Thursday, January 13

The Far-Ultraviolet Cosmos Unveiled

Once upon a time astronomers observed only in visible light. Much of the

story of astronomy in the last half century has been the opening up of the

rest of the electromagnetic spectrum, from radio waves to gamma rays. One

of the last regions to be opened, and still one of the most mysterious, is the

far ultraviolet.

The only instrument working in this part of the spectrum is NASA's Far

Ultraviolet Spectroscopic Explorer (FUSE), which was launched just last

June. FUSE examines the sky at wavelengths from about 900 to 1200

angstroms (90 to 120 nanometers), and with greater resolving power and

10,000 times the sensitivity of its only predecessor, the Copernicus

satellite, which died more than 20 years ago. Two dozen groups of

scientists presented a slew of early results from FUSE on January 12th at

the meeting of the American Astronomical Society in Atlanta. Among the

highlights:

Our Milky Way galaxy is surrounded by hot gas blown out by

supernovae in the galaxy's disk. The gas extends some 5,000 to

10,000 light-years above and below the disk and is heated to

300,000° Kelvin, reported Blair D. Savage (University of Wisconsin,

Madison) and William Steigerwald (NASA/Goddard Space Flight

Center). The gas is patchy and only a thousandth as dense as the

average of the interstellar medium in the Milky Way's disk itself. Its

existence has long been known, "but we weren't sure how it stayed

hot," Savage told a press conference at the meeting.

FUSE found lots of five-times-ionized oxygen in the gas, indicating

temperatures produced by blast waves from supernovae. The gas,

which Savage described as "the atmosphere of the Milky Way,"

represents fountains of material being thrown up from, and raining

back onto, the disk.

Cold molecular hydrogen gas, H2, is common throughout the Milky

Way and the universe. Until recently it was almost impossible to

detect directly, because unlike atomic hydrogen (H) it does not

radiate or absorb at easily observed wavelengths. But it does have

many strong absorption lines in the far ultraviolet.

Michael Shull and Jim Scott (University of Colorado, Boulder) used

FUSE to probe for H2 in dense gas clouds in the Milky Way and

nearby galaxies. "Molecular hydrogen is the lifeblood of a galaxy,"

said Shull. "It completes the cycle of gas to form new stars," since

hydrogen passes through this phase on its way into star-forming

clouds and globules. In fact H2 is the most abundant molecule in the

universe; "everywhere we look we see it," remarked Warren Moos

(Johns Hopkins University). Some of it is even scattered in thin

wisps far from the dense clouds where molecules would normally be

expected.

Nevertheless, according to FUSE researchers, H2 represents less

than 1 percent of the Milky Way's mass (about 10 percent of

interstellar matter). Thus it cannot account for the Milky Way's

mysterious budget of dark matter. This finding throws some cold

water on a recent report, based on infrared observations, that H2 in

another galaxy, NGC 891 in Andromeda, might account for all of a

galaxy's dark mass (see the January Sky & Telescope, page 20).

FUSE is proving especially apt at dissecting dense stellar winds, the

outflows of gas streaming from hot young stars at speeds up to 3,000

kilometers per second. John Hutchings (National Research Council

of Canada) described a surprising finding: seemingly identical stars

in different galaxies produce winds of different velocities even close

to the stars themselves. Clues may lie here to the ways in which hot

stars mold their environments, and vice versa, in galaxies at different

stages of cosmic evolution.