<!--intro-->It's as big as Manhattan Island, is 10 trillion times denser than steel and is hurtling our way at speeds over 100 times faster than a supersonic jet. An alien spaceship? No, it's a runaway neutron star, called RX J185635-3754, forged in a stellar explosion that would have been visible to our distant ancestors in 1 million B.C.<!--/intro--> <a HREF="http://oposite.stsci.edu/pubinfo/PR/2000/35/content/0035w.jpg"><img SRC="http://oposite.stsci.edu/pubinfo/PR/2000/35/content/0035x.jpg" border=0 hspace=7 align=left></a>Precise observations made with NASA's Hubble telescope confirm that the interstellar interloper turns out to be the closest neutron star ever seen. Now located 200 light-years away in the southern constellation Corona Australis, it will swing by Earth at a safe distance of 170 light-years in about 300,000 years. A light-year is the distance traveled by light in a full year (about 6 trillion miles). Because it is the closest neutron star ever seen and its distance has been well established by Hubble, astronomers can compare stellar theories against a variety of its physical properties such as size, inherent brightness, and true age. Since the object has no companion star that would affect its appearance, this discovery will allow future astronomers to more easily confirm stellar theories. The results are being presented today at the 2000 meeting of the American Astronomical Society's High Energy Astrophysics Division (HEAD) in Honolulu, HI. "The scientific importance of this object lies in the fact that the neutron star is isolated," says Frederick M. Walter of the State University of New York (SUNY), in Stony Brook, NY. "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. 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." The neutron star's wayward trajectory was caught in three Hubble snapshots taken in 1996 and 1999. The three Hubble images show that the star moves across the sky with a characteristic apparent "wobble" (a reflection of the Earth's own orbital motion, an effect called parallax), which is expected of an object located about 200 light-years away. In addition, the observations reveal that the neutron star is streaking across the sky from west to east at a rate of 1/3 of an arc second per year. (An arc second is a unit of angular measure. There are 3,600 arc seconds in a degree and 360 degrees in a full circle.) In 5,400 years, RX J185635-3754 travels a distance equal to the diameter of the Moon. Although this apparent motion may seem slow, it is actually one of the fastest moving stars in the sky. The fastest, Barnard's star, moves 10 arc seconds each year). The apparent motion, combined with the distance, means that the neutron star is moving at a speed of about 240,000 miles per hour (389,000 kilometers per hour). This neutron star may be approaching from a grouping of young stars in the constellation Scorpius. About 1 million years ago, 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, which is also zooming away from the region. Because 1 million years ago the neutron star and Zeta Ophiuchus were in about the same location in space, the neutron star may be the remnant of the original binary companion of Zeta Ophiuchus, the star which exploded. 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, Walter and S.J. Wolk (Stony Brook) and R. Neuhaeuser (Max-Plack-Institut fuer Extraterrestrische Physik) surmised that it was likely to be a neutron star, a hot, dense stellar corpse with a six-mile radius. 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, as expected from the bright X-ray emission. The temperature is about 1 million degrees Fahrenheit (600,000 degrees Kelvin). In September 2000, 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.