Search For Crushed Lonely Stars Continues
Contact:
Christopher Wanjek
wanjek@gsfc.nasa.gov
301-286-4453September 6, 1999
Greenbelt, Md. -- Of the estimated 100 million isolated neutron stars in the Milky Way galaxy, astronomers have found only about 700, detected by their pulsed radio emissions.
However, a large fraction of them can't be detected this way. Recent X-ray imaging of the sky is an efficient means to find more of these stars, especially the relatively close ones. The identification of these tiny embers greatly strengthens the theory of stellar evolution and death.
Astronomers have identified the fifth of this kind. The difficulty and recent success in locating these dead, collapsed stars no larger than 10 miles across in the dark, empty regions of a galaxy that stretches for 500,000 trillion miles is the subject of a presentation today at the X-ray Astronomy 1999 meeting in Bologna, Italy. The meeting will be chaired by Dr. Nicholas White, head of NASA Goddard Space Flight Center's X-ray Astrophysics Branch in the Laboratory for High Energy Astrophysics.
Dr. Christian Motch of the University of Strasbourg and CNRS in France detected the latest isolated neutron star candidate with ROSAT, a German/UK/US X-ray satellite launched in 1990.
"In light visible to the human eye, these small stars are extremely faint and only the closest or hottest ones may be detected by the largest ground-based telescopes or by the Hubble Space Telescope," said Motch. "In many cases, the objects are so dim that the identification of the X-ray source with an isolated neutron star can only be claimed after all other possibilities have been safely ruled out by long and careful observations."
A neutron star is the stellar endpoint of a star once several times larger than the Sun. After the star exhausts its nuclear fuel, it blows off its outer shell of gas in a fiery display known as a supernova. The remaining dense core, still as massive as the Sun, collapses further into a compact, city-sized sphere.
This sphere, called a neutron star because the core's tightly squeezed protons and electrons essentially form a single neutron, is characterized by a strong gravitational field and rapid spin. A teaspoon of neutron star matter would weigh three billion tons on Earth.
Scientists find some neutron stars within the glow their supernova remnant. Other neutron stars are part of a binary star system, with the gas of one star falling onto the nearby neutron star due to the neutron star's gravitational field, thus lighting the path to the tiny sphere.
Isolated neutron stars, however, are in the middle of nowhere, without a supernova remnant or companion star to reveal their presence. The isolated neutron star was either blown away from its companion star in the supernova explosion or perhaps never had a neighbor. Its supernova remnant has since faded or dispersed. Thus, the theory is that these neutron stars are millions of years old embers from stellar explosions.
Dr. Motch said that isolated neutron stars can radiate energy by one of two ways. Despite their age, they could still glow X-ray hot from the initial explosion. Or, the star's gravitational pull could attract nearby gas from the interstellar medium, the rush of which onto the surface of the neutron star would glow in X-rays. Dr. Motch's recent discovery, a source called RX J1605+3249, is likely glowing in X-ray by itself from thermal emission and not by accreting interstellar gas.
Regardless of the radiation emission process, isolated neutron stars can be detected by X-ray telescopes. Hot X-ray stars do fade to optical light over time, but the emission is too faint to detect from Earth. Dr. Motch said that NASA's Chandra X-ray Observatory, launched in July, and ESA's XMM X-ray satellite, scheduled for a December launch, will greatly resolve the diffuse galactic X-ray radiation into myriad point sources and perhaps locate thousands of other isolated neutron stars.
Indeed, Chandra's very first X-ray image -- to be featured at the X-ray Astronomy 1999 meeting along with other first Chandra events - shows what is likely a tiny neutron star in the middle of the Cassiopeia-A supernova remnant.
The X-ray Astronomy 1999 meeting in Bologna, held from Sept. 6-10, is sponsored by NASA Goddard Space Flight Center, the University of Bologna, and Consiglio Nazionale delle Ricerche Istituto di Tecnologie e Studio delle Radiazioni Extraterrestri. The meeting features the latest discoveries on black holes, neutron stars, pulsars, Active Galactic Nuclei, X-ray background radiation, and X-ray astronomy theory.
EDITOR'S NOTE: The Chandra stock photo gallery is available on the internet at:
http://www1.msfc.nasa.gov/NEWSROOM/photos/photogallery/chandra/chandra5.htm.-30-