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Sham supernovas. This giant star in a nearby galaxy (left) erupted in 1954, spawning a massive cloud of dust resembling Eta Carinae in our Milky Way (right).

The Supernova That Wasn't

MINNEAPOLIS, MINNESOTA--Giant stars sometimes fake their deaths. Thousands of years before they blow up, the biggest stars belch gas in bright eruptions that only look like fatal supernovas. Now for the first time, a telescope has fingered the star that spawned such a "supernova imposter" in another galaxy. The detective work should help astronomers discriminate between real and faux supernovas in the distant universe.

The research concerns a dramatic blaze of light seen in 1954 in the nearby galaxy NGC 2403. Some astronomers thought it was a supernova, but odd pulsations before the outburst convinced others that the object was a giant star that survived an extreme but temporary episode of brightening. In recent years, astronomers focused on the spot of the blast with telescopes on the ground, but the images were too fuzzy to be conclusive.

Now, teamwork between the Hubble Space Telescope and the Keck II Telescope in Hawaii has pinpointed the culprit. Researchers led by astronomer Schuyler van Dyk of NASA's Spitzer Science Center in Pasadena, California, used Hubble to resolve four individual stars. Then, analysis of each star's pattern of light with Keck showed that one star was blowing a vigorous wind of gas and dust into space at more than 700 kilometers per second. "It clearly was a bright blue supergiant star behind a screen of dust," says van Dyk--just the type of object that a supernova imposter would create. Van Dyk described the research here on 2 June at a meeting of the American Astronomical Society.

The team thinks the object is a distant cousin to Eta Carinae, one of the biggest stars in our own Milky Way galaxy. Eta Carinae shed about 20 times the mass of our sun during a spectacular outburst in the 1840s. Astronomers might be fooled into thinking that similar events in more distant galaxies are supernovas, van Dyk notes. By reconstructing the past behaviors of the newly identified star, astronomers may improve their success rate at picking out the genuine supernovas in remote galaxies, he says. Those explosions are vital tools for tracing the evolutionary history of the universe.

Further monitoring of the star will help reveal why massive stars act so erratically before their dooms, says astronomer Roberta Humphreys of the University of Minnesota in Minneapolis. "These things are on the verge of becoming true supernovas, yet here they are throwing off all this mass in nonfatal outbursts," she says. Researchers don't yet know how many such "warnings" a star endures before it finally blows up, Humphreys adds.

Related sites
Abstract of paper, with link to full text
Background on Eta Carinae
Hubble Advanced Camera for Surveys, used to pinpoint star