General Discussion

Astronomers have spotted a cataclysmic explosion that marked the death of a huge, distant star in a blast five times as bright and powerful as any they had seen previously. They said Monday that a similar fate may be imminent for a star in Earth's galactic neighborhood.

The size and energy of the newly recorded blast, 240 million light-years away, have already begun to transform scientific understanding of how especially large stars explode, and have left awestruck researchers concerned - and a little excited - about what might happen to the similarly enormous and unstable star closer to home.

If that nearer star, named Eta Carinae, blows up like the one just discovered, they said, it could possibly spew dangerous radiation in Earth's direction. More likely, however, it would erupt into the most luminous star in our sky - visible during the day and bright enough to let people read unaided at night for weeks and perhaps months.

The new discovery of a massive star exploding in a runaway thermonuclear reaction is especially exciting for scientists, who said it gives them important clues to the nature of the early universe and the formation and destruction of the earliest stars.

"This was a truly monstrous explosion, a hundred times more energetic than a typical supernova," astronomer Nathan Smith of the University of California at Berkeleysaid of the exploded star, called SN 2006gy, which was believed to be as massive as a star can be.

"It's so powerful that it requires a new explanation of how massive stars explode," said Smith. Unlike most supernovas, which fade quickly, this exploded star has remained extremely bright since it was spotted in September.

Stars approaching this enormous size generally implode when they become unstable, leaving behind a black hole or a neutron star, a cold, extremely dense remnant. Astronomers considered it theoretically possible that such a massive star could instead explode and throw vast amounts of matter into space instead of collapsing, but they had never before witnessed it.

"Discoveries don't get more exciting than this for a theorist," said Mario Livio of the Space Telescope Science Institute in Baltimore, Maryland,who was not involved in the research but spoke at a NASA news conference Monday.

"The suggestion is that we may be seeing a new type of explosion mechanism, never seen before, and that the first stars in the universe exploded with this mechanism," he said. "At the same time, there may be a star in our own galaxy that could do the same thing - tomorrow."

Livio said the enormous Eta Carinae, which is only 7,500 light-years away in our own Milky Way galaxy, has many features similar to those of the newly discovered exploded star and has been showing signs of instability that could lead to a similar supernova.

The potential danger comes from the fact that explosions of massive stars generally emit jets of intense gamma radiation, among the most powerful and harmful forces in the universe. If Eta Carinae did explode and a jet was pointed in the general direction of the solar system, Livio said, Earth could be endangered. But because the gamma-ray jets tend to be relatively narrow, like the beam of a lighthouse, the odds are that the jet would miss Earth.

Far more likely, Livio said, is that an Eta Carinae explosion "could be the best star show in the history of modern civilization." That show, however, would be seen only in the Southern Hemisphere.

Stars as massive as Eta Carinae and the distant one that exploded have seldom been found - only a dozen or so have been detected among the 400 billion stars in the Milky Way. Eta Carinae was first observed as an especially bright star in 1843, when it was in the midst of a smaller surge in brightness. The extra luminosity faded around 1870, but the star began to brighten noticeably a century later.

Unlike Eta Carinae, the newly discovered exploding star is not prominent in the sky because it is so far away.

As astronomers peer to the farther reaches of the universe and back into time, they expect to find many more of the superlarge stars, which coalesced and were destroyed in a relatively short time after the Big Bang. These early stars are of enormous interest to astrophysicists because they can help explain how the original few elements of the universe - primarily hydrogen and helium - were fused together inside stars to form the heavier elements. Those stars also had to explode, rather than collapse into black holes, in order for the universe to become populated with the many elements it has.

The SN 2006gy supernova was first detected last September during a routine telescope sweep of the night skies by University of Texasgraduate student Robert Quimby.

The extraordinary brightness of the star attracted the interest of astronomers at Berkeley and the University of Texas at Austin. The supernova was also examined at the Lick Observatory in California, at the Keck Observatory in Mauna Kea, Hawaii, and, perhaps most important, by NASA's orbiting Chandra X-ray Observatory, operated by the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts.

Using the Chandra instruments, researchers were able to rule out the most likely alternative explanation for the huge supernova: that a white dwarf star with a mass about that of the sun exploded in a hydrogen-rich environment, leading to the unprecedented brightness.

The Chandra instruments, however, found that the exploded star was not producing the X-ray radiation expected under those conditions. With that established, Berkeley astrophysicist Dave Pooley said, the team became confident that SN 2006gy "was, in fact, the death of an extremely massive star."