A supernova has been discovered that may help to explain starburst galaxies, realms where it is thought that stars are rapidly born and die as two galaxies collide.

The supernova, in galaxy NGC 3690 in the Big Dipper about 150 million light-years from Earth, was discovered by a team of astronomers from the California Institute of Technology's Infrared Processing and Analysis Center (IPAC) and NASA's Jet Propulsion Laboratory.

The light observed by the astronomers, at NASA's Infrared Telescope Facility on Mauna Kea, Hawaii, left the galaxy during the middle of the age of dinosaurs.

Although supernovas, or violently exploding stars, are fairly common -- about 20 to 30 are spotted per year -- this one is unique because it occurred in a type of galaxy called a starburst galaxy, said Dr. David Van Buren, who heads the astronomer team.

Starburst galaxies are actually the product of two separate galaxies that collide. In the process, clouds of gas are brought together that rapidly form stars that burst into life.

About two years ago the Caltech and JPL astronomers began searching among about 40 such starburst galaxies for any evidence of supernovas, which could help them understand the mechanics of the birth and death of stars in the galaxies.

The supernova in NGC 3690 was spotted by Van Buren's collaborator Dr. Thomas Jarret, also of IPAC, who noticed that the object appeared in an image captured on March 9, 1992, but had disappeared in images taken in December 1993.

"Only now, almost two years later, are we confident that the object seen in 1992 was a supernova because it has completely disappeared," Van Buren said.

Infrared astronomy is a key way of studying starburst galaxies because they are surrounded by dense clouds of dust and gas that make them invisible in optical telescopes. They are extremely bright, on the other hand, in the infrared spectrum.

One theory held that starburst galaxies are bright in the infrared because extremely rapid star formation is taking place in them. But another theory holds that they are bright because huge amounts of energy are released by matter falling onto massive black holes near their centers.

The IPAC astronomers undertook their study to distinguish between those two possibilities, said Van Buren.

The brilliant explosions of supernovas, he added, are the most easily observed event of the life of a massive star of the type thought to inhabit starburst galaxies. But because of the dense clouds surrounding such galaxies, those events are usually not found by traditional supernova search programs.

The occurrence of the supernova in NGC 3690, along with the discovery last year of an optically visible supernova in the same galaxy, gives scientists confidence that the galaxy's brightness in the sky comes largely from massive stars.

Very massive stars, those at least eight times the mass of our own sun, become supernovas at the ends of their lives as they exhaust the last bits of their nuclear fuel and collapse catastrophically in on themselves.

The galaxy can be seen with telescopes near the center of the bowl of the Big Dipper, in the constellation Ursa Major.

Also on the astronomer team are Susan Terebey and Chas Beichman of IPAC, and Mark Shure and Charlie Kaminski of the NASA Infrared Telescope Facility at the University of Hawaii.

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