Spaceflight Now: Breaking News

Hubble sees lone neutron star streaking across galaxy
NASA NEWS RELEASE
Posted: November 10, 2000

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.

Hubble
his photograph is the sum of three Hubble Space Telescope images. All stars line up in this composite picture, except the neutron star, which moves across the image in a direction 10 degrees south of east. The three images of the neutron star are labeled by date. Credit: NASA and F.M. Walter (State University of New York at Stony Brook)
 
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 were presented this week 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.