What is Eta Carinae?

Eta Carinae is a remarkable hypergiant binary star located 7,500 to 8,000 light-years from the Earth in the constellation Carina, the keel. It is one of the most luminous 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 thought to be the most luminous star in the Galaxy, is actually only the thirteenth. Like many of the other most massive stars, Eta Carinae is a luminous blue variable (LBV).

Eta Carinae consists of two closely orbiting companion stars, Eta Carinae A and Eta Carinae B. The first contains 80 solar masses, the second 30. They maintain an eccentric orbit around each other, coming as close as 5 AU and separating as far as 30 AU. When the primary star formed, it may have contained as much as 200 solar masses, but the more massive a star is, the more energy it produces in its core, ejecting substantial portions of its outer envelope in the form of solar wind. Because many fusion reactions have speeds that accelerate exponentially as the temperature and density are increased linearly, the fusion reactions in the core of a hypergiant star are quick. Eta Carinae is only about 2.6 million years old, and may only last another 10,000 - 20,000 years before it explodes catastrophically as a hypernova.

In 1843, Eta Carinae increased in brightness tremendously, becoming the second brightest star in the sky after Sirius, even though Eta Carinae is about a thousand times further away. During this time it was the brightest star in the galaxy. This was 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 doubled unexpectedly, so now it is visible again.

Eta Carinae is surrounded by a large cloud of gas expelled during the Great Eruption. Because of its vague resemblance to the shape of a man, it is called the Homunculus Nebula.