Telescope Discovers Bright Supernova
Telescope Discovers Bright Supernova
Sanders, Public Affairs
A supernova dubbed 1999em, recently discovered at Lick Observatory, may become one of the century's best observed supernovae, according to Alexei Filippenko, the Berkeley astronomer whose group found the exploding star.
The supernova is a coup for Filippenko's automated supernova search, which employs a robotic telescope, the Katzman Automatic Imaging Telescope at Lick Observatory, to take nightly pictures of large numbers of nearby galaxies in search of new points of light. 1999em is the 30th supernova that he and his team have discovered so far this year -- twice what any other search for nearby supernovae has turned up in a 12-month period.
"This has the potential to become the best observed supernova since the one in 1987," said Filippenko, professor of astronomy and director of the supernova search. He's referring to the most spectacular supernova of the century, a naked-eye exploding star that wowed astronomers and casual observers alike in 1987.
The supernova was photographed before dawn on Oct. 29, and postdoctoral fellow Weidong Li immediately e-mailed news of it around the world.
Six hours later, astronomers at the Beijing Astronomical Observatory confirmed it was a very bright Type II supernova located in a nearby galaxy called NGC 1637, in the constellation Eridanus near its border with Orion.
Though SN 1999em outshines the core of its own galaxy and is extremely bright compared to normal stars, because it is 25 million light years away it isn't visible to the naked eye.
Astronomers nevertheless jumped at the chance to observe an explosion in progress. The Chandra X-ray Observatory turned its eye on the supernova on Nov. 2, while the Hubble Space Telescope wheeled around to look at the brightening explosion on Nov. 5.
"Supernovae expand quickly and cool quickly, so each day we delay observing the supernova it has changed irretrievably," Filippenko said. "We caught this really early, only a day or two after the explosion. We were lucky."
Discoveries like 1999em make Berkeley's automated supernova search exciting, Li said. From the top of Mt. Hamilton outside San Jose, Calif., the telescope -- a 30-inch reflector -- photographs a few thousand galaxies every three to five days.
"Among the galaxies that we monitor, we are discovering essentially every exploding star within about 100 million light years of Earth," Filippenko said. The only ones they miss are those too near other bright objects, like galactic centers, or those buried in haze near the horizon.
The point of the automated supernova search is to learn as much as possible about the physics of exploding stars. To do that, Filippenko, Li and their colleagues need to find them shortly after they explode so they can chart the course of brightening and dimming, and also obtain spectra -- the range and brightness of colors in the star's light.
The 50 supernovae they have found since the telescope came online in 1997 are giving them information on the types of supernovae in the cosmic zoo and providing clues to the fate of the cosmos.
Filippenko and an international crew of astronomers -- the High-Z Supernova Search Team -- last year drew upon the distances of Type Ia supernovae to conclude, along with a competing team, that the expansion of the universe is accelerating. The Type Ia supernovae were used as a "standard candle" to estimate galactic distances.
Though none of the supernovae employed in that study were found by the telescope, which looks only for nearby supernovae, Filippenko has been compiling data on nearer Type Ia supernovae to bolster confidence in the use of these supernovae to estimate distance.
In the December issue of Astronomical Journal, Filippenko and his colleagues will report a worrisome finding -- nearby supernovae might take longer than more distant supernovae to reach their maximum brightness.
Is this an indication of inherent differences between the nearby supernovae astronomers think they understand, and the older and more distant ones? Filippenko asks. If so, can we really say that their maximum brightness is the same, and use them as a standard candle?
"Perhaps we will find that we can explain this possible difference, and that there really is no error in our study of distant supernovae," Filippenko said.