X-Sender: kwright@pop100.gsfc.nasa.gov Date: Wed, 24 Sep 1997 15:11:54 -0500 To: 130all@listserv.gsfc.nasa.gov From: Kisha Wright Subject: Hubble Sees a Neutron Star Alone in Space Sender: owner-130all@listserv.gsfc.nasa.gov Reply-To: Kisha Wright >Date: Wed, 24 Sep 1997 14:19:42 -0400 (EDT) >From: NASANews@hq.nasa.gov >Subject: Hubble Sees a Neutron Star Alone in Space >Sender: owner-press-release@lists.hq.nasa.gov >To: undisclosed-recipients:; > >Don Savage >Headquarters, Washington, DC September 24, 1997 >(Phone: 202/358-1547) EMBARGOED UNTIL 2 P.M. EDT > >Tammy Jones >Goddard Space Flight Center, Greenbelt, MD >(Phone: 301/286-5566) > >Ray Villard >Space Telescope Science Institute, Baltimore, MD >(Phone: 410/338-4514) > >RELEASE: 97-213 > >HUBBLE SEES A NEUTRON STAR ALONE IN SPACE > > Astronomers using NASA's Hubble Space Telescope have taken >their first direct look, in visible light, at a lone neutron star. >This offers a unique opportunity to pinpoint its size and to >narrow theories about the composition and structure of this >bizarre class of gravitationally collapsed, burned-out stars. > > By successfully characterizing the properties of an >isolated neutron star, astrophysicists have an opportunity to >better understand the transitions matter undergoes when subjected >to the extraordinary pressures and temperature found in the >intense gravitational field of a neutron star. > > The Hubble results show the star is very hot, and can be no larger >than 16.8 miles (28 kilometers) across. These results prove that the object >must be a neutron star, for no other known type of object can be this >hot and small. > > "This puts the neutron star uncomfortably close to the >theoretical limit of how small a neutron star should be," says >Fred Walter of the State University of New York (SUNY) at Stony >Brook. "With this observation we can begin to rule out some of the >many models of the internal structure of neutron stars." The >observation results, made by Walter and Lynn Matthews (also of >SUNY), are reported in the Sept. 25 issue of Nature magazine. > > Neutron stars, which are created in some supernovae, are so >dense because the electrons and protons that form normal matter >have been squeezed into neutrons and other exotic subatomic >particles. Neutron star matter is the densest form of matter known >to exist. (Theoretically, a piece of neutron star surface weighing >as much as a fleet of battleships would be small enough to be held >in the palm of your hand.) > > The Hubble observations, combined with earlier data, >promise to help astronomers refine the mathematical description -- >called the equation of state -- of the complex transformations >matter undergoes at extraordinary densities not found on Earth. >Equations of state are well understood for "everyday" matter such >as water, which can transition between gaseous, liquid and solid >states. But the behavior of matter under extreme temperature and >pressure found on a neutron star is not well understood. > > Several hundred million neutron stars should exist in our >galaxy. However, all neutron stars now known have either been >found orbiting other stars in X-ray binary systems or emitting >machine-gun blasts of radio energy as pulsars (a class of neutron >star). The neutron star seen by Hubble is not a member of a >binary system, and is not known to pulse at X-ray or radio >wavelengths (it has not been detected as a radio source). Pulsars >are young neutron stars born with strong magnetic fields; non- >pulsing neutron stars may be old, dead pulsars, with ages of more >than a million years, or they may never have been pulsars. Only a >few lone neutron star candidates have been pinpointed through X- >ray observations, and this is the first optical counterpart to be >identified. > > The first clue that there was a neutron star at this >location came in 1992, when the ROSAT (the Roentgen Satellite) >found a bright X-ray source without any optical counterpart in >optical sky surveys. It drew the attention of astronomers because >objects this hot and bright, without counterparts at other >wavelengths, are extremely rare. > > Hubble's Wide Field Planetary Camera 2 was used in October >1996 to undertake a sensitive search for the optical object, and >found a stellar pinpoint of light within only 2 arc seconds >(1/900th the diameter of the Moon) of the X-ray position. > > Astronomers haven't directly measured the neutron star's >distance but fortunately the neutron star lies in front of a >molecular cloud known to be about 400 light-years away in the >southern constellation Coronae Australis. > > Using the distance to the cloud as an upper limit, the >astronomers calculated a diameter by next comparing the neutron >star's brightness and color as measured by Hubble, along with X- >ray brightness from the ROSAT and EUVE (Extreme Ultraviolet >Explorer) satellites. > > The object is brightest at X-ray wavelengths. In the two >Hubble images, the object is brighter at ultraviolet wavelengths >than at visible wavelengths. They concluded they are directly >seeing an ultracompact surface sizzling at about 1.2 million >degrees Fahrenheit. > > To be so hot, yet so dim (below 25th magnitude in visual >light) and relatively close to Earth, the object must be extremely >small -- below the size of a white dwarf, a more common stellar >cinder. A hot white dwarf at this magnitude would lie 150,000 >light-years away (outside our galaxy), and have 1/70,000 as much >X-ray emission. > > The 16.8-mile diameter estimate comes from assuming the >neutron star is at the farthest it can be, just in front of the >obscuring "wall" of the molecular cloud. If instead the neutron >star is significantly closer to us, say midway to the molecular >cloud, it would be smaller still, and present an even bigger >challenge to the theories of the equation of state of nuclear matter. > > Although neutron stars in binary systems allow astronomers >to measure their mass, which turn out to be consistent with >theory, it's much harder for astronomers to estimate the diameter >of the neutron stars. Since the neutron stars "feed" on their >companion stars in these systems, the light does not come >exclusively from the surface but from jets, disks and other >phenomenon that occur around the star. This can lead to >inaccurate size estimates. > > Over the next year, planned observation with the Hubble >will be used in an attempt to determine exactly how far away and >how large the star is. > > - end - > > A photo and caption are available via the World Wide Web at URLs: > >http://oposite.stsci.edu/pubinfo/PR/97/32.html and via links in >http://oposite.stsci.edu/pubinfo/Latest.html or >http://oposite.stsci.edu/pubinfo/Pictures.html. Images are >available via the World Wide Web at >http://oposite.stsci.edu/pubinfo/gif/nscra.gif (GIF), >http://oposite.stsci.edu/pubinfo/jpeg/nscra.jpg (JPEG). > > Image files also may be accessed via anonymous ftp from >oposite.stsci.edu in /pubinfo: gif/nscra.gif (GIF) and >jpeg/nscra.jpg (JPEG). Higher resolution digital versions (300 >dpi JPEG) of the release photograph are available in >/pubinfo/hrtemp: 97-32.jpg (color) and 97-32bw.jpg (black & >white). Full resolution TIFF image is available in >/pubinfo/tiff/1997/32.tif. >