ETA CAR (Eta Carinae). "Magnificent;" "Grandest in the Galaxy of stars"; "None like it:" so would go critical reviews were Eta Car a stage actor rather than a star. Hyperbole? Yes there are other stars that are similar, but none that can really claim ascendancy. As we speak (so to speak), Eta Carinae, in Carina (the Keel), one of the triparted sections of Argo (the Ship), is of modest fifth magnitude. It was not ever so. In the 1840s, this grand star of the deep southern hemisphere was the second brightest of the sky, beating out Canopus (the current luminary of Carina) and nearly equalling Sirius, the result of a great outburst. Even in the 1600s and 1700s the star glowed between second and fourth magnitude. And here it is around fifth, but without a real magnitude base, as it is now slowly brightening from the sixth to seventh magnitude it had sunk to after the outburst. Eta Car is one of the rare "hypergiants," a star so luminous that mere "supergiant" does it no justice. Others in this rarefied category include P Cygni and Rho Cas, though neither makes it to the level of Eta. Actually of hot and blue class B (probably B0 with a temperature around 30,000 Kelvin), the star's seeming current faintness is an illusion of sorts. Around 1840, it ejected an expanding bi-lobed cloud that had near the mass of the Sun and that is now a light year in diameter (a smaller ejection following in 1890). Dust condensing in the cloud absorbs optical starlight and re-radiates it in the infrared where the human eye cannot see it. The stellar luminosity is difficult to know as there is no good way of getting the distance. From its surroundings (it is part of a dense cluster called Tr 16), we estimate a distance of an amazing 8000 light years. If that be the case, Eta Carinae shines with the luminosity of as much as five million Suns! From the luminosity, we estimate a mass of as much as 100 times solar, close to the upper allowed limit (much above which stars tear themselves apart by their own radiation). A couple other hypergiants could be brighter, but their distances are not known well either, so let Eta Car be King, at least for now. It is in fact the epitome of the class of "luminous blue variables," or "LBVs," of which there are but a handful known. Or is it two stars? Strong spectral evidence suggests a companion with a 5.6 year period. The suggestion is enhanced by a similar periodicity in X-ray radiation that is caused by the powerful winds from the two stars colliding, the collision becoming stronger when the pair approach each other on elliptical orbits. If this is the case, Eta Car may be an 80-60 solar mass pair. Whether one star or two, given its mass and clear instability, Eta Car is one of the sky's prime candidates to become a supernova, the result of nuclear burning to an iron core, which collapses and blows the star apart. Indeed, it may become a "hypernova," one of a kind that produces a powerful burst of gamma rays similar to those seen coming from ultradistant galaxies billions of light years away. Keep your eye to the southern sky, for (at least on an astronomical time-scale) Eta Car is not long for this Galaxy. Thanks to the several people who suggested this star. Eta Car was selected by Jim Kaler as one of the sky's "Hundred Greatest Stars."
Written by Jim Kaler. Return to STARS.