The quarrelsome Brahe lost most of his nose in a duel over a mathematical dispute. Credit: © SPL

Observations around a 400-year-old supernova have revealed the 'companion star' that may have caused the blast. Astronomers believe the star fed a white dwarf until, massive and unstable, the dwarf exploded in a gargantuan ball of light and energy.

The 1572 supernova was bright enough to intrigue Danish astronomer Tycho Brahe, who meticulously recorded its position and changing brightness in a tract called De Nova et Nullius Aevi Memoria Prius Visa Stella, which translates as On the New and Never Previously Seen Star.

Brahe, who famously lost a chunk of his nose in a duel and replaced the missing bit with moulded silver alloy, was a great stickler for accurate measurement. Thanks to him, modern astronomers can classify this explosion as a 'type 1a' supernova.

Type 1a supernovae are all approximately of the same actual brightness. This useful homogeneity makes it possible to guess how far away they are by measuring how bright they seem to our eyes here on Earth.

In another 400 years time Tycho will still be remembered," he says. "I don't know if we will be. Astronomer Stephen Smartt , Queen's University Belfast

Like most supernovae, they are thought to come about when a white dwarf (a smallish, old, cooling star) accretes mass by sucking it off another star that it rotates around, until it gets too big and blows up. However, no one knew what kind of star this companion might be: a giant, a regular star, or perhaps another dwarf.

Kicked out

Pilar Ruiz-Lapuente of the University of Barcelona, Spain, wanted to know, so she gathered together an international team of astronomers to look at all the stars around the now-dim blast site for certain unusual features.

The former companion would have been kicked out of its binary orbit and been accelerated by the power of the supernova, so the researchers knew it would be moving faster than the other stars in the vicinity. At the far edge of their search radius, they found a speedy little star not unlike our own Sun. The suspect star is dubbed 'Tycho G' in a paper published in Nature this week1.

"There is a strong suggestion that the companion was a solar-type star," says team member Stephen Smartt, now at Queen's University Belfast. The next step is to take measurements to work out the star's chemical composition. If it is the companion, it would have been showered with heavy-metal shrapnel formed in the explosion, so it would be expected to contain lots of nickel and iron.

Doomed relationship

William Blair, an astrophysicist at Johns Hopkins University in Baltimore, has worked with Kepler's supernova, which exploded in 1604. He says that Ruiz-Lapuente's results help prove the almost universally held theory that two stars in a tight binary system are the ingredients of a supernova.

So, Brahe saw the explosion 400 years ago, and Ruiz-Lapuente and her team have now seen the cause: the other half of the doomed relationship between the two stars. Smartt, for one, is modest about the discovery. "In another 400 years time Tycho will still be remembered," he says. "I don't know if we will be."