ETA CARINAE

'Eta Carinae' (η Carinae or η Car) is a highly luminous hypergiant double star. Estimates of its mass range from 100–150 times the mass of the Sun, and its luminosity is about four million times that of the Sun.

Contents

Significance

Brightness variations

Future prospects

See also

References

External links

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 weight class of Eta Carinae, with more than 100 times the mass of the Sun, produce more than a million times as much light. 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 its radiation and gas, resulting in a possible supernova or hypernova in the near future.
Eta Carinae's chief significance for astrophysics is based on its giant eruption or ''supernova impostor 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 impostors have been seen in other galaxies, for example the false supernovas SN 1961v in NGC 1058 and SN 2006jc in UGC 4904, which produced a false supernova in Oct 2004. Significantly, SN 2006jc was destroyed in a supernova explosion two years later, on October 9, 2006^[1]. The supernova impostor phenomenon may represent a surface instability^[2] 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 Car.
# "Trumpler 16" open cluster, to which Eta Carinae belongs, is itself located within the Carina Nebula. The nebula includes other open clusters, e.g., Trumpler 14

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 10km (6.2miles) 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.^[3]
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^[4]) 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^[5] have found good agreements between the X-rays' light curve and the evolution on a wind-wind collision zone of a binary system. Their results were complemented by new tests on radio wavelengths.
Spectrographic monitoring of Eta Carinae^[6] showed that some emission lines faded precisely every 5.52 years, and that this period was stable for decades. The star's radio emission,^[7] along with its X-ray brightness,^[8] 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.^[9]
Kashi and Soker^[10] 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 4.7.

Future prospects

.
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 after about 1 million years or less from now, or perhaps less than several millennia, but as its current age is uncertain, it could explode at any time. However, 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.^[11]
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.^[12]
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 Sept 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 humans directly, who are protected from gamma rays by the atmosphere. The damage would likely be restricted to the upper atmosphere, the ozone layer, spacecraft, including satellites, and any astronauts in space. At least one scientist has claimed that if the star were to explode, "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".^[13] A supernova or hypernova produced by Eta Carinae would probably shoot a gamma ray burst out on both sides in the direction of its rotation axis. This catastrophic burst would probably not hit earth though, because the rotation axis does not currently point at us. Since Eta Carinae is at least a double star, or even a ''triple star'', examined due to its short brightness and X-ray variation period, this may either increase or decrease the intensity of the supernova or hypernova it produces depending on the circumstances.[1]

See also

★ Pistol Star

★ LBV 1806-20

★ SN 2006gy

References

1. Star's odd double explosion hints at antimatter trigger, Shiga, D., , , New Scientist, 2007
2. .
3. light curve
4. http://lilen.fcaglp.unlp.edu.ar/EtaCar/
5. Wind-wind collision in the η Carinae binary system: a shell-like event near periastron, Falceta-Gonçalves, D.; Jatenco-Pereira, V.; Abraham, Z., , , MNRAS, 2005
6. The 5.52 Year Cycle of Eta Carinae, Damineli, A., , , ApJ, 1996
7. http://www.astro.umd.edu/~white/images/eta_time_full.html
8. http://lheawww.gsfc.nasa.gov/users/corcoran/eta_car/etacar_rxte_lightcurve/
9. http://www.nasa.gov/home/hqnews/2005/nov/HQ_05353_massive_star.html
10. Modelling the Radio Light Curve of Eta Carinae, Kashi, A.; Soker, N., , , , 2007
11. http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2006ApJ...645L..45S&db_key=AST&data_type=HTML&format=&high=451e33df7e10366
12. Massive star burps, then explodes, Robert S., , , UC Berkeley News, 2007
13. http://news.bbc.co.uk/2/hi/science/nature/6633609.stm

External links

★ 'ETA CARINAE — an evolved triple-star system?' by Wolfgang Kundt and Christoph Hillemanns

★ HST Treasury Project and General Information on Eta Carinae

★ Eta Carinae profile

★ Is there a "clock" in Eta Carinae? - Brazilian research about the star

★ Broad Band Optical Monitoring

★ X-ray Monitoring by RXTE

★ Possible Hypernova could affect Earth

★ ESO press release about the possibility of a supernova in 10 to 20 millennia

★ The 2003 Observing Campaign

★ An Unusual Brightening Of Eta Carinae

★ The Behemoth Eta Carinae: A Repeat Offender