Tycho Brahe's 1572 supernova classified

One of the most famous supernovae in the history of astronomy, Tycho Brahe's supernova of 1572 (SN 1572), first identified more than 400 years ago, is a normal type Ia supernova, according to research published in the current (4 December 2008) issue of Nature. An international team of astronomers, led by Oliver Krause of the Max Planck Institute for Astronomy, have used light echoes as a time machine to unearth secrets of one of the most influential events in the history of astronomy.

A brilliant new star appeared on the sky in early November 1572. The new star outshined all other stars in brightness and was even visible during daylight. It was widely observed by astronomers all around the world and it helped to change our understanding of the Universe forever. Precise measurements of the star position by the great Danish astronomer Tycho Brahe reported in his book 'Stella Nova' revealed that the star was located far beyond the Moon.

This contradiction to the Aristotelian concept, that a change on the sky can only occur in the sublunar regime, ultimately led to the abandonment of the notion of the immutability of the heavens.

The 16th century scientists did not know what kind of star they had observed. Only in 1940 it was concluded that it must have been a supernova - an explosion blasting apart a star at the end of its life.

Supernovae, in general, comprise the most intense stellar explosions. But not all supernovae are of the same kind. Some of them are related to the sudden collapse of very massive stars at the end of their lives, the so-called type II events. Other supernovae are produced in a process of cataclysmic interaction between the members of a binary stellar system. The most important of this group are the so-called type Ia supernovae, explosions of white dwarf stars. They play an important role as cosmological distance indicators and have led to the discovery of the accelerated expansion of the Universe.

Among the most important unsolved questions about supernovae are how the explosion actually proceeds and whether accretion occurs from a companion or by the merging of two white dwarfs.

Tycho Brahe's supernova of 1572 (SN 1572) is thought to be one of the best candidates for a type Ia supernova in the Milky Way. The proximity of the SN 1572 remnant has allowed detailed studies, such as the possible identification of the binary companion, and provides a unique opportunity to test theories of the explosion mechanism and the nature of the progenitor.

The authors report an optical spectrum of Tycho Brahe's supernova near maximum brightness, obtained from a scattered-light echo more than 400 years after the direct light from the explosion swept past the Earth.

The spectroscopic analysis of the light echo showed the signatures of the atoms present when the supernova exploded. The resulting spectrum of light revealed silicon but no hydrogen, telltale signs that Tycho's supernova resulted from a type Ia explosion of a white dwarf star. All supernovae of type Ia show practically the same intrinsic luminosity and, for this reason, they are used as cosmological probes to measure the large distances among the galaxies in the vastness of the Universe.

Oliver Krause said: 'We show that the supernova of 1572 was a standard type Ia event that shows indications of an aspherical explosion. This puts new stringent constraints on explosion models, which can be now compared in great detail to observations of both the explosion and the remnant at the same time.'

The new data include an exact classification of the event, and new estimates of the power of the explosion, its geometry, and an independent measure of its distance. Authors' observations reveal that the supernova, now known as SN 1572, belongs to the majority class of normal type Ia supernovae.