Eta Carinae or Eta Car is a Supernova Imposter near the Southern Cross. There are ominous signs that this Luminous Blue Variable could become a supernova soon. Nobody can tell when this will be nor what impact this would have upon life on Earth.
To the ancients it was the star Foramen and it was the flickering lamp hanging on the stern of the mythical vessel Argo.
Eta Car brightened in 1835 and again erupted in 1843, creating the gigantic double-lobed brown cloud that is now one of the most famous of the Hubble images. After several more cycles of brightening and fading to invisibility Eta Car suddenly started to get brighter in 1998. By 2007 it had dimmed again.
The scary thing about Eta Car is that it is the biggest star known to astronomy, being 120 times heavier than our own Sun. It is more than 7,500 light-years away but even that may be too close for comfort.
“Eta” is hardly a reliable appellation for this star. The Greek letter suggests that it is the seventh brightest star in the constellation Carinae, as it was in 1677 when first catalogued. However it has sometimes become the brightest star in the constellation and at other times it has been too dim to see. Some time soon Eta Car might become the brightest star in the night sky. Yet even as it prepares to die, Eta Car is producing vast quantities of the elements that could one day form solid planets with perhaps Earth-like atmospheres, water – and life.
The extreme variations in luminosity and the huge amount of matter being boiled off Eta Car’s surface seems to indicate that something may happen to Eta Car sooner rather than later. It is the pressure of outbound photons that keeps stars like Eta Car from collapsing under their own weight. A supernova takes place when a giant star has used up most of its hydrogen fuel and starts to “burn” other elements, at which stage the star implodes and hotly consumes much of its remaining fuel all at once, detonating with a blast that illuminates entire galaxies. Eta Carina could be one of the biggest and the event is certain, be it 7,500 years ago or a million years in the future.
Most stars are binaries. X-rays and UV rays show that there is a small variation in a 5.5 year cycle. It is possible that Eta Car is also a binary star. If so then there may be a very tight elliptical orbit involved and when the winds of the two stars meet, extraordinary events occur. Alternatively it is one star with a destructive wobble.
In October 2004 a far more distant star brightened in galaxy UGC 4904 and caught the attention of astronomers on Earth. The outburst faded for two years. Then in October 2006 star SN 2006jc blossomed into a true supernova. At a distance of 77 million light years it was much further away, but it reminded astronomers that they cannot predict supernovae.
The experts differ on the degree and extent of the danger. Some say Eta Car could go supernova “in the next few years” Others assert there is “material left to burn” and that Eta Car will not go supernova for thousands of years. Some say we would be “protected from gamma rays by the atmosphere”. Others are not so sure. Intense bursts of gamma rays emerge in a cone from the poles of Eta Car. Fortunately neither pole is pointing in our general direction. However the detonation of a star is a messy business and it can be seen that Eta Car is firing out “bullets” in many directions. A very intense burst of gamma rays striking our atmosphere could generate so many muons that there would be no protection, even deep underground.In such an event more than half the world would surely perish.
Unfortunately much of the action is hidden behind those billowing brown clouds. When Eta Carinae finally does go supernova we are not likely to receive much warning. Perhaps that is just as well.
