On November 11 1572, Tycho looked up into the constellation of Cassiopeia and found a dazzling new star.

The star lingered for 18 months. Its appearance was traumatic for many astronomers at the time, for it destroyed the notion, set in stone by Aristotle, that the Universe was fixed and unchanging.

Nor was it a stunner just for sky-watchers.

The star had such an effect on the popular mind that it reputedly was the inspiration for the celestial portent in Shakespeare's "Hamlet."

What Tycho had witnessed, in fact, was not the birth of a star, as he mistakenly thought, but a star in death-bed agony.

It was a supernova, one of the most powerful physical forces in the Universe.

Most supernovae occur when a very massive star runs out of fuel, its core collapses and dramatically explodes, leaving behind a neutron star or black hole.

But Tycho's star is believed to belong to a different type of supernovae, a category called 1a, of which only one other has ever been witnessed in our galaxy.

The theory behind Type 1 supernovae is that the phenomenon starts with a binary system of two stars that rotate around each other.

One is a white dwarf -- the fiercely hot, tiny but incredibly dense core of an old star -- and other is a "red giant", a cooling ageing star that is running out of fuel.

Little by little, mass flows from the red giant to the white dwarf.

Eventually, the white dwarf can no longer support this swelling mass and collapses in on itself, crushing its atoms into an electron mass so dense that the result is a massive thermonuclear explosion, a flash as bright as a billion Suns.

Of this cataclysm, nothing of the white dwarf remains except stardust and glowing embers, visible today, although only in the radio and X-ray part of the energy spectrum.

In a remarkable piece of detective work, a European-American team of astronomers led by Pilar Ruiz-Lapuente of the University of Barcelona, Spain, believe they have now identified the companion star that unleashed the 1572 supernova.

Probing into deep space using optical telescopes and calculating the likely velocity and trajectory of the star on its course around the Milky Way, they believe they have spotted its depleted remains some 10,000 light years from Earth.

If they are right, though, the star is rather similar to the Sun in surface temperature and brightness, and thus cannot be a red giant.

And it would also knock out a minor theory about 1a supernovae -- that they are triggered by the coalescence of two binary white dwarves.

The study is published on Thursday in Nature, the British weekly science journal.

In a commentary also published in Nature, University of Oklahoma astronomer David Branch said the discovery "will intrigue many for the sheer historical significance" of the 1572 supernova.

"It will excite supernova enthusiasts because of the new information it provides about the nature of progenitor binary systems for type 1a supernovae."

The next step should be to check for signs that the atmosphere of the companion star has been contaminated with tell-tale isotopes ejected by Tycho's supernova, he suggested.

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