Today, Eta Carinae, in the keel of the great ship Argo Navis, is a modest star of magnitude 6.21. But it was not always so. As the result of a great outburst, Eta Carinae was once the second brightest star in the sky.
In the 1840s, η Car was brighter than Canopus, and nearly equalled Sirius. In the 1600s and 1700s ,the star glowed between 2nd and 4th magnitude. It is now slowly brightening from 6th or 7th magnitude, where it had sunk after the outburst in the 1840s.
Properties
Of spectral class B0, with a hot blue surface around 30,000 K, η Car's seeming current faintness is an illusion of sorts. Around 1840, the star ejected an bi-lobed cloud containing nearly a solar mass of material; a smaller ejection followed in 1890. The expanding cloud, called the "Homunculus Nebula", is now a light year in diameter. Dust condensing in the cloud absorbs optical starlight and re-radiates it in the infrared where the human eye cannot see it.
The star's luminosity is difficult to know precisely as there is no good way of determining its distance. It is part of a dense open cluster called Trumpler 16, located within the Eta Carinae nebula, NGC 3372. This is a large, bright, star-forming region that produced a number of very massive stars, including η Car itself. If at the cluster's distance of 7500 - 8000 light years, then η Car shines with the luminosity of five million Suns!
Eta Car is a "hypergiant," a star so luminous that mere "supergiant" status is inadequate to describe it. In fact, η Car has the highest confirmed mass and luminosity of any star that has been studied in detail. Eta Carinae is the prototype of the "luminous blue variables," or "LBVs," of which only a handful are known. Others in this rare category include P Cygni and S Doradus, though neither makes it to the level of η Car.
From its luminosity, η Car has an estimated mass as much as 100 to 150 times the Sun's. This is close to the maximum theoretical "Eddington limit" of 120 suns, above which stars tear themselves apart by their own radiation.
Companion
Spectral evidence strongly suggests a companion with a 5.52-year period. The suggestion is enhanced by a similar periodicity in X-ray radiation caused by the collision of powerful winds from the two component stars, becoming stronger when the pair approach each other on their elliptical orbits. If this is the case, η Car may be a 60-80 solar mass pair.
Evolution
Whether one star or two, given its mass and clear instability, η Car is one of the sky's prime candidates to become a supernova, or even a "hypernova", similar to those observed in other galaxies. Recent possible η Car supernova analogues observed in other galaxies include SN 2006jc in UGC 4904, and SN 2006gy in NGC 1260. On an astronomical timescale, η Car is not long for this Galaxy. It is expected to explode as a supernova or hypernova some time within the next million or so years, possibly in our lifetimes, or even in the next few years.
Even at a distance of 7,500 light years away, an Eta Carinae hypernova could affect the Earth's upper atmosphere, ozone layer, spacecraft, and astronauts; it would be bright enough to see during the daytime. The hypernova would probably eject a gamma ray burst along both poles of its rotational axis, similar to the γ ray bursts seen coming from galaxies billions of light years away. This catastrophic burst would probably not hit Earth, though, because η Car's rotational axis does not point toward us.
[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]