Could A Faraway Supernova Threaten Earth?

Supernovae, just like any other explosions, are really cool. But, just like any other explosion, it’s preferable to have them happen at a good distance. The star T Pyxidis, which lies over 3,000 light-years away from the Earth in the constellation Pyxis, was previously thought to be far enough away that if anything happened in the way of a supernova, we’d be pretty safe.

According to Edward Sion, Professor of Astronomy and Physics at Villanova University, T Pyxidis may be in fact a “ticking time bomb,” and potential threat to the Earth if it were to go supernova, which it may do sometime in the future, though very, very far in the future on our timescale: by Scion’s calculations, at least 10 million years.

Sion presented his findings at the American Astronomical Society Meeting in Washington, D.C. earlier today. T Pyxidis, which lies in the constellation Pyxis, is what is called a recurring nova. The star, which is a white dwarf, accretes gas from a companion star. As the amount of matter increases in the white dwarf, it occasionally builds up to the point where there is a runaway thermonuclear reaction in the star, and it ejects large quantities of material.

T Pyxidis has had five different outbursts over the course of observations of the star. It was the American Association of Variable Star Observers’ variable star of the month in April, 2002.  The first was in 1890, followed by another outburst in 1902 (these two were discovered much later on photographic plates in the Harvard plate collection). The next three were in 1920, 1944 and 1967. Its average for outbursts is about 19 years, but there hasn’t been one since the 1966 brightening.

The distance estimate to T Pyxidis, revised to 3,260 light-years from the previously estimated distance of 6,000 light-years has prompted a reconsideration of the details about the white dwarf. Hubble images that have been taken of the star would then have to be re-examined so as to revise the amount of mass the star is expected to be ejecting.

If the recurring novae are ejecting enough material, then the white dwarf would stay small enough to continue to go through the phase of recurring novae. However, if the shells of gas repeatedly ejected by the star do not carry enough mass away, it would eventually build up to pass the Chandrasekhar limit – 1.4 times the mass of the Sun – and become a Type Ia supernova, one of the most destructive events in our Universe.

Sion concluded the presentation with the statement (shown here on his last powerpoint slide) that “A Type Ia supernova exploding within 1000 parsecs of Earth will greatly affect our planet”

A supernova within 100 light-years of the Earth would likely be a catastrophic event for our planet, but something as far out as T Pyxidis may or may not damage the Earth. One of the journalists in attendance pointed out this possibility during the questions session and Sion said that the main danger lies in the amount of X-rays and gamma rays that stream from such an event, which could destroy the protective ozone layer of the Earth and leave the planet vulnerable to the ultraviolet light streaming from the Sun.

There remains some doubt as to whether T Pyxidis will go supernova at all. There is a good treatment of this subject in “The Nova Shell and Evolution of the Recurrent Nova T Pyxidis” by Bradley E. Schaefer et al. on Arxiv.

If you’re worried about the dangers of exploding stars, you should check out this video by Phil Plait, the Bad Astronomer. He’ll calm you down.

Source: AAS Press Conference on USTREAM, Space.com