The first images of supernova SN2007if - the faint host galaxy could not be made out at the time the supernova was discovered.
Credit: NBNL
SYDNEY: A star that exceeded its known upper mass limit before turning supernova could change the way scientists measure the expansion of the universe and study dark energy, scientists said.
The researchers, led by experts from Yale University, measured the mass of a supernova thought to belong to a unique subclass, type Ia, and found that it significantly exceeded the upper limit, known as the Chandrasekhar limit - which is 1.4 times the mass of our Sun.
"Our findings present a challenge for theories of stellar evolution to explain," said lead author of the study, Richard Scalzo, an astrophysicist from Yale University.
White dwarf grows too large
"The theorists will have to tell us how our white dwarf got to be so massive [without exploding]," he said.
A supernova is the incredibly violent and bright explosion that follows the collapse of a massive star and leads to its death. It is one of the most energetic events in the universe. Type Ia Supernovae are specific to the explosion of white dwarf stars - the smaller and denser product of exhausted Sun-like stars.
Up until now, scientists have believed that there is a 'critical mass' for white dwarfs, the Chandrasekhar limit, above which they are supposed to become unstable and explode in a Type Ia supernova.
Handful suspected to exceed upper mass limit
Since 2003 cosmologists have discovered four Type Ia supernovae that were so bright they were thought to have possibly exceeded the Chandrasekhar limit, and were subsequently dubbed the "super-Chandrasekhar" supernovae.
Now 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.
They found that it significantly surpasses the Chandrasekhar limit with a mass of at least 2.1 times that of our Sun - give or take 10%.
Type Ia points to dark energy
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.
While Scalzo stresses that over-bright supernovae such as SN 2007if would not be used to measure stellar distances and universal expansion, he is concerned that there could be other similarly anomalous supernovae, which are not as extreme and could therefore end up in a study of dark energy.
