The physicist Enrico Fermi once asked referring to visits to Earth by extraterrestrial civilizations: Where are they? The accurate answer might well be: destroyed by radiation from supernova explosions.
It has long been was thought that white dwarfs could not exceed what is known as the Chandrasekhar limit, a critical mass equaling about 1.4 times that of the Sun, before exploding in a supernova. Since 2003, four supernovae have been discovered that were so bright, cosmologists wondered whether their white dwarfs had surpassed the Chandrasekhar limit, dubbed the "super-Chandrasekhar" supernovae.
American and French cosmologists from a collaboration called the Nearby Supernova Factory have used telescopes in Chile, California and Hawaii to measure the mass of one of these stars - called SN 2007if that significantly surpasses the Chandrasekhar limit with a mass of at least 2.1 times that of our Sun - give or take 10%.
Cosmologists use Type Ia supernovae as 'standard candles' to measure distances to faraway galaxies because of their uniform intrinsic brightness. This, in turn, can be used to measure the expansion of the universe by observing the supernovae as they fade away.
Type Ia supernovae were therefore instrumental in inferring the presence of 'dark energy', when it was discovered that the expansion of the universe was accelerating rather than slowing down, as should have been the case if gravity were the only force influencing the evolution of the universe.
Being able to measure masses for all parts of the star system tells the physicists about how the system may have evolved—a process that is currently poorly understood. "We don't really know much about the stars that lead to these supernovae," said Richard Scalzo, an astronomer at Yale. "We want to know more about what kind of stars they were, and how they formed and evolved over time."
Theorists continue to explore how stars with masses above the Chandrasekhar limit, which is based on a simplified star model, could exist without collapsing under their own weight. Either way, a subclass of supernovae governed by different physics could have a dramatic effect on the way cosmologists use them to measure the expansion of the universe.
"Supernovae are being used to make statements about the fate of the universe and our theory of gravity," Scalzo said. "If our understanding of supernovae changes, it could significantly impact of our theories and predictions."A massive white dwarf star in our galaxy may become a supernova several million years from now, and could possibly destroy life on Earth.
Most astronomers today believe that one of the plausible reasons we have yet to detect intelligent life in the universe is due to the deadly effects of local supernova explosions that wipe out all life in a given region of a galaxy.
While there is, on average, only one supernova per galaxy per century, there is something on the order of 100 billion galaxies in the observable Universe. Taking 10 billion years for the age of the Universe (it's actually 13.7 billion, but stars didn't form for the first few hundred million), Dr. Richard Mushotzky of the NASA Goddard Space Flight Center, derived a figure of 1 billion supernovae per year, or 30 supernovae per second in the observable Universe!
Certain rare stars -real killers -type 11 stars, are core-collapse hypernova that generate deadly gamma ray bursts (GRBs). These long burst objects release 1000 times the non-neutrino energy release of an ordinary "core-collapse" supernova. Concrete proof of the core-collapse GRB model came in 2003.
It was made possible in part to a fortuitously "nearby" burst whose location was distributed to astronomers by the Gamma-ray Burst Coordinates Network (GCN). On March 29, 2003, a burst went off close enough that the follow-up observations were decisive in solving the gamma-ray burst mystery. The optical spectrum of the afterglow was nearly identical to that of supernova SN1998bw. In addition, observations from x-ray satellites showed the same characteristic signature of "shocked" and "heated" oxygen that's also present in supernovae. Thus, astronomers were able to determine the "afterglow" light of a relatively close gamma-ray burst (located "just" 2 billion light years away) resembled a supernova.
It isn't known if every hypernova is associated with a GRB. However, astronomers estimate only about one out of 100,000 supernovae produce a hypernova. This works out to about one gamma-ray burst per day, which is in fact what is observed.
What is almost certain is that the core of the star involved in a given hypernova is massive enough to collapse into a black hole (rather than a neutron star). So every GRB detected is also the "birth cry" of a new black hole.
Scientists at the American Astronomical Society's 215th meeting, in Washington DC, said that new observations of T Pyxidis in the constellation Pyxis (the compass) using the International Ultraviolet Explorer satellite, indicate the white dwarf is part of a close binary system with a sun, and the pair are 3,260 light-years from Earth and much closer than the previous estimate of 6,000 light-years.
The white dwarf in the T Pyxidis system is a recurrent nova, which means it undergoes nova (thermonuclear) eruptions around every 20 years. The most recent known events were in 1967, 1944, 1920, 1902, and 1890. These explosions are nova rather than supernova events, and do not destroy the star, and have no effect on Earth. The astronomers do not know why the there has been a longer than usual interval since the last nova eruption.
Astronomers believe the nova explosions are the result of an increase of mass as the dwarf siphons off hydrogen-rich gases from its stellar companion. When the mass reaches a certain limit a nova is triggered. It is unknown whether there is a net gain or loss of mass during the siphoning/explosion cycle, but if the mass does build up the so-called Chandrasekhar Limit could be reached, and the dwarf would then become a Type 1a supernova. In this event the dwarf would collapse and detonate a massive explosion resulting in its total destruction. This type of supernova releases 10 million times the energy of a nova.
Observations of the white dwarf during the nova eruptions suggest its mass is increasing, and pictures from the Hubble telescope of shells of material expelled during the previous explosions support the view. Models estimate the white dwarf's mass could reach the Chandrasekhar Limit in around 10 million years or less.
According to the scientists the supernova would result in gamma radiation with an energy equivalent to 1,000 solar flares simultaneously - enough to threaten Earth by production of nitrous oxides that would damage and perhaps destroy the ozone layer. The supernova would be as bright as all the other stars in the Milky Way put together. One of the astronomers, Dr Edward Sion, from Villanova University in Pennsylvania, said the supernova could occur "soon" on the timescales familiar to astronomers and geologists, but this is a long time in the future in human terms.
Astronomers think supernova explosions closer than 100 light years from Earth would be catastrophic, but the effects of events further away are unclear and would depend on how powerful the supernova is. The research team postulate it could be close enough and powerful enough to damage Earth, possibly severely, although other researchers, such as Professor Fillipenko of the Berkeley Astronomy Department, disagree with the calculations and believe the supernova, if it occurred, would be unlikely to damage the planet
The image at the top is a composite Chandra X-ray (blue) and Palomar infrared (red and green) image of the supernova remnant W49B -a barrel-shaped nebula consisting of bright infrared rings around a glowing bar of intense X-radiation along the axis. W49B was created when a massive star formed from a dense cloud of dust and gas, shone brightly for a few million years while spinning off rings of gas and pushing them away to form a nearly empty cavity around the star. The star then exhausted its nuclear fuel and its core collapsed to form a black hole. Much of the gas around the black hole was pulled into it, but some, including material rich in iron and nickel was flung away in oppositely directed jets of gas traveling near the speed of light. When the jet hit the dense cloud surrounding the star, it flared out and drove a shock wave into the cloud.
An observer aligned with one these jets would have seen a gamma-ray burst, a blinding flash in which the concentrated power equals that of ten quadrillion Suns for a minute or so. The view perpendicular to the jets would be a less astonishing, although nonetheless spectacular supernova explosion. For W49B, the jet is tilted out of the plane of the sky by about 20 degrees, but the remains of the jet are visible as a hot X-ray emitting bar of gas.
W49B is about 35 thousand light years away, whereas the nearest known gamma-ray burst to Earth is several million light years away - most are billions of light years distant. And safe to Earthlings.
Casey Kazan via NASA/JPL.
Citation: Scalzo et al., 'Nearby Supernova Factory Observations of SN 2007if: First Total Mass Measurement of a Super-Chandrasekhar-Mass Progenitor', Astrophysical Journal, 2010; doi: http://arxiv.org/abs/1003.2217