November 9, 2000
RELEASE: 00-176

HUBBLE SEES LONE NEUTRON STAR STREAKING ACROSS GALAXY

     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 
diameter.

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, NY.

"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 Ophiuchus.

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 
emission. 

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 space.

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. 

Image and artwork associated are available at: 

         http://oposite.stsci.edu/pubinfo/pr/2000/35 

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