ASCA probes the X-ray universe near and far.

From studies of celestial powerhouses far beyond the Milky Way to the nature of exploded stars within, it , the X-ray eyes of the orbiting Japanese telescope ASCA have uncovered several surprises.

Astronomers described these and d other intriguing discoveries last week at a meeting of the American Physical Society in Crystal City, Va.

ASCA's observations of the diffuse glow of X rays, known as the cosmic X-ray background, suggest that the output of quasars and to the brightest active galaxies can't by themselves account for most of the background at X-ray energies below 10,000 electronvolts. According to ASCA astronomer Hajime Inoue of the Institute of Space and Astronautical Science in Tokyo, the emissions from active galaxies at these low energies don't match the relatively flat X-ray spectrum recorded by the telescope.

Instead, he suggests, the combined X-ray emissions from the vast number of seemingly mundane galaxies in the universe produce the low-energy part of the X-ray background. The smoothness of the observed background suggests that the X-ray glow is formed by many relatively dim galaxies rather than a few very bright ones.

Inoue proposes that most galaxies have at their center a small black hole and that these central powerhouses provide the galaxy's X-ray background. Thus, most galaxies may be active. Thus, most galaxies may be active. ASCA researcher Stephen S. Holt of NASA's Goddard Space Flight Center in Greenbelt, Md., says that less exotic sources of radiation, such as a cluster of rotating neutron stars, could produce the galactic X-ray emissions.

Closer to home, ASCA observations of the Milky Way have prompted researchers to refine their view of what happens when a star explodes as a supernova, hurling its remains through space.

Spectra taken with ASCA reveal that the remnants of some supernovas, though born in a powerful explosion, manage to preserve the layered structure of their parent star. This finding might provide a new tool for analyzing supernovas, says Robert Petre, an ASCA investigator at the Goddard Space Flight Center.

Stars shine by squeezing together the nuclei of lighter elements, such as hydrogen or hellum, to make heavier elements. As a star ages, its center -- the hottest, densest area -- becomes the birthplace of the heaviest elements. Indeed, the classic model of a massive, older star depicts a layered structure akin to an onion -- an iron core surrounded by concentric shells of lighter elements.

In examining the elderly supernova remnant W49B, ASCA found that this exploded star retains the onionlike layering of light and heavy elements. ASCA images show iron concentrated close to the core, with silicon and sulfur lying nearer the outskirts of the remnant, report Ryuichi Fujimoto of the Institute of Space and Astronautical Science and his colleagues.

"It is astonishing that supernova remnants can retain the memory of the explosion for several thousand years," comments Petre.

In contrast, Cassiopeia A, the youngest known supernova remnant in the Milky Way, appears messier. ASCA images indicate that sulfur and silicon concentrate in lumps rathe than shells. In addition, material from the southern half of the remnant moves toward Earth, whereas the northern half recedes. This suggests that the supernova explosion flung a broken ring of material, rather than a spherical remnant, into space, says Petre.

Holt says that the ASCA images of Cassiopeia A reveal for the first time two distinct types of X-ray emission in a single remnant. The X-ray glow from specific elements, including sulfur and silicon, lies in the northern and eastern parts of the remnant. But at higher X-ray energies, which may indicate emissions from hot electrons and ions, the southwest section appears brightest.

According to Hold, the X-ray-emitting electrons and ions represent material from the interstellar medium that got heated and swept up by the expanding blast wave from the supernova explosion. In contrast, the X-ray glow from the elements stems from heating by a reverse shock wave -- the secondary wave of energy created whent he original, outgoing blast slams into interstellar space.