A rare event that can only be seen about three times every hundred years just became even more intriguing.
Two supernovas side by side in one galaxy were found by scientists using NASA's Swift satellite. Large galaxies only see about three supernovas per century, said Neil Gehrels, Swift principal investigator at NASA's Goddard Space Flight Center.
"Supernovas are massive stars that explode," said Gehrels. Also, they do not fade for a few months, he said.
Galaxy NGC1316 has had two supernovas in less than five months and a total of four in the last 26 years, said Gehrels.
"It is the most supernova-active galaxy we know," he said.
The first supernova was discovered on June 19 and named SN 2006dd. The second was detected Nov. 5 and named SN 2006mr, according to a Penn State press release.
"It is exciting to be able to study two supernovas at the same time," Gehrels said.
NGC1316, which stands for New Galactic Catalog, is a large elliptical galaxy about 80 million light years away, said John Nousek, director of Swift's mission operations center and professor of astronomy and astrophysics at Penn State.
Scientists are still trying to determine what could have caused the rare occurrence and why this galaxy has seen such a high supernova rate. The galaxy, which recently collided with a spiral galaxy, can produce supernovas, but all four supernovas in NGC1316 appear to be Type Ia, which is not connected to galaxy mergers and massive star formation, Nousek said.
One explanation could be that it simply was an accident, Nousek said. He said it is like playing with a deck of cards -- if the deck keeps getting shuffled, eventually when drawing cards from the pile there will be two aces together.
Another explanation is that collision with the spiral galaxy created the supernovas, Gehrels said. He said these collisions can produce supernovas by creating new massive stars, which soon after die and explode.
The Swift, which was launched in November 2004, carries the Burst Alert Telescope, the X-ray Telescope, and the Ultraviolet/Optical Telescope. The X-ray Telescope and UV/Optical Telescope were developed and built by international teams led by Penn State researchers, Nousek said.
Swift, short for Swift Gamma-Ray Burst Explorer, was built to find gamma-rays and follow gamma-ray bursts but does not limit itself to only observe gamma-rays, Nousek said.
"Supernovas are a very good way for us to study the expansion of the universe," he said.
One of the ways to study this is by figuring out the change in brightness in the supernova, he said.
"When you look at anything -- let's say a light bulb, bright or dim -- the closer light bulb looks brighter than the far one," Nousek said.
He said if scientists could figure out the change in brightness and figure out which supernovas are the same levels of brightness and distance, they could figure out how fast the universe is expanding.
