The star that outshined it's own Galaxy: SN 2006gy

photo: SN 2006gy and the core of its home galaxy, NGC 1260, viewed in x-ray light from the Chandra X-ray Observatory. The NGC 1260 galactic core is on the lower left and SN 2006gy is on the upper right.

SN 2006gy was an extremely energetic supernova, sometimes referred to as a hypernova or quark-nova, that was discovered on September 18, 2006. It was first observed by Robert Quimby and P. Mondol, and then studied by several teams of astronomers using facilities that included the Chandra, Lick, and Keck Observatories. On May 7, 2007, NASA and several of the astronomers announced the first detailed analyses of the supernova, describing it as the "brightest stellar explosion ever recorded". In October 2007 Quimby announced that SN 2005ap had broken SN 2006gy's record as the brightest ever recorded supernova. Time magazine listed the discovery of SN 2006gy as third in its Top 10 Scientific Discoveries for 2007.

Characteristics:

SN 2006gy occurred in a distant galaxy (NGC 1260), approximately 238 million light years (72 megaparsecs) away. Therefore, due to the time it took light from the supernova to reach Earth, the event occurred about 238 million years ago.Preliminary indications are that it was an unusually high-energy supernova of a very large star, around 150 solar masses , possibly of a type referred to as a pair-instability supernova.The kinetic energy released by the explosion has been estimated at 10⁵² ergs 10⁴⁵ J.A pair instability supernova can only happen in stars that are very massive—having a range of around 130 to 250 solar masses. The massive star's core can produce high energy gamma rays which have a greater energy than the rest mass of two electrons (mass-energy equivalence).

photo: This diagram illustrates the pair production process that astronomers think triggered the explosion in SN 2006gy. A sufficiently massive star can produce gamma rays of such high energy that some of the photons convert into pairs of electrons and positrons causing a runaway reaction which destroys the star.

These gamma rays interact with electromagnetic fields of the atomic nuclei in the star, and become particle and anti-particle pairs of electrons and positrons. This causes the average travel distance of the gamma rays to become shorter, causing the temperature of the interior of the star to rise. This causes an even larger fraction of the produced gamma rays to be of high enough energy for pair production, causing more of the energy to be reabsorbed closer to its source. This creates a runaway reaction. As the energy is concentrated more and more into the star's core, the outer layers start to fall inwards, which then compress the core. The compression and heating produce a rapid (few seconds) thermonuclear burn or explosion of the core material. The explosion blows the star completely apart without leaving a black hole remnant behind.Although the SN 2006gy supernova was intrinsically about one hundred times as luminous as SN 1987A, which was bright enough to be seen by the naked eye, SN 2006gy was more than 1,400 times as far away as SN 1987A, and too far away to be seen without a telescope.Denis Leahy and Rachid Ouyed, Canadian scientists from the University of Calgary have proposed that SN 2006gy was birth of a quark star.


Similarity to Eta Carina:

Eta Carina (η Carinæ or η Car) is a highly luminous hypergiant star located approximately 7,500 light years from Earth in the Milky Way galaxy. Since Eta Carinæ is 32,000 times closer than SN2006gy, the light from it will be about a billion-fold brighter. It is estimated to be similar in size to the star which became SN2006gy. Dave Pooley, one of the discoverers of SN2006gy, says that if Eta Carinæ exploded in a similar fashion, it would be bright enough that one could read by its light here on Earth nights, and would even be visible during the day time. SN2006gy's Apparent magnitude (m) is 15, so a similar event at Eta Carinæ will have an m of about -7.5. According to astrophysicist Mario Livio, this could happen at any time, but the risk to life on Earth would be low.


photo: Light curve of SN 2006gy (uppermost intermittent squares) compared with other types of supernovae.