Eta Carinae
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 lightyears). 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.
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