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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 hypernovaA hypernova is a theoretical type of supernova produced when exceptionally large stars collapse at the end of their lifespan. In a hypernova, the core of the star collapses directly into a black hole and two extremely energetic jets of plasma are emitted 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 starA binary star system consists of two stars both orbiting around their barycenter. The term "binary star" was apparently first coined by Sir William Herschel in 1802 to designate "a real double star the union of two stars that are formed together in one sy, the two stars orbiting each other with a period of around 5.5 years.
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