Eta
Carinae is a very large (100-150 times as
much mass as the Sun) and
bright (about 4 million times as bright) star, a luminous blue
variable, in the constellation Carina (right ascension 10 h 45.1 m,
declination -59°41m). The star is surrounded by a large, bright
nebula, known as the Eta Carinae Nebula, the Keyhole Nebula, or NGC3372
One remarkable
aspect of Eta Carinae is its changing brightness. When
it was first catalogued in 1677 by Edmond Halley, it was of the 4th
magnitude, but later it brightened, reaching its greatest brightness in
April 1843, when with a magnitude of -0.8 it was the second brightest
star in the sky (after Sirius), despite its enormous distance
(7,000-10,000 light-years). After that, it faded away, and between
about 1900 and 1940 it was only of the 8th magnitude, and thus not
visible to the unaided eye. Currently (2002) it is of magnitude 5-6,
having suddenly and unexpectedly doubled its brightness in 1998-1999.
Eta Carinae
sometimes has large outbursts, the last one just around its
brightness maximum, in 1841. The reason for these outbursts is not yet
known. The most likely possibility is believed to be that they are
caused by built-up radiation pressure from the star's enormous
luminosity.
Very large stars
like Eta Carinae use up their fuel very fast because
of their high luminosity, and the star is expected to go supernova or
hypernova within about 1 million years of its first appearance (the
total lifetime of the Sun is estimated at 10,000 million years, half of
which has passed now).
Recent
observations seem to indicate that eta Carinae is actually a
binary star, the two stars orbiting each other with a period of around
5.5 years.
