Eta Carinae is one of the most luminous and massive stellar systems known in our Milky Way galaxy. It consists of at least two stars with a combined mass estimated to be more than 150 times that of our Sun. These stars are orbiting each other at a distance of about five astronomical units (AU, the Earth-Sun distance) and take 5.54 years to complete one orbit. The primary star, Eta Carinae A, is an unstable supergiant that has undergone multiple outbursts during which it became one of the brightest stars in the night sky. The secondary star, Eta Carinae B, is a much smaller companion that orbits closer to the primary star.
Eta Carinae is located about 7500 light-years away from Earth in the constellation Carina (The Keel). It was first catalogued by Edmond Halley in 1677 but was not widely known until 1843 when it underwent a major outburst that made it brighter than Rigel (the brightest star in Orion). This event, called the Great Eruption, ejected at least 10–30 times the mass of our Sun into space and created two large lobes of material (now known as the Homunculus Nebula) around the binary system. The Great Eruption was likely caused by instability in the primary star’s outer layers due to its high mass and luminosity.
Since 1843, Eta Carinae has been slowly fading but remains one of the most interesting objects for astronomers to study. Its proximity and extreme brightness make it an ideal target for telescopes such as NASA’s Hubble Space Telescope and ESO’s Very Large Telescope which have been used to study its structure and dynamics in detail. In 2009, new observations made with Hubble showed that Eta Carinae may be on the verge of another outburst similar to what occurred in 1843. If this happens, it will provide astronomers with a unique opportunity to study how such events unfold and affect stellar evolution.