Biggest Supernova till date Seen
The biggest star explosion yet seen may be the best known example of a rare type of star death that leaves no “body” behind, astronomers say.
The unusual blast, dubbed SN 2007bi, appears to be a textbook example of a pair-instability supernova, a theoretical type of explosion proposed for very massive stars—those more than 140 times the mass of the sun.
Although most supernovae leave behind black holes or dense stellar corpses called neutron stars, pair-instability explosions would be so intense that the whole star would be obliterated.
Pair-instability supernovae have been hard to spot, however, because stars more than a hundred times the sun’s mass are extremely rare.
Spied in images of a distant dwarf galaxy taken by an automated telescope, SN 2007bi was about 40 times brighter than a typical supernova, and it took about three times longer to reach its maximum brightness.
“Anything that takes that long to rise and is that bright has to have a lot of mass,” said study co-author Peter Nugent, an astrophysicist at the Lawrence Berkley National Laboratory in California.
Hot Core, Unstable Star
Massive stars normally die when they run out of material to sustain nuclear fusion, and all that’s left in their cores is inert iron.
This means the core is no longer producing a steady stream of photons, which in a living star creates outward pressure, keeping the star from being crushed by its own gravity.
Without this stable outward pressure, the star collapses, generating a supernova in which the core gets crunched down to form a black hole or a neutron star.
But for even more massive stellar titans, astronomers think the cores quickly get so hot that their photons start to split apart into pairs of electrons and positrons.
This leads to an instability between the star’s temperature and pressure, sparking a devastating explosion that flings the star’s remains into space.
The star effectively vanishes, although a lingering cloud of expanding gas can remain visible for a while.
Star Explosion
Pair-instability supernovae were first predicted more than 30 years ago. But evidence from previous candidates—including a bright explosion seen in 1999 and another in 2006—were inconclusive, Nugent said.
By contrast, SN 2007bi seems to fit the bill almost perfectly. If confirmed, studies of the newfound supernova could have major implications for computer models of star formation in the early universe, Nugent said.
That’s because pair-instability supernovae were likely much more frequent in the early universe, when stars with masses several hundred times that of our sun are thought to have existed.
When these megastars exploded, the ancient, powerful outbursts scattered debris that might have sown the seeds for future stars.
A pair-instability supernova “may be a one-in-a-trillion type of event,” Nugent said, “but they may actually be very important” in understanding the evolution of the universe.
Giant exploding star outshines previous supernovas
Scientists have just released images of the brightest stellar explosion recorded.
The supernova, known as SN 2006gy, was believed to be about 150 times as massive as the sun.
The explosion could help astronomers better understand how the first generation of stars in the universe died.
“This supernova stands out as the brightest supernova that’s ever been observed,” said Nathan Smith, astrophysicist at the University of California at Berkeley.
“The reason we’re so excited is because it was so powerful we think it may require a new type of explosion mechanism that we’ve never observed before,” said Smith at a news conference from NASA headquarters in Washington.
A supernova is a rare and often dramatic phenomenon that involves the explosion of most of the material within a star. Supernovas can be very bright for a short time and usually release huge amounts of energy.
A graduate student using a robotic telescope that was part of the Texas Supernova Search project first detected SN 2006gy on September 18, 2006.
For about 70 days it got brighter, peaking with a brightness comparable to 50 billion suns, much brighter than most other supernovas. Supernovas are usually bright for a couple of weeks at most.
Astronomers captured the star’s demise using NASA’s Chandra X-Ray Observatory, and ground- based telescopes at the Lick Observatory in California and the Keck Observatory in Hawaii. The explosion was estimated about 238 million light years away from Earth.
Scientists believe supernova SN 2006gy expelled many of its outer layers in an eruption before its violent collapse.
When it exploded, it plowed into the cooler gases that had already been expelled, creating the brightest light show ever from a supernova.
Mario Livio, astrophysicist from the Space Telescope Science Institute in Baltimore, Maryland, calls the discovery “extraordinarily intriguing” for a theorist such as himself.
“We may be learning something entirely new about how massive stars explode,” said Livio.
“We are seeing a new type of explosion that has so far only existed in theory, and we believe the first stars in the universe exploded by this mechanism. It may provide a new lesson in the evolution of stars, and a star in our own galaxy may do the same thing tomorrow,” he said.
Astronomers say the star that became SN 2006gy expelled a large amount of its mass before it exploded. A huge star in the Milky Way, Eta Carinae, has already erupted in a similar fashion. So if it explodes as a supernova, it could present the best light show seen, because it is a mere 7,500 light years away from our planet.
The orbiting Chandra X-ray Observatory, which began its mission July 23, 1999, can detect and capture images of X-ray sources that are billions of light years away. Scientists all over the world use it to get insight about high temperature events, such as black holes and collapsed stars that are millions of degrees Celsius.
Chandra is the largest satellite deployed from the space shuttle. The observatory was named after Subrahmanyan Chandrasekhar, the Indian-American astrophysicist who worked at the University of Chicago and won the 1983 Nobel prize in physics. “Chandra” means “moon” or “luminous” in the Sanskrit language.
 Scientists believe supernova SN 2006gy expelled many of its outer layers in an eruption before its violent collapse. |