WASHINGTON (AFP) - US stargazers beheld the most spectacular supernova ever seen when a massive star exploded in a cosmic flash 50 billion times brighter than the sun, the space agency NASA said on Monday. "Of all exploding stars ever observed, this was the king," said Alex Filippenko, one of the leaders of the NASA-backed operations which observed the phenomenon that began last fall, in a statement presenting the find. "We were astonished to see how bright it got, and how long it lasted," said Filippenko, who headed teams at the Lick Observatory at Mount Hamilton, California, and the Keck Observatory in Mauna Kea, Hawaii. The supernova, named SN 2006gy, was likely caused by "the death of an extremely massive star," said Dave Pooley of the University of California at Berkeley, who headed the overall observation project by NASA's Chandra X-ray Observatory. The resulting stellar light show lasted 70 days. A supernova usually occurs when a vast star collapses under its own gravity because it has ceased to produce nuclear energy in its core. But the astronomers think that this explosion may have been due to a drop in energy caused by particle changes brought about by radioactive gamma rays. It takes a star at least eight times the size of the sun to burst into a supernova, but SN 2006 appears to have been much bigger, the report said -- around the size of 150 suns. "Astronomers think many of the first generation of stars were this massive, and this new supernova may thus provide a rare glimpse of how the first stars died," the team's statement said. "It is unprecedented, however, to find such a massive star and witness its death." The stellar blast some 240 million light years away may also have consequences closer to home, the astronomers suggested. The star that produced SN 2006 apparently expelled a large amount of material before exploding, with a mass suspiciously similar to that of another massive star, Eta Carinae, part of Earth's own Milky Way galaxy. This raises the prospect that Eta Carinae "may be poised to explode as a supernova," the astronomy team said in its statement of the findings to be published in the Astrophysical Journal. "We don't know for sure if Eta Carinae will explode soon, but we had better keep a close eye on it just in case," said Mario Livio of the Space Telescope Science Institute in Baltimore, who did not take part in the research. "Eta Carinae's explosion could be the best star-show in the history of modern civilization."