Astronomers See Evidence of New Type of Supernova

09 May 2007

(Click for larger image)

The top panel is an artist's illustration showing what SN 2006gy may have looked like if viewed at a close distance. The bottom left panel is an infrared image, using adaptive optics at the Lick Observatory, of NGC 1260, the galaxy containing SN 2006gy. The panel to the right shows Chandra's X-ray image of the same field of view, again showing the nucleus of NGC 1260 and SN 2006gy. Image credit: Illustration: NASA/CXC/M.Weiss; X-ray: NASA/CXC/UC Berkeley/N.Smith et al.; IR: Lick/UC Berkeley/J.Bloom & C.Hansen

The brightest stellar explosion ever recorded may be a long-sought new type of supernova, according to observations by NASA's Chandra X-ray Observatory and ground-based optical telescopes. This discovery indicates that violent explosions of extremely massive stars were relatively common in the early Universe, and that a similar explosion may be ready to go off in our own galaxy.

"This was a truly monstrous explosion, a hundred times more energetic than a typical supernova," said Nathan Smith of the University of California at Berkeley, who led a team of astronomers from California and the University of Texas in Austin. "That means the star that exploded might have been as massive as a star can get, about 150 times that of our sun. We've never seen that before."

This supernova was unusual not only because of its brightness, but also because of how long it remained bright - around 100 days. Supernovae from white dwarfs or massive stars (10 to 20 times the mass of our sun) stay bright for at most tens of days.

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. It is unprecedented, however, to find such a massive star and witness its death. The discovery of the supernova, known as SN 2006gy, provides evidence that the death of such massive stars is fundamentally different from theoretical predictions.

"Of all exploding stars ever observed, this was the king," said Alex Filippenko, leader of the ground-based observations at the Lick Observatory at Mt. Hamilton, Calif., and the Keck Observatory in Mauna Kea, Hawaii. "We were astonished to see how bright it got, and how long it lasted."

The Chandra observation allowed the team to rule out the most likely alternative explanation for the supernova: that a white dwarf star with a mass only slightly higher than the sun exploded into a dense, hydrogen-rich environment. In that event, SN 2006gy should have been 1,000 times brighter in X-rays than what Chandra detected.

"This provides strong evidence that SN 2006gy was, in fact, the death of an extremely massive star," said Dave Pooley of the University of California at Berkeley, who led the Chandra observations.

This animation shows an artist's rendition of how SN 2006gy exploded.(Credit: NASA/G.Bacon) (Video Description)

The star that produced SN 2006gy apparently expelled a large amount of mass prior to exploding. This large mass loss is similar to that seen from Eta Carinae, a massive star in our galaxy, raising suspicion that Eta Carinae may be poised to explode as a supernova. Although SN 2006gy is intrinsically the brightest supernova ever, it is in the galaxy NGC 1260, some 240 million light years away. In cosmological terms, that's not too far away. However, Eta Carinae is only about 7,500 light years away in our own Milky Way galaxy.

"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 was not involved in the research. "Eta Carinae's explosion could be the best star-show in the history of modern civilization."

Supernovas usually occur when massive stars exhaust their fuel and collapse under their own gravity. In the case of SN 2006gy, astronomers think that a very different effect may have triggered the explosion. Under some conditions, the core of a massive star produces so much gamma ray radiation that some of the energy from the radiation converts into particle and anti-particle pairs. This robs the star of radiation pressure from the photons which counter-act the force of the star's own gravity. Hence, the star begins to collapse.

As the star collapses, it's interior temperature rises, causing runaway thermonuclear reactions. The entire star explodes, spewing the remains into space. The energy of the explosion creates and disperses heavy elements into surrounding gas clouds, which then become the next generation of stars. While the other supernovae disperse heavy elements, much of their material remains locked within the left over black hole or neutron star.

"In terms of the effect on the early Universe, there's a huge difference between these two possibilities," said Smith. "One pollutes the galaxy with large quantities of newly made elements and the other locks them up forever in a black hole."