When a famous Hollywood star gains weight, people tend to notice. The same can be said about stars in the sky that are larger than normal. The recent discovery of a massive supernova-larger than anyone thought possible-is moving astronomers to rethink how exploding stars behave.
A white dwarf occurs when a star of low to medium mass expands into a red giant at the end of its life, then collapses in on itself, resulting in a much smaller star that has an extremely dense core. Our sun may become a white dwarf at the end of its life, many millions of years from now. Due to their large gravitational pull, these white dwarf stars are able to pull matter from other nearby stars, increasing their mass until they explode in a violent and impressive fashion. The explosion, called a supernova, remains bright for weeks or months.
“Nobody knows exactly how white dwarfs work,” said Andy Howell a postdoctoral researcher at U of T’s department of astronomy and astrophysics. The discovery of a massive supernova only complicates matters.
In the 1930’s, the Indian physicist and Nobel laureate Subrahmanyan Chandrasekhar calculated that the highest mass a white dwarf star could attain before exploding would be 1.4 solar masses (1.4 times the mass of our sun). This value, known as the Chandrasekhar limit, had been true for supernovae since then. Supernovae are used as the “standard candle” of the universe as they emit a generally uniform light that allows astronomers to determine how far away they and other objects are from Earth.
However, the size of the new supernova, which reached two solar masses before exploding, and its corresponding increase in luminosity, suggests that the Chandrasekhar limit may not hold true in all circumstances.
“The scariest part is that we thought we understood how supernovae work,” Howell said. “Then, we find this one that behaves differently from anything else we have ever seen.”
Using data from the Supernova Legacy Survey, a research group that is a joint effort between France, Canada and the United States, Howell recently published the implications of the finding in Nature though observations of the supernova were reported in 2003. The star (or SNLS-03D3bb, as it is officially known) is identified as a type 1a supernova that lies four billion light years away from Earth. Typically, type 1a supernovae are used not only as distance markers, but also in calculating the expansion of the universe, the need for dark energy, and the relative youth of the stars around it.
With this new finding, Howell is confident that the entire field of research need not be reworked, though future studies and calculations will have to account for the possibility of oversized white dwarfs.
“It doesn’t erase what happens before, it just adds an extra dimension,” said Howell. And the dimensions of this supernova are certainly impressive.
“If you converted [the 2.58 x 1030 kg of nickel in the star] into nickels, it would be … 28 nonillion dollars Canadian,” said Howell. That is, 28 dollars with 30 zeroes behind it.
Howell is confident that more of these large-scale exceptions will be found in the future.
“It’s a new direction to look for our research,” said Howell. “We’ve got to go and look back with fresh eyes.”
