Eta Carinae

Eta Carinae is a star located 7,500 light-years away from Earth in the constellation Carina. Previously a 4th-magnitude star, it brightened in 1837 to become brighter than Rigel, marking the start of its so-called “Great Eruption”. It became the second-brightest star in the sky between 11 and 14 March 1843 before fading well below naked eye visibility after 1856. In a smaller eruption, it reached 6th magnitude in 1892 before fading again. It has brightened consistently since about 1940, becoming brighter than magnitude 4.5 by 2014. Eta Carinae is a binary star system. It is expected to explode as a supernova in the astronomically near future. This is the only star known to produce ultraviolet laser emission. The secondary star is hot and also highly luminous, probably of spectral class O, around 30–80 times as massive as the Sun. The system is heavily obscured by the Homunculus Nebula, material ejected from the primary during the Great Eruption. It is a member of the Trumpler 16 open cluster within the much larger Carina Nebula.

## Name

The star was also known by the Bayer designations Eta Roboris Caroli, Eta Argus or Eta Navis. In 1751 Nicolas-Louis de Lacaille gave the stars of Argo Navis and Robur Carolinum a single set of Greek letter Bayer designations within his constellation Argo, and designated three areas within Argo for the purposes of using Latin letter designations three times over. Eta fell within the keel portion of the ship which was later to become the constellation Carina. It was not generally known as Eta Carinae until 1879, when the stars of Argo Navis were finally given the epithets of the daughter constellations in the *Uranometria Argentina* of Gould.

## Discovery

There is no reliable evidence of Eta Carinae being observed or recorded before the 17th century, although Dutch navigator Pieter Keyser described a fourth-magnitude star at approximately the correct position around 1595–1596, which was copied onto the celestial globes of Petrus Plancius and Jodocus Hondius and the 1603 *Uranometria* of Johann Bayer. Frederick de Houtman's independent star catalogue from 1603 does not include Eta Carinae among the other 4th-magnitude stars in the region. The earliest firm record was made by Edmond Halley in 1677 when he recorded the star simply as *Sequens* (i.e. “following” relative to another star) within a new constellation Robur Carolinum. His *Catalogus Stellarum Australium* was published in 1679.

## The Great Eruption

In 1827, Burchell specifically noted Eta Carinae's unusual brightness at 1st magnitude, and was the first to suspect that it varied in brightness. John Herschel, who was in South Africa at the time, made a detailed series of accurate measurements in the 1830s showing that Eta Carinae consistently shone around magnitude 1.4 until November 1837. On the evening of 16 December 1837, Herschel was astonished to see that it had brightened to slightly outshine Rigel. This event marked the beginning of a roughly 18-year period known as the Great Eruption.

Eta Carinae was brighter still on 2 January 1838, equivalent to Alpha Centauri, before fading slightly over the following three months. Herschel did not observe the star after this, but received correspondence from the Reverend W.S. Mackay in Calcutta, who wrote in 1843, “To my great surprise I observed this March last (1843), that the star Eta Argus had become a star of the first magnitude fully as bright as Canopus, and in colour and size very like Arcturus.” Observations at the Cape of Good Hope indicated it peaked in brightness, surpassing Canopus, over 11 to 14 March 1843 before beginning to fade, then brightened to between the brightness of Alpha Centauri and Canopus between 24 and 28 March before fading once again. For much of 1844 the brightness was midway between Alpha Centauri and Beta Centauri, around magnitude +0.2, before brightening again at the end of the year. At its brightest in 1843 it likely reached an apparent magnitude of −0.8, then −1.0 in 1845. The peaks in 1827, 1838 and 1843 are likely to have occurred at the periastron passage—the point the two stars are closest together—of the binary orbit. From 1845 to 1856, the brightness decreased by around 0.1 magnitudes per year, but with possible rapid and large fluctuations.