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Astronomers working with the Astro-2 observatory on the space shuttle Endeavour have obtained important new data that will permit them to learn more about a halo of ultra-hot gases scientists believe surrounds our Milky Way galaxy.

On Sunday scientists used the Hopkins Ultraviolet Telescope (HUT) to observe the brightest quasar in the sky, called 3C273, about two billion light years away. As ultraviolet light from the quasar shines through interstellar space, it is absorbed by the halo that surrounds the Frisbee-like disk of stars that is the Milky Way galaxy.

By analyzing the absorption characteristics with HUT's spectrograph, Johns Hopkins University astronomers will be able to test theories about this halo of gas, said astrophysicist William Blair, a member of the HUT team.

Accepted theories suggest that materials from stellar winds and powerful supernova shock waves are thrown out of the plane of the Milky Way -- above and below the Frisbee of stars -- puffing hot gas into the halo.

"This stuff eventually cools down and comes raining back down onto the disk because of the gravitational attraction," Dr. Blair explained. "The material eventually enriches the galactic gas, feeding new generations of star formation."

It's called a galactic fountain -- flows of gas as hot as 300,000 degrees that bubble up and out of the plane of the galaxy and into the halo. Major questions center on whether the halo exists, and if so, how hot the gas is and how far it extends. Data from HUT supports evidence for the halo's existence, but other details will be forthcoming, after scientists analyze the massive amount of data during the months and years following the Astro-2 mission, Dr. Blair said.

A portion of the hot gas is made up of "oxygen VI," oxygen that has been stripped of five electrons by radiation and bombardment with fast-moving particles blasted into space by shock waves.

Astronomers also gained high-quality spectrographic data from the quasar itself, providing information about this mysterious object, he noted. Quasars, the most luminous objects in the universe, are extremely distant. They look like stars when viewed through a conventional telescope, but actually emit much more energy than an entire galaxy. Quasars were common long ago, when the universe was young, and some scientists believe they played a role in the formation of galaxies.

In other Astro-2 news Sunday, astronomers for the first time recorded ultraviolet spectrographic data from a point where a supernova's exploding material expands into interstellar space at more than 1,200 miles per second. Scientists used HUT to observe one of the youngest supernova remnants, called SN 1006, first documented by Chinese astronomers in the year 1006.

SN1006 is much younger than the expanding clouds of other supernovas observed by Astro-2 so far. That means astronomers are seeing the immediate blast wave, right after the explosion that tore the dying star apart, producing the expanding cloud of gas and debris called a supernova remnant.

"This shock is right at the main blast wave, where the main blast wave is encountering fresh interstellar material for the first time," Dr. Blair said. "That's why it's so interesting."

The typical supernova remnant is moving outward at speeds of from 150 to 200 kilometers per second (about 100 miles per second). Observations of those supernova remnants yield information about gases that are well behind the original shock wave from the star's explosion. The gas is cooling down, and it's cooling down by emitting the bright radiation astronomers observe.

By comparison, because SN1006 is only about 1,000 years old, the speed of its debris is blasting outward at 2,000 km per second.

"It's very young and is still expanding very rapidly," Dr. Blair noted. "That is a different kind of shock wave than we have ever had an opportunity to observe in the ultraviolet.

"We actually will get at the physics of the shock front itself, as opposed to the cooling flow well behind the main blast wave."

The supernova blast wave has not been slowed down much by its interaction with interstellar gas. Spectrographic data will provide new insights into the characteristics of the interstellar material surrounding the site of the supernova explosion.

Also Sunday, astronomers planned to use HUT to study the atmospheres of Venus and Mars. Both planets have atmospheres primarily of carbon dioxide -- but Venus's atmosphere is very thick, and Mars has a very thin atmosphere. The HUT team is looking for traces of other gases in the atmospheres of Mars and Venus, gases such as argon and neon.

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