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Stellar survivor supports supernova theory

| 10 November 2004

An international team of astronomers announced last month that they have identified the probable surviving companion star to a titanic supernova explosion witnessed in the year 1572. The discovery provides the first direct evidence supporting the long-held belief that Type Ia supernovae come from binary star systems containing a normal star and a burned-out white dwarf star. The normal star spills material onto the dwarf, which eventually triggers an explosion.

The results of this research were published in the Oct. 28 issue of the British science journal Nature. “The suspect star is moving away at breakneck speed and is at the same distance as the supernova remnant,” says Alex Filippenko, Berkeley professor of astronomy and a co-author on this research.

Type Ia supernovae are used to measure the history of the expansion rate of the universe and so are fundamental to helping astronomers understand the behavior of dark energy, an unknown force that is accelerating the expansion of the universe. Finding evidence to confirm the theory as to how Type Ia supernovae explode is critical to assuring astronomers that the objects can be better understood as reliable calibrators of the expansion of space.

Even though today’s astronomers arrived at the scene of the disaster 432 years later, using astronomical forensics they have nabbed what they believe to be one of the perpetrators rushing away from the location of the explosion (which is now enveloped in a vast bubble of hot gas called Tycho’s Supernova Remnant). For the past seven years the runaway star and its surroundings were studied with a variety of telescopes, with the Hubble Space Telescope playing a key role by precisely measuring the star’s motion against the sky background. (The star is breaking the speed limit for that particular region of the Milky Way by moving three times faster than the surrounding stars.)

On Nov. 11, 1572, Tycho Brahe noticed a star in the constellation Cassiopeia that was as bright as the planet Jupiter, and that soon came to equal Venus in brightness. No such star had ever been observed at this location before. For about two weeks the star could be seen in daylight. At the end of November it began to fade and change color, from bright white to yellow and orange to faint reddish light, finally fading away from visibility in March 1574. Tycho’s meticulous record of the brightening and dimming of the supernova now allows astronomers to identify its “light signature” as that of a Type Ia supernova.