutron stars
cool off."

The neutron star’s wayward trajectory was caught in
three Hubble snapshots taken in 1996 and 1999. The
results of the observations are being presented today by
the American Astronomical Society’s High Energy
Astrophysics Division (HEAD) in Honolulu, HI.

The images also show the star moves across the sky
with an apparent wobble, caused by a reflection of the
Earth’s own orbital motion, called parallax.

In addition, the images reveal that the neutron star
is streaking across the sky from west to east at a rate
equal to the diameter of the Moon every 5,400 years.
Although this apparent motion may seem slow, it is
actually one of the fastest-moving stars in the sky. The
apparent motion, combined with the distance, means the
energetic ember is moving at a speed of about 240,000
miles per hour.

This neutron star may have formed about 1 million
years ago when a massive star in a binary star system
exploded as a supernova, releasing its companion star, an
ultra-hot, blue star now known as Zeta Ophiuchus. Because
the neutron star and Zeta Ophiuchus were in about the
same location in space, RX J185635-3754 may be the
remnant of the original binary companion of Zeta

The runaway neutron star was first reported in 1992,
when astronomers detected a very bright source of X-ray
emission with the Roentgen Satellite (ROSAT). Because it
was not seen in optical light and appeared to be within
500 light-years of the Earth, researchers surmised it was
likely to be a neutron star.

Four years later, Stony Brook astronomers Walter and
L.D. Matthews reported the optical identification of the
star using the Hubble telescope. The object is very faint
(26th magnitude or about 20 billion times fainter than
the bright star Vega), and has a blue color. The blue
color indicates that the object is hot, about one million
degrees Fahrenheit, as expected from the bright X-ray

In September, images taken with the European Southern
Observatory’s Very Large Telescope showed a small,
cone-shaped "bowshock" in front of the neutron
star, created as the star plowed through interstellar

The Hubble results have been accepted for publication
in the Astrophysical Journal.

The Space Telescope Science Institute is operated by
the Association of Universities for Research in
Astronomy, Inc., for NASA, under contract with NASA’s
Goddard Space Flight Center, Greenbelt, MD. The Hubble
Space Telescope is a project of international cooperation
between NASA and the European Space Agency.

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