Eight thousand light years from the Earth, the star Eta Carinae is in its death throes. This is a binary star system with a luminous blue variable star in the last stages of its life, which could explode into a supernova within the next several centuries – or even the next several years. Fortunately, when it does, the risk of a gamma ray burst hitting the Earth is low.
– Discovery –
Eta Carinae is over a hundred times the mass of our own Sun, and is a million times brighter. This makes it one of the biggest stars in the Milky Way Galaxy – in fact, according to current astronomical theory, it’s also nearly as big as a star is physically capable of growing, before its own gravity becomes too weak to hold it together.
One of the consequences of being at this upper limit may be the star’s bizarre variable behaviour: it dims and brightens very noticeably. During the early 1900s, after a century’s worth of regular observations, the star dimmed to the point that star-gazers needed telescopes to find it. It then brightened again. In general, the observations reveal that Eta Carinea has a roughly five-year cycle of dimming and brightening, except that some cycles somehow get interrupted, causing either extreme brightening or extreme dimming.
– The Danger –
In 1843, Eta Carinae blew up – or so it seemed at the time. Nova explosions were not new to astronomers: in fact, the brightest supernova ever detected lit up the skies as bright as the moon for several months in 1006 A.D. What was odd about the Eta Carinae explosion that year was that, a few years later, Eta Carinae was still present – and shining more or less as normal. Scientists now recognize this event as a “supernova impostor,” a rare category of nova-like explosion which blows off part of a star but leaves the rest shining, intact.
That averted the danger for the moment, but only for the moment. Eta Carinae’s behaviour indicates that it is probably coming to the end of its available supply of hydrogen, which stars fuse into helium and thus produce energy and light. We currently lack the tools and the exact theoretical knowledge to calculate when this will happen. However, some time within the next million years (and almost certainly much less), Eta Carinae will explode again, this time as a full supernova. When this will happen, not whether it will, is the real question.
At several thousand light years’ distance, the risk of exposure to Earth from the supernova should be minimal. The radiation from the explosion itself will probably not make it past the ozone layer, so that the harm to life on the surface would be limited. A more serious risk from supernovae are gamma ray bursts – short, incredibly powerful bursts of gamma ray radiation which shoot out of a star’s poles as it explodes. A nearby gamma ray burst could be lethal to life on Earth. However, the best estimates are that neither pole faces Earth, so we should be unharmed.
The more chilling thought, perhaps, is that thanks to the problems of relativity and the speed of light, Eta Carinae may already have gone supernova. The light we are seeing took eight thousand years to reach us – so if Eta Carinae blew up a thousand years ago, we wouldn’t even know about it for many millennia.
