WHEN Danish astronomer Tycho Brahe saw a new star in 1572 and described it as 鈥渟urpassing all the other stars in brilliancy鈥�, little did he realise that he had seen a supernova. Now, some 450 years later, astronomers say they have found the companion star that fed the explosion.
Brahe saw what is known as a type 1a supernova. These are considered to be the most uniform of stellar explosions in the universe, and observations of distant type 1a supernovae have led to the conclusion that the expansion of the universe is accelerating.
Astronomers have various models to explain these explosions. In one version, two white dwarfs slowly spiral into each other, merge and explode. In another scenario, a white dwarf sucks matter from a companion star 鈥� either a red giant or a normal star 鈥� and eventually explodes.
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Now Pilar Ruiz-Lapuente of the University of Barcelona in Spain and her colleagues have scoured the region surrounding Brahe鈥檚 supernova 鈥� one of only two type 1a supernovae in the Milky Way 鈥� and say they have found the companion star that fuelled it. Using the Hubble Space Telescope, the team discovered a sun-like star moving three times as fast as its neighbours, and travelling away from the supernova鈥檚 centre. This could mean it was forcefully expelled from its orbit by the exploding star (Nature, vol 431, p 1069).
鈥淭hey have shown that the donor star could not have been a red giant. There simply aren鈥檛 any red giants at the appropriate distance and direction,鈥� says David Branch of the University of Oklahoma. 鈥淭hey may well have identified the donor star. If so, that鈥檚 important, because it tells us that not all type 1a supernovae are triggered by the merger of two white dwarfs.鈥�
Astronomer and team member Alex Filippenko of the University of Berkeley, California, says that it is possible, though somewhat unlikely, that the star they found 鈥渏ust happened to be zooming through that region, and is unrelated to [Brahe鈥檚] supernova鈥�.