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."