White dwarfs are dead stars, whereas stars are huge gaseous balls having continuous nuclear burning and resulting in emission of light. When all helium and hydrogen of a star of mass about 5 times or less than mass of sun burn out, it leads to a white dwarf.
Chandrasekhar showed by calculations that mass of stable white dwarfs is maximum 1.4 times mass of sun, which is known as the Chandrasekhar limit. When a mass of white dwarf reaches the Chandrasekhar limit, its nuclear burning becomes highly dramatic, resulting in an explosion called supernova (type Ia). As all the exploding white dwarfs have same mass and hence energy, the supernova explosions have the same luminosity (a measurement of brightness). Hence, the supernovae which are observed fainted through the existing telescopes are understood to be at further distance than brighter supernovae. This can be used to calculate the size and subsequently the expansion rate of the Universe.
According to the new paper, white dwarfs can burst at 2.6 times the mass of sun, which is its new limit called "Mukhopadhyay-Das limit". "Chandrasekhar limit is a point of our research. We are first persons showcasing new generic mass limit of white dwarfs as 2.6 times solar mass. This result explains enigmatic, super-luminous supernovae observed in last few years," says Mukhopadhyay, who arrived to the conclusion based on his theoretical calculations along with students Upasana Das and Aritra Kundu.
Super-luminous supernovae, which were first discovered in 2006, are most energetic of these explosions and are more than twice as bright as and more powerful than "normal" supernovae that would have been explained based on the Chandrasekhar's work. "Chandrasekhar didn't take into account the effects of magnetic fields in his calculations which we did," says Mukhopadhyay, who's currently an associate professor with IISc's Physics Department.
"In order to correctly interpret the expansion of history of universe and then dark energy, one might need to carefully sample the observed data from the supernovae explosions, especially if the over-luminous supernovae (type Ia) are eventually found to be enormous in number. However, it is probably too early to comment whether our discovery has any direct implications on the current dark energy scenario which is based on the observation of ordinary supernovae," says Mukhopadhyay.
Currently, about few hundreds of supernovae have been observed and only 5-10 percent of them are super-luminous supernovae. "In future, with evolution of technology, if we see more over-luminous supernova, the universe expansion theory might need to be redone. As a result, dark energy scenario might need to be rethought," says Mukhopadhyay.
Meanwhile, major astrophysicists across the country have applauded the recent work. "The paper has given a very different perspective. The existence of super luminous supernova can now be explained," said Arikkala Raghuram Rao, an astronomer at the Tata Institute of Fundamental Research, Mumbai. Rao is hoping to launch a collaborative search for white dwarfs with extremely high magnetic fields.
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