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Astronomers finger culprit in 1572 supernova

Star spotted fleeing the scene

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An international team of astronomers has identified the surviving companion star to the 1572 supernova explosion witnessed by the Danish astronomer Tycho Brahe. This looks to be the first piece of direct, physical evidence that supports the long-standing theory of how this particular type of supernova explosion actually occurs.

Supernovae are classified according to the elements that can be identified in their spectra. Broadly, they are split into Type I and Type II. Type II has a so-called Balmer line, a spectral line indicating a particular transition within the hydrogen atom, while Type I does not. Each main type is further divided — Type I into a, b or c classifications. Type Ia supernovae are characterised by strong silicon emission lines.

Type Ia supernovae, of which the Tycho Brahe explosion is an example, occur when a white dwarf star begins to suck in matter from a companion star, usually when it reaches its red-giant stage. This continues until the white dwarf star becomes so massive that its internal pressure is no longer sufficient to support the outer layers, and the star collapses in on itself. The threshold is known as the Chandrasekar limit, after the Indian physicist who first calculated it.

In the case of the 1572 explosion, when the white dwarf exploded, the companion star was released from its gravitational influence like a stone being thrown by a sling. It went hurtling off into space with its orbital velocity being retained. This is what alerted the astronomers to its existence.

This new research, published today in the journal Nature, identifies the likely companion star to the now-exploded white dwarf. The team studied the star for seven years, using a variety of telescopes, including Hubble, to track how it is moving against the background. Their results show that it is moving three times faster than anything else in the region.

Pilar Ruiz-Lapuente of the University of Barcelona, led the research. She notes that this is the first evidence of any specific type of companion star: "Here we have identified a clear path: the feeding star is similar to our sun, but slightly older," she said.

However, there are alternative suggestions, both as to how the star could be moving so fast, and of how a type Ia supernova can occur. It is possible that the star is falling to the area from another region known as the galactic halo. Type Ia supernovae can also be generated by a double white dwarf system in which the stars fall into one another, and their combined mass sends them over the Chandrasekar limit. ®

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