|Designation|| η Carinae|
|Right ascension||10h 45m 03.591s|
|Declination||−59o 41′ 04.26″|
|Type of object||Hypergiant star|
|Magnitude||Variable, as of 2007 around +5|
|Radial velocity||−25.0 km/s|
|Proper motion|| RA: −7.6 mas/yr|
Dec.: 1.0 mas/yr
Eta Carinae is a massive luminous blue variable star and probable binary. The star is one of the most massive and luminous known in the galaxy and is located in the constellation Carina. In addition to its historical variation in luminosity and incredible mass, Eta Carinae is an example of a star that may go supernova at any time.
History of Observation
Edmund Halley is the first person known to catalog Eta Carinae, when in 1677, he classified it as a 4th magnitude star. The star brightened over time, reaching a peak in 1730 before fading once again to the original magnitude by 1782. Observers noted the star again started to brighten in 1820. Eta Carinae reached a maximum brightness of magnitude -0.8 in 1843, temporarily becoming the second brightest star in the night sky, only Sirius being brighter. This was the result of the "Great Eruption" that expelled some 10 solar masses worth of material from the star's polar regions at 700 km/s. Today, the expanding gas appears as two large, grayish lobes of material moving away from the star's poles, and is nicknamed the Homonculus Nebula, which has since expanded to a diameter of 6.4 trillion kilometers since. This event was unusual as it released as much light as a supernova explosion, but the star itself remained intact, making it a supernova impostor event. After 1843, the star again faded to around a 7th magnitude star. Since around 1940, the star has been increasing in brightness, and as of 2007 is a 5th magnitude star, making it again visible to the unaided eye.
Eta Carinae itself is located within the massive Carina Nebula, which surrounds many similar extremely bright and young stars.
Eta Carinae A
Eta Carinae A appears to be a B class hypergiant (luminosity class 0) of spectral type B0-1 with an absolute barometric magnitude of -12, making it some 4-5 million times as luminous as our own Sun. The surface temperature is estimated around 20,000 to 30,000 K. The star is so large that it has a diameter larger than the orbit of Jupiter at 11 AU. This diameter isn't exact though, as the star is rapidly losing its outer layers of gas at a rate of to some 500 Earth masses every year. This makes the boundary between the outer edge of the star and the surrounding stellar wind difficult to determine. In addition to shedding mass at an extremely high rate, the star may be rotating at some 90 percent of the star's theoretical breakup speed, and has a pronounced oblate shape.
The exact mass of the star is unknown, but estimated around 100 solar masses. It was thought to be around 120 solar masses before evidence of a binary companion was discovered. The star could already be approaching the end of its life, exploding in a supernova.
Eta Carinae B
In 2005 astronomers using NASA's Far Ultraviolet Spectroscopic Explorer (FUSE) satellite, believe they located a companion star to Eta Carinae that, while dimmer overall, is actually hotter and emits even more brightly in the ultraviolet. Some astronomers disputed this, believing that only the dense stellar winds of Eta Carinae A were being detected. In February 2008, astronomers at the European Space Agency found evidence of x-rays generated by what is believed to be the shockwaves created by the collision of stellar winds from Eta Carinae A and its companion. It is also believed that the instability and outbursts of Eta Carinae A is caused by the companion. Further evidence of the existence of Eta Carinae B was inferred from a repeating pattern of changes in visual, X-ray, radio, and infrared light in a 5.5 year cycle.
If the companion, Eta Carinae B, exists, it would be around 60 solar masses (With Eta Carinae A being around 80 solar masses). The two stars would be orbiting each other once every 5.53 years with a mean orbital distance of 11 AU, but approach as close as 5 AU and as far apart as 30 AU. Because of the distance of the system from our vantage point, it is not possible for telescopes to resolve the two stars separately.