Eta Carinae

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**Eta Carinae**
Observation data Epoch J2000
Hubble Space Telescope image showing Eta Carinae and the bipolar Homunculus Nebula which surrounds the star. The Homunculus was partly created in an eruption of Eta Carinae whose light reached Earth in 1843. Eta Carinae itself appears as the white patch near the center of the image, where the 2 lobes of the Homunculus touch.
Constellation (pronunciation)	Carina
Right ascension	10^h 45^m 03.591^s^[1]
Declination	−59° 41′ 04.26″^[1]
Apparent magnitude (V)	6.21^[1]
Characteristics
Spectral type	Peculiar
U-B color index	-0.45
B-V color index	0.61
Variable type	LBV binary or multiple star
Astrometry
Radial velocity (R_v)	−25.0^[1] km/s
Proper motion (μ)	RA: −7.6^[1] mas/yr Dec.: 1.0^[1] mas/yr
Details
Mass	100–150^[2] M_☉
Radius	80–180 R_☉
Luminosity	5 × 10⁶(bolometric) L_☉
Temperature	36–40,000 K
Age	~ <3 × 10⁶ years
Other designations
Foramen, Tseen She, HR 4210, CD−59°2620, HD 93308, SAO 238429, WDS 10451-5941, IRAS 10431-5925, GC 14799, CCDM J10451-5941

Eta Carinae (η Carinae or η Car) is a hypergiant Luminous blue variable star in the Carina constellation. Its luminosity is about four million times that of the Sun and, with an estimated mass of between 100 and 150 times the solar mass, it is one of the most massive stars yet discovered. Because of its mass and the stage of life, it is expected to become a supernova in the "near" future.

1 Significance
2 Brightness variations
3 Future prospects
4 See also
5 References
6 External links

[edit] Significance

This object is currently the most massive nearby star that can be studied in great detail. While it is possible that other known stars might be more luminous and more massive, Eta Carinae has the highest confirmed luminosity based on data across a broad range of wavelengths; former prospective rivals such as the Pistol Star have been demoted by improved data.

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^[3] and SN 2006jc in NGC 4904^[4], which produced a false supernova in October 2004. Significantly, SN 2006jc was destroyed in a supernova explosion two years later, on October 9, 2006^[5]. The supernova impostor phenomenon may represent a surface instability^[6] 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.

This object is located in the constellation Carina (right ascension 10 h 45.1 m, declination −59°41m), about 7,500 to 8,000 light-years from the Sun. It is not typically visible north of latitude 27°N.

Related names have caused much confusion:

"Eta Carinae" means the star itself.
The "Homunculus Nebula" is the bipolar cloud of debris ejected in the great eruption, portrayed in images such as those from the Hubble Space Telescope.
"The Keyhole Nebula" is a much larger, nearby diffuse structure.
"The Carina Nebula," NGC 3372, is a large, bright star-formation region that produced a number of very massive stars including Eta Carinae.
"Trumpler 16" open cluster, to which Eta Carinae belongs, is itself located within the Carina Nebula. The nebula includes other open clusters, for example, Trumpler 14.

[edit] Brightness variations

One remarkable aspect of Eta Carinae is its changing brightness. It is currently classified as a luminous blue variable (LBV) double star.

When Eta Carinae was first catalogued in 1677 by Edmond Halley, it was of the 4th magnitude, but by 1730, observers noticed it had brightened considerably, and was at that point one of the brightest stars in Carina. Subsequently it dimmed again, and by 1782 was back to its former obscurity, but in 1820 it started growing in brightness again. By 1827 it had brightened more than tenfold, and reached its greatest brightness in April 1843: with a magnitude of −0.8 it was the second brightest star in the night-time sky (after Sirius at 8.6 light years away), despite its enormous distance of 7,000–8,000 light-years. (To put the relationship in perspective, the relative brightness would be like comparing a candle (Sirius) at 14.5 meters (48 feet) to another light (Eta Carinae) on the horizon of our planet 10 kilometers (6 mi) away, which would appear almost as bright as the candle.)

Eta Carinae sometimes has large outbursts, the last one just around its brightness maximum, in 1841. The reason for these outbursts is not yet known. The most likely possibility is believed to be that they are caused by built-up radiation pressure from the star's enormous luminosity.

After 1843 Eta Carinae faded away, and between about 1900 and 1940 it was only of the 8th magnitude: invisible to the naked eye.^[7]

A composite X-ray (orange and yellow) — optical (blue) image of Eta Carinae from Chandra X-ray Telescope and Hubble Space Telescope.

A "spectroscopic minimum" or "X-ray eclipse" occurred in the midsummer of 2003. Astronomers organized a large observing campaign, which included every available ground-based (e.g. CCD optical photometry^[8]) and space observatory, including major observations with the Hubble Space Telescope, the Chandra X-ray Observatory, the INTEGRAL Gamma-ray space observatory, and the Very Large Telescope. Primary goals of these observations were to determine if in fact Eta Carinae is a binary star; if so, to identify its companion star; to determine the physical mechanism behind the "spectroscopic minima"; and to understand their relation (if any) to the large scale eruptions of the 19th century.

Falceta-Gonçalves and co-workers^[9] have found good agreements between the X-rays' light curve and the evolution of a wind-wind collision zone of a binary system. Their results were complemented by new tests on radio wavelengths.

Spectrographic monitoring of Eta Carinae^[10] showed that some emission lines faded precisely every 5.52 years, and that this period was stable for decades. The star's radio emission,^[11] along with its X-ray brightness,^[12] also drop precipitously during these "events" as well. These variations, along with ultra-violet observations gives very high probability for the scenario that Eta Carinae is actually a binary star, in which a hot, lower mass star revolves around η Carinae in a 5.52-year, highly eccentric elliptical orbit.^[13]

Kashi and Soker^[14] studied the propagation of the ionizing radiation emitted by the secondary star in Eta Carinae. A large fraction of this radiation is absorbed by the primary stellar wind, mainly after it encounters the secondary wind and passes through a shock wave. The amount of absorption depends on the compression factor of the primary wind in the shock wave. The compression factor is limited by the magnetic pressure in the primary wind. The variation of the absorption by the post-shock primary wind with orbital phase changes the ionization structure of the circumbinary gas and can account for the radio light curve of Eta Car. Fast variations near periastron passage are attributed to the onset of the accretion phase.

Eta Carinae suddenly and unexpectedly doubled its brightness in 1998–1999. Currently (2007) it can be easily seen with the naked eye, because it is brighter than magnitude 5 ^[15].

[edit] Future prospects

Carina nebula (NGC 3372) containing Eta Carinae. Photograph by the Hubble Space Telescope.

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.^[16]

More recently another possible Eta Carinae analogue was observed; namely SN 2006jc some 77 million light years away in UGC 4904, in the constellation of Lynx.^[17] It brightened on 20 October 2004 and was reported by amateur astronomer Koichi Itagaki as supernova. However, it survived and finally exploded two years later as a Mag 13.8 type Ib supernova on 9 October 2006. Its earlier brightening was a supernova impostor event; the initial explosion hurled 0.01 solar masses (~20 Jupiters) of material into space.

Due to the similarity of Eta Carinae and SN 2006jc, Stefan Immler of NASA's Goddard Space Flight Center suggests that Eta Carinae could explode in our lifetime or even in the next few years. However, Stanford Woosley of the University of California in Santa Cruz disagrees with Immler’s suggestion, and he says it is likely that Eta Carinae is at an earlier stage of evolution and that it has several kinds of material left for nuclear fusion.

Another recent analog star explosion was supernova SN 2006gy, observed starting on September 18, 2006 in NGC 1260 (a spiral galaxy in the constellation Perseus) 238 million light years from earth. A number of astronomers modelling supernova events have suggested that the explosion mechanism for SN 2006gy may be very similar to the fate that awaits Eta Carinae.

It is possible that the Eta Carinae hypernova or supernova could affect Earth, about 7,500 light years away, but would not likely affect the remaining terrestrial humans directly, who will be protected from gamma rays by the atmosphere, as well from some other cosmic rays by the magnetosphere. The damage would likely be restricted to the upper atmosphere, the ozone layer, spacecraft, including satellites, and any astronauts in space, although a certain few claim that radiation damage to the upper atmosphere would have catastrophic effects as well. At least one scientist has claimed that when the star explodes, "it would be so bright that you would see it during the day, and you could even read a book by its light at night".^[18] A supernova or hypernova produced by Eta Carinae would probably eject a gamma ray burst out on both polar areas of its rotational axis. This catastrophic burst would probably not hit Earth, though, because the rotation axis does not currently point at us. Recently, Eta Carinae has been examined and while many agree that it is a double star, or possibly even a triple star system, it has not been confirmed. This theory was adopted after examinations of its unusual periodic electromagnetic radiation variations of 5.3-year cycles. This may affect the future intensity and orientation of the supernova or hypernova explosion that it produces, depending on the circumstances.^[19]