22 September 2006

Rebellious supernova confronts dark energy

By
Cosmos Online
A rebellious supernova in a youthful galaxy is breaking all the cosmological rules, according to an international team of astronomers who published their study in the British journal Nature.
Rebellious supernova confronts dark energy

Supernovae are among the brightest events in the universe ... the bright spot in the lower left is a supernova observed in 1994, and is almost as bright as all the other stars in the centre of the spiral galaxy. This galaxy and supernova are 52 million lights years from Earth.

Credit: NASA

SYDNEY: A rebellious supernova in a youthful galaxy is breaking all the cosmological rules, according to an international team of astronomers who published their study in the British journal Nature.

One of the most energetic events possible in the universe is a supernova – the explosion of a massive star that ultimately leads to its death. Type 1a supernovae are explosions of small, dense stars that were attracting matter towards themselves from a nearby star. Until now, these supernovae were thought to show little variation in their important characteristics, especially mass and brightness.

Type 1a supernova explosions were throught to occur when the mass of the star approaches 1.4 times the mass of our Sun, otherwise known as the Chandrasekhar limit. This new study describes a supernova in a distant galaxy 4 billion light years away, in which the star reached twice the mass of our Sun before exploding.

“It should not be possible to break this limit,” says Andy Howell from the University of Toronto in Canada, lead author of the study. “But nature has found a way. Now we have to figure out how nature did it.”

The rebellious supernova has been dubbed a ‘Champagne Supernova’ – an explosion that offers significant insights and is cause for celebration. Its official name is the less ebullient SNLS-03d3bb.

Until now, the brightness of Type 1a supernovae was also thought to show little variation, consistently measuring around 5 billion times the brightness of our Sun. But little SNLS-03d3bb is more than twice as bright as usual.

Because the brightness of Type 1a supernovae was considered constant, comparisons between the known brightness and the brightness as seen from Earth have been used to calculate the distances to supernovae. These distances have also been considered cosmological yardsticks.

Most well known are the distances to supernovae in distant galaxies that sparked the hypothesis of dark energy back in 1998. At that time, it was generally believed that cosmic expansion was slowing down since the Big Bang as galaxies were gravitationally pulling on each other.

But atronomers found that supernovae were further away than theory predicted, suggesting that the universe was expanding faster since the Big Bang and that some mysterious force was overcoming the gravitational pull and causing galaxies to fly away from each other at faster and faster speeds. That mysterious force was dubbed ‘dark energy’.

While the authors of the study are confident that the discovery of a rebellious supernova does not undermine the need for dark energy, it will make astronomers more cautious about using Type 1a supernovae to measure distances in future.

According to Richard Ellis from the California Institute of Technology in the U.S., “We have much more to learn about supernovae if we want to use them with the necessary precision in the future.”

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