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.

A photo and caption are available via the World Wide Web at URLs:

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