The sun's environs: a bubble burst?

The bulk of interstellar space in the sun's neighborhood appears to be occupied by an extremely diffuse gas. This gas is so widely dispersed that astronomers have suggested that the sun and its stellar neighbors reside within a low-density .gas bubble, about 600 light-years across, immersed in gas of somewhat greater density.

Now, the Roentgen satellite (ROSAT) and the Extreme Ultraviolet Explorer (EUVE) spacecraft have provided new details of the sun's environment. These observations - at wavelengths between 60 and 740 angstroms -- reveal striking variations in the density of interstellar gas. They call into question some of the assumptions underlying the notion that the sun sits within a hot bubble, perhaps blasted out by the explosion of a nearby star hundreds of thousands of years ago (SN: 1/2/93, p.4).

"There may be serious problems with the local bubble picture," says Donald P. Cox of the University of Wisconsin-Madison. Researchers described the new observations and some of their implications at an American Physical Society meeting held this week in Washington, D.C.

Hydrogen and helium atoms-the main constituents of interstellar gas - readily absorb ultraviolet light. Nonetheless, the gas in the sun's vicinity is sufficiently thin and ionized that instruments aboard spacecraft can detect emissions of extreme ultraviolet (EUV) radiation from a variety of nearby sources, including various types of stars.

"The recent EUV studies are so exciting because they can probe the gas distribution at very low densities," Cox says. Such data enable astronomers to begin mapping the size and shape of the local bubble of low-density gas.

Both ROSAT and EUVE have now completed the first surveys covering the whole sky at EUV wavelengths. Analysis oi data from ROSAT's wide-field camera has produced a catalog of 383 sources of EUV radiation. Most of these sources correspond to white dwarf stars or to "active, late-type" stars (SN: 5/23/92, p.344).

However, the ROSAT survey reveals significant differences in the distribution of these two types of stars. A deficiency of white dwarfs detected in the direction of the Milky Ways center indicates strong absorption of ultraviolet light, suggesting that the local bubble doesn't extend very far in that direction. In contrast, a surfeit of white dwarfs in the direction of the constellation Ursa Major apparently reveals a significant bulge in the bubble.

"And there's an excess of active stars in a region towards Orion," says B.A. Cooke of the University of Leicester in England. "The only way we can explain that is to reduce [previous estimates of] the gas density in that region."

The EUVE results, which cover a broader wavelength range than the ROSAT data, confirm this parchiness in the absorption and density of interstellar gas. In one instance, researchers observed a particularly hot star about 600 light-years away. For that star to be detected at EUV wavelengths, the density of interstellar gas had to be much lower in the direction of the galactic plane than astronomers had expected.

Earlier ROSAT findings at X-ray wavelengths had already indicated that absorption is low enough along lines of sight at right angles to the galactic plane that the region of hot, ionized gas characteristic of the local bubble may stretch beyond 3,000 light-years from Earth. Indeed, such "windows" have enabled both ROSAT and EUVE to detect EUV sources from outside our galaxy.

"We live in a very complex region [of the interstellar medium]," Cooke remarks.

Taken together, the findings indicate that astronomers may have to modify their picture of the hot bubble of low-density gas in which the sun apparently resides. "The question is how much is this bubble really a description of reality, and how many bubbles are there?" says C. Stuart Bowyer of the University of California, Berkeley, who presented the EUVE results.

"With EUV studies, we can begin to test these structures for their reality. their states, and their dimensions," Cox adds.