Eta Carinae (η Carinae, abbreviated to η Car), formerly known as η Argus, is a stellar system containing at least two stars with a combined luminosity greater than five million times that of the Sun, located around away 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 night sky between 11–14 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.
At declination −59° 41′ 04.26″, η Carinae is circumpolar from locations on Earth south of latitude 30°S (for reference, the latitude of Johannesburg is 26°12′S), and is not visible north of about latitude 30°N, just south of Cairo (which is at a latitude of 30°02′N).
The two main stars of the η Carinae system have an eccentric orbit with a period of 5.54 years. The primary is an extremely unusual star, similar to a luminous blue variable (LBV). It was initially , of which it has already lost at least , and 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, which consists of material ejected from the primary during the Great Eruption. It is a member of the open cluster, itself embedded in the much larger Carina Nebula.
Although unrelated to the star and nebula, the weak Eta Carinids meteor shower has a radiant very close to η Carinae.
Observational history η Carinae was first recorded as a fourth-magnitude star in the 16th or 17th century. It became the second-brightest star in the sky in the mid-19th century, before fading below naked-eye visibility. During the second half of the 20th century, it slowly brightened to again become visible to the naked eye, and by 2014 was again a fourth-magnitude star.
Discovery and naming There is no reliable evidence of η 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 η 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 star was also known by the Bayer designations η Roboris Caroli, η Argus, or η 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. The letter η fell within the keel portion of the ship which was later to become the constellation Carina. It was not generally known as η Carinae until 1879, when the stars of Argo Navis were finally given the epithets of the daughter constellations in the Uranometria Argentina of Gould.
η Carinae is too far south to be part of the mansion-based traditional Chinese astronomy, but it was mapped when the Southern Asterisms were created at the start of the 17th century. Together with s Carinae, λ Centauri and λ Muscae, η Carinae forms the asterism (Sea and Mountain). Its modern Chinese name is (, ). R. H. Allen's 1899 book Star Names claimed the Chinese name Tseen She (, "Heaven's altar") for η Carinae, but this is actually the name of an asterism. The Latin name Foramen ("opening", "hole") is occasionally seen; it may have originated in a 1923 astrology book.
Halley gave an approximate apparent magnitude 4 at the time of discovery, which has been calculated as magnitude 3.3 on the modern scale. The handful of possible earlier sightings suggest that η Carinae was not significantly brighter than this for much of the 17th century. Further sporadic observations over the next 70 years show that η Carinae was probably around 3rd magnitude or fainter, until Lacaille reliably recorded it at 2nd magnitude in 1751. It is unclear whether η Carinae varied significantly in brightness over the next 50 years; there are occasional observations such as William Burchell's at 4th magnitude in 1815, but it is uncertain whether these are just re-recordings of earlier observations.
Great Eruption
In 1827, Burchell specifically noted η 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 η 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.
η Carinae was brighter still on 27 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 η 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, from 11 to 14 March 1843, then began 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.
In their oral traditions, the Boorong clan of the Wergaia people of Lake Tyrrell, north-western Victoria, Australia, told of a reddish star they knew as Collowgullouric War "Old Woman Crow", the wife of War "Crow" (Canopus).Or more accurately gala-gala gurrk waa, with the onset of gurrk "woman" lost in Stanbridge. In 2010, astronomers Duane Hamacher and David Frew from Macquarie University in Sydney showed that this was η Carinae during its Great Eruption in the 1840s. From 1857, the brightness decreased rapidly until it faded below naked-eye visibility by 1886. This has been calculated to be due to the condensation of dust in the ejected material surrounding the star, rather than to an intrinsic change in luminosity.
Lesser Eruption A new brightening started in 1887, peaked at about magnitude 6.2 in 1892, then at the end of March 1895 faded rapidly to about magnitude 7.5. Although there are only visual records of the 1890 eruption, it has been calculated that η Carinae was suffering 4.3 magnitudes of visual extinction due to the gas and dust ejected in the Great Eruption. An unobscured brightness would have been magnitude 1.5–1.9, significantly brighter than the historical magnitude. Despite this, it was similar to the first one, even almost matching its brightness, but not the amount of material expelled.
Twentieth century Between 1900 and at least 1940, η Carinae appeared to have settled at a constant brightness of around magnitude 7.6, but in 1953 it was noted to have brightened again to magnitude 6.5. The brightening continued steadily, but with fairly regular variations of a few tenths of a magnitude.
In 1996, the variations were first identified as having a 5.52 year period, later measured more accurately at 5.54 years, leading to the idea of a binary system. The binary theory was confirmed by observations of radio, optical and near-infrared radial velocity and line profile changes, referred to collectively as a spectroscopic event, at the predicted time of periastron passage in late 1997 and early 1998. At the same time there was a complete collapse of the X-ray emission presumed to originate in a colliding wind zone. The confirmation of a luminous binary companion greatly modified the understanding of the physical properties of the η Carinae system and its variability.
A sudden doubling of brightness was observed in 1998–99 bringing it back to naked-eye visibility. During the 2014 spectroscopic event, the apparent visual magnitude became brighter than magnitude 4.5. The brightness does not always vary consistently at different wavelengths, and does not always exactly follow the 5.5 year cycle. Radio, infrared and space-based observations have expanded coverage of η Carinae across all wavelengths and revealed ongoing changes in the spectral energy distribution.
In July 2018, η Carinae was reported to have the strongest colliding wind shock in the solar neighbourhood. Observations with the NuSTAR satellite gave much higher resolution data than the earlier Fermi Gamma-ray Space Telescope. Using direct focussing observations of the non-thermal source in the extremely hard X-ray band that is spatially coincident with the star, they showed that the source of non-thermal X-rays varies with the orbital phase of the binary star system and that the photon index of the emission is similar to that derived through analysis of the γ-ray (gamma) spectrum. Alt URL
Visibility
As a fourth-magnitude star, η Carinae is comfortably visible to the naked eye in all but the most light-polluted skies in inner-city areas according to the Bortle scale. Its brightness has varied over a wide range, from the second-brightest star in the sky for a few days in the 19th century, to well below naked-eye visibility. Its location at around 60°S in the far southern celestial hemisphere means it cannot be seen by observers in Europe and much of North America.
Located between Canopus and the Southern Cross, η Carinae is easily pinpointed as the brightest star within the large naked-eye Carina Nebula. In a telescope the "star" is framed within the dark "V" dust lane of the nebula and appears distinctly orange and clearly non-stellar. High magnification will show the two orange lobes of a surrounding reflection nebula known as the Homunculus Nebula on either side of a bright central core. Variable star observers can compare its brightness with several 4th- and 5th-magnitude stars closely surrounding the nebula.
Discovered in 1961, the weak Eta Carinids meteor shower has a radiant very close to η Carinae. Occurring from 14 to 28 January, the shower peaks around 21 January. Meteor showers are not associated with bodies outside the Solar System, making the proximity to η Carinae merely a coincidence.
Visual spectrum