Here’s another extremely explosive supernova that can be chalked up to the production of antimatter in the core of the star: Y-155. These types of supernova explosions – which can be ten times brighter than the already spectacular explosion of a Type Ia supernova – have been theorized to exist for over forty years. About a month ago, we reported on the first observations of one of these types of supernovae, and at the American Astronomical Society super-meeting yesterday, Peter Garnavich of the University of Notre Dame presented on the observation of a second.
The star Y-155 was a whopping large star, with a mass of over 200 times that of our Sun. In these types of stars, energetic gamma rays can be created by the intense heat in the core of the star. These gamma rays in turn make pairs of electrons and positrons, or antimatter pairs. Since so much energy goes to the creation of these pairs, the pressure pushing outwards on the star weakens, and gravity swoops in to collapse the star, generating a supernova of enormous proportions.
These types of supernovae have been dubbed “pair-instability” supernovae, and once they explode, there is nothing left: in other types of supernovae, a neutron star or black hole can form out of the remnants of the star, but pair-instability supernovae explode with such force that there is nothing left where the core of the star once existed. In addition to supernova 2007bi, which we reported on in December of 2009, the supernova 2006gy is another candidate for this type of supernova.
Y-155, which lies in the constellation Cetus, was discovered as part of the Equation of State: SupErNovae trace Cosmic Expansion,”ESSENCE”, search for stellar explosions. During the 6-year search, a team of international astronomers led Christopher Stubbs of Harvard University collaborated to find Type Ia supernovae as a means to measure the expansion of the Universe. These types of supernovae explode with a characteristic luminosity, making them excellent candidates to measure distances in the Universe. The team utilized the National Optical Astronomy Observatory’s (NOAO) 4-m Blanco telescope in Chile.
Y-155 was discovered in November of 2007, during the last weeks of the project, using the Blanco telescope. Once the initial discovery was made, followup observations using the Keck 10-m telescope in Hawaii, the Magellan telescope in Chile, and the MMT telescope in Arizona revealed the redshifting of the light due to the expansion of the Universe to be about 80%, meaning that the star is very far away, and thus very old. Y-155 is estimated to have undergone a supernova approximately 7 billion years ago.
According to Garnavich, the team calculated the star to be generating 100 billion times the energy of the Sun at its peak. To accomplish this, it must have synthesized between 6 and 8 solar masses of nickel 56, which is what gives Type Ia supernovae their brightness. For comparison, the typical Type Ia supernova burns 0.4-0.9 solar masses of nickel 56.
Y-155 has been shown by deep imaging with the Large Binocular Telescope in Arizona to reside in a galaxy that is rather small. Smaller galaxies are usually low in heavier atoms. The gas out of which this and other types of ultra-massive stars form is relatively pristine, composed largely of hydrogen and helium. Supernova 2007bi, the first-observed pair-instability supernova, grew up in a galaxy remarkably like that of Y155.
This means that when astronomers look for other types of pair-instability supernovae, they should find more of them in smaller galaxies that existed near the beginning of the Universe, before other supernovae synthesized heavier elements and spread them around.