Eta Carinae is a hypergiant binary star in the Carina constellation, consisting of two close orbiting companion stars with a combined mass of 110 solar masses. It experienced a supernova imposter event in 1843 and is surrounded by a gas cloud called the Homunculus Nebula. It is only 2.6 million years old and could erupt as a hypernova in the next 10,000-20,000 years.
Eta Carinae is a striking hypergiant binary star located 7,500 to 8,000 light-years from Earth in the constellation Carina, the Keel. It is one of the brightest stars known: either the fourth or the third, after LBV 1806-20 (according to some models), Cyg OB2-12 and HD 93129A. The Pistol Star, once believed to be the brightest star in the Galaxy, is actually only the 13th. Like many of the other more massive stars, Eta Carinae is a luminous blue variable (LBV).
Eta Carinae consists of two close orbiting companion stars, Eta Carinae A and Eta Carinae B. The former contains 80 solar masses, the latter 30. They maintain an eccentric orbit around each other, reaching up to 5 AU and separating until to 30 AU. When the primary star formed, it may have contained up to 200 solar masses, but the more massive a star, the more energy it produces in its core, ejecting substantial portions of its outer envelope as solar wind. Because many fusion reactions have rates that accelerate exponentially as temperature and density increase linearly, fusion reactions in the core of a hypergiant star are fast. Eta Carinae is only about 2.6 million years old and could only last another 10,000 – 20,000 years before it catastrophically erupts as a hypernova.
In 1843, Eta Carinae increased dramatically in brightness, becoming the second brightest star in the sky after Sirius, even though Eta Carinae is about a thousand times farther away. During this period it was the brightest star in the galaxy. This has been called a supernova imposter event, because it looked like a supernova, but the star survived. This event is often called the Great Eruption. After the Great Eruption, the star faded to become invisible to the naked eye, but in 1999 its brightness unexpectedly doubled, so it is now visible again.
Eta Carinae is surrounded by a large gas cloud ejected during the Great Eruption. Because of its vague resemblance to the shape of a man, it is called the Homunculus Nebula.
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