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The Discovery crew
The seven astronauts to fly the return to flight space shuttle mission hold a news conference at the Kennedy Space Center runway Jan. 7 to talk about delivery of the external tank, tile/RCC repair options and other issues. (44min 24sec file)

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Shuttle news conference
Senior space shuttle program officials hold a news conference at Kennedy Space Center on Jan. 6 following delivery of the redesigned external fuel tank to be used on the return-to-flight launch. (51min 47sec file)

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External tank arrives
The external tank for space shuttle Discovery's return-to-flight launch arrives at Kennedy Space Center. The tank is offloaded from the barge and moved into the Vehicle Assembly Building. (3min 15sec file)
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Tank leaves New Orleans
The redesigned external fuel tank to be used on the return-to-flight space shuttle launch is rolled out of the Michoud Assembly Facility and place on a barge for shipment from New Orleans to Kennedy Space Center. (1min 29sec file)
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Final touches
Technicians put the final touches on the Lockheed Martin-built space shuttle external fuel tank in advance of its shipment to the Cape. (1min 44sec file)
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Mars rover cake
NASA Administrator Sean O'Keefe is presented with a commemorative birthday cake marking the one-year anniversary of the Mars rover Spirit's landing. (1min 21sec file)
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Rover news briefing
On the one-year anniversary of Spirit's landing on Mars, mission officials hold a status news conference on the twin exploration rovers to discuss the latest findings and future plans for the craft. (31min 20sec file)
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NASA chief speech
During celebrations marking the Mars rover milestone on Jan. 3, NASA Administrator Sean O'Keefe gave this speech at the Jet Propulsion Laboratory. (10min 20sec file)
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The Mars rover story
Storyteller Syd Lieberman presents "Twelve Wheels on Mars" that describes the adventure to build, launch and explore with the Mars rovers Spirit and Opportunity. (54min 57sec file)
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Delta 4-Heavy launch
The Boeing Delta 4-Heavy rocket is launched from Cape Canaveral on its demonstration flight. (4min 35sec file)
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Onboard the Heavy
An onboard camera records the launch of Boeing's Delta 4-Heavy rocket from liftoff through separation of the outer boosters. (4min 40sec file)
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See-through galaxy
Posted: January 10, 2005

The center of our galaxy is hidden behind a "brick wall" of obscuring dust so thick that not even the Hubble Space Telescope can penetrate it. Astronomers Silas Laycock and Josh Grindlay (Harvard-Smithsonian Center for Astrophysics) and colleagues have lifted that veil to reveal a beautiful vista swarming with stars. Moreover, their hunt for specific stars associated with X-ray-emitting sources has ruled out one of two options for the nature of these X-ray sources: most apparently are not associated with massive stars, which would have shown up as bright counterparts in their deep infrared images. This points to the X-ray sources being white dwarfs, not black holes or neutron stars, accreting matter from low-mass binary companion stars.

This deepest-ever infrared view of a region near the galactic center shows thousands of stars crowded into an area only 6 light-years across. Locations of bright stars don't match locations of X-ray sources, indicating that the galactic center may contain many faint Sun-like stars with X-ray-emitting white dwarf companions. Credit: Silas Laycock (Harvard-Smithsonian Center for Astrophysics)
Their study is being presented today at a press conference at the 205th meeting of the American Astronomical Society in San Diego, Calif.

To peer into the galactic center, Laycock and Grindlay used the unique capabilities of the 6.5-meter-diameter Magellan Telescope in Chile. By gathering infrared light that more easily penetrates dust, the astronomers were able to detect thousands of stars that otherwise would have remained hidden. Their goal was to identify stars that orbit, and feed, X-ray-emitting white dwarfs, neutron stars or black holes - any of which could yield the faint X-ray sources discovered originally with NASA's Chandra X-ray Observatory.

Chandra previously detected more than 2000 X-ray sources in the central 75 light-years of our galaxy. About four-fifths of the sources emitted mostly hard (high-energy) X-rays. The precise nature of those hard X-ray sources remained a mystery. Two possibilities were suggested by astronomers: 1) high-mass X-ray binary systems, containing a neutron star or black hole with a massive stellar companion; or, 2) cataclysmic variables, containing a highly magnetized white dwarf with a low-mass stellar companion. Determining the nature of the sources can teach us about the star formation history and dynamical evolution of the region near the galactic center.

"If we found that most of the hard X-ray sources were high-mass X-ray binaries, it would tell us that there had been a lot of recent star formation because massive stars don't live long," says Laycock. "Instead, we found that most of the X-ray sources are likely to be older systems associated with low-mass stars."

That conclusion comes from a null result: that is, most of the counterparts to the X-ray sources must be fainter than the brightness expected if the X-ray sources had massive companions. Since massive stars are both rare and bright, an association with the X-ray sources would have been easy to spot. Smaller stars are more common and fainter, making it difficult to match them to a specific X-ray source. Analysis of the infrared images found only a chance number of matches between stars and the locations of X-ray sources. Many of those matches likely were due to the crowded field of view.

"The fact that we found no significant excess of bright infrared counterparts means that the galactic center Chandra sources are probably low-mass binaries. Since by far the most common low-mass binaries with X-ray luminosities, spectra, and variability similar to the galactic center Chandra sources are accreting magnetic white dwarfs, we conclude these are the most likely identification," says Grindlay.

If the X-ray sources near the galactic center are accreting white dwarfs, the large numbers of compact low-mass binaries required could suggest that they formed in the very dense star cluster around the galactic center or that they have been "deposited" there by the destruction of globular clusters. Deeper infrared observations and spectra of the sources are needed to make actual identifications and constrain the masses of the accreting compact objects.

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.