Stars in the mass class of Eta Carinae, with more than 100 times the mass of the Sun, produce more than a million times as much light as the Sun. They are quite rare — only a few dozen in a galaxy as big as the Milky Way. They are assumed to approach (or potentially exceed) the Eddington limit, i.e., the outward pressure of their radiation is almost strong enough to counteract gravity. Stars that are more than 120 solar masses exceed the theoretical Eddington limit, and their gravity is barely strong enough to hold in their radiation and gas.

Eta Carinae's chief significance for astrophysics is based on its giant eruption or supernova imposter event seen around 1843. In a few years, Eta Carinae produced almost as much visible light as a supernova explosion, but it survived. Other supernova imposters have been seen in other galaxies, for example the false supernovas SN 1961v in NGC 1058 and SN 2006jc in NGC 4904, which produced a false supernova in October 2004. Significantly, SN 2006jc was destroyed in a supernova explosion two years later, on October 9, 2006. The supernova impostor phenomenon may represent a surface instability or a failed supernova. Eta Carinae's giant eruption was the prototype for this phenomenon, and after 160 years the star's internal structure has not fully recovered.

One remarkable aspect of Eta Carinae is its changing brightness. It is currently classified as a luminous blue variable (LBV) double star due to peculiarities in its pattern of brightening and dimming.
Eta Carinae sometimes has large outbursts, the last one just around its brightness maximum, in 1841. After 1843 Eta Carinae faded away, and between about 1900 and 1940 it was only of the 8th magnitude: invisible to the naked eye. Eta Carinae suddenly and unexpectedly doubled its brightness in 1998–1999. As of 2007 it can be easily seen with the naked eye, because it is brighter than magnitude 5.
The formerly clockwork regularity of the dimming was upset in 2008. Following the 5.52 year cycle, the star would have started its next dimming in January 2009, but the pattern was noticed starting early in July 2008 by the southern Gemini Observatory in La Serena, Chile. Spectrographic measurements showed an increase in blue light from superheated helium, which was formerly assumed to occur with the wind shock.

Very large stars like Eta Carinae use up their fuel very quickly because of their disproportionately high luminosities. Eta Carinae is expected to explode as a supernova or hypernova some time within the next million or so years. As its current age and evolutionary path are uncertain, it could explode within the next several millennia or even in the next few years. LBVs such as Eta Carinae may be a stage in the evolution of the most massive stars; the prevailing theory now holds that they will exhibit extreme mass loss and become Wolf-Rayet Stars before they go supernova, if they are unable to hold their mass to explode as a hypernova.

Sources: nasa.gov