Cosmology tackles big questions, such as: How was the universe created? Is there such a thing as dark matter? Since ultimate answers for questions like these are not forthcoming, it’s not surprising that from time to time new information appears that sends a moiety of cosmologists and astrophysicists back to the white board for a furious round of theoretical mathematics. Such may be the case with new evidence about the universe provided by a rarity – a super-supernova. It’s one that appears to be in violation of Chandrasekhar’s limit.
Everybody knows the old saw: The speed of light, it’s not only the limit, it’s the law. Chandrasekhar’s limit, though not quite as binding as the speed of light, has been a standard held by astrophysicists as the upper limit for the mass of a star before it collapses and goes supernova. That mass is commonly given as about 1.4 times the mass of our Sun. The stars in question are called white dwarfs, which make up about 6% of the stars in our visible universe. No non-rotating white dwarf can be heavier than the Chandrasekhar limit. So it was thought.
Since 2003 four supernovae have been discovered that were so bright, scientists considered that they originally may have had mass above the Chandrasekhar limit. Confirmation has now been provided for at least one case, the supernova called SN 2007if. It was above the Chandrasekhar limit before it went supernova.
The research work was done by a team of American and French physicists, led by Richard Scalzo of Yale University (New York, USA). The project was called the ‘Nearby Supernova Factory,’ which measured the mass of SN 2007if. Using observations from massive telescopes in Chile, Hawaii, and California, they measured the mass of the central star. They also discovered that not only did the star have an unusually bright center, but also a shell of material that was ejected during the nova explosion. They found that the mass of the star was 2.1 times the mass of the Sun (give or take 10 percent). According to the Chandrasekhar limit, that star should have gone supernova at a mass 0.7 times the mass of the Sun before it actually did. Impossible, according to theory, but there’s the evidence with a nice wide margin.
Why should the discovery of stars that break the Chandrasekhar limit bother cosmologists? Well, that limit wasn’t picked out of a hat. Behind it is the work of Nobelist Subrahmanyan Chandrasekhar and many other physicists, who produced mathematical models showing how the behavior of physical materials could not exist beyond a certain limit of concentration. If the gravitational attraction (because of size and density) becomes too great – at 1.4 times the mass of the Sun, the atomic structures would collapse – suddenly and with a very big explosion, a supernova. If, however, stars exist that don’t collapse at that mass, perhaps the mathematical models are wrong. Worse yet, some of the assumed principles of physics behind the models might be wrong. In any case, astrophysicists need to come up with plausible explanations for how something like SN 2007if can exist.
Scalzo believes there’s a good chance that SN 2007if resulted from the merging of two white dwarfs, rather than the explosion of a single white dwarf and hopes to study the other super-Chandrasekhar supernovae to determine whether they, too, could have involved a merger of two white dwarfs.
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.”
[Source: Yale University]
As Spock would say: “Fascinating.”