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Nailing a Nova
01.29.08
 
Artist depiction of RS Ophiuchi binary system This artist rendering depicts the RS Ophiuchi binary system shortly after the white dwarf (right) has exploded as a nova. The other star is a red giant. Note the spiral dust lanes. Credit: Casey Reed > Larger image For decades astronomers have been keenly interested in ticking time bombs called novae. These are powerful star explosions that shoot off millions of times more energy than our Sun, but amazingly, the star survives the blast. In a new study, a large team of scientists has made surprising discoveries about a nova that erupted in February 2006.

The team was able to make its discovery because of a state-of-the-art new instrument at the W. M. Keck Observatory in Hawaii. This instrument was built by NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, Calif. The instrument, known as the Keck Nuller, collects the starlight from Keck’s two 10-meter (33-feet) telescopes and combines them into a single "mega-telescope." By doing so, the team’s observations are 10 times sharper than those taken by either telescope alone. Better yet, the light from the two telescopes can be combined in such a way that the nova’s bright light can be canceled out, or "nulled," allowing astronomers to see the surrounding environment.

"Because a star is so much brighter than its surroundings, something has to block the light, which is what the nuller does," says Rachel L. Akeson, Keck Interferometer project scientist at the California Institute of Technology.

View of the Keck telescopes at dusk Summit view of the twin Keck telescopes, located at 13,800 feet atop the Mauna Kea volcano in Hawaii. Credit: NASA/JPL/Keck
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In the new study, Richard K. Barry and William Danchi of NASA’s Goddard Space Flight Center in Greenbelt, Md., along with a large team of colleagues, discovered that the nova is surrounded by dust that appears to be arranged in a spiral pattern, sort of like a pinwheel.

The Keck Nuller saw no dust in the bright zone, presumably because the nova’s blast wave destroyed the dust. But farther from the explosion, at distances starting around 20 times the Earth-Sun distance, the nuller recorded the presence of dust made of silicate materials, similar to the stuff that makes up sand on Earth. The blast wave had not yet reached this zone, so the dust must have formed before the explosion.

"This flies in the face of what we expected. Astronomers had previously thought that nova explosions actually create dust," says Barry.

The nova itself flared up in the constellation Ophiuchus on February 12, 2006. The system, named RS Ophiuchi, consists of a giant red star and an Earth-sized star called a white dwarf. But don’t let the white dwarf’s size fool you. It contains more material than the Sun, squeezed by gravity to such high pressures that a cup of white dwarf material would weigh hundreds of tons. The red giant gives off a wind, and some of this gas collects on the white dwarf’s surface and heats up. When it reaches a critical temperature, the accumulated gas explodes, producing the nova.

A fisheye view of the combining lab in the basement of the Keck Observatory A fisheye view of the combining lab in the basement of the Keck Observatory, where starlight from each telescope is steered through an intricate network of mirrors and optics, then combined and manipulated. Credit: NASA/JPL/Keck
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"We were extremely lucky, because we had astronomers in place at two mountain-top interferometers, Keck in Hawaii and IOTA in Arizona," says the team’s principal investigator, Wes Traub of JPL. "Within minutes of hearing about the discovery of the nova, we alerted both teams to start observing it that night."

The Keck Nuller is helping astronomers develop the technology that might someday allow them to take actual pictures of Earth-sized planets around nearby stars. "The RS Ophiuchi observations are just a small taste of the power and potential we expect from the Keck Nuller," says Danchi. "But ultimately we want to launch a nuller into space to image extrasolar planets. These Keck results are a technological and scientific pathfinder toward that future."