Hubble Space Telescope image showing Eta Carinae and the bipolar Homunculus Nebula which surrounds the star. The Homunculus was partly formed in an eruption of Eta Carinae, the light from which reached Earth in 1843. Eta Carinae itself appears as the white patch near the center of the image, where the two lobes of the Homunculus touch.
Epoch J2000 Equinox J2000
|Right ascension||10h 45m 03.591s|
|Declination||−59° 41′ 04.26″|
|Apparent magnitude (V)||−0.8 to 7.9 (4.6 February 2012)|
|Spectral type||BIae-0 / OI|
|U−B color index||-0.45|
|B−V color index||0.61|
|Variable type||LBV & binary|
|Radial velocity (Rv)||−25.0 km/s|
|Proper motion (μ)||RA: −7.6 mas/yr
Dec.: 1.0 mas/yr
|Absolute magnitude (MV)||-7 (current)|
|Mass||120 / 30 M☉|
|Radius||~240[n 1] / 24 R☉|
|Luminosity||5,000,000 / 1,000,000 L☉|
|Temperature||~15,000 / 37,200 K|
|Age||~ <3 × 106 years|
Eta Carinae (η Carinae or η Car) is a stellar system in the constellation Carina, about 7,500 to 8,000 light-years from the Sun. The system contains at least two stars, of which the primary is a luminous blue variable (LBV) that initially had around 150 solar masses, of which it has lost at least 30. A hot supergiant of approximately 30 solar masses is in orbit around the primary, although an enormous thick red nebula surrounding Eta Carinae makes it impossible to see this companion optically. The Eta Carinae system is enclosed in the Homunculus Nebula, itself part of the much larger Carina Nebula, and currently has a combined bolometric luminosity of over five million times the Sun's. It is not visible north of latitude 30°N and is circumpolar south of latitude 30°S. Because of its mass and the stage of its life, it is expected to explode in a supernova or hypernova in the astronomically near future.
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. In the middle of the 18th century, Nicolas Louis de Lacaille mapped and gave the stars of Argo Navis Bayer designations. He assigned the then second-magnitude star the Greek letter Eta.
Subsequently Eta Carinae dimmed, and by 1782 it appeared to have reverted to its former magnitude. In 1820, it was observed to be growing in brightness again. By 1827, it had brightened more than tenfold and reached its greatest apparent 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. (To put the relationship in perspective, the relative brightness would be like comparing a candle (representing Sirius) at 14.5 meters (48 feet) to another light source (Eta Carinae) about 10 kilometers (6.2 mi) away, which would appear almost as bright as the candle.)
Eta Carinae sometimes has large outbursts, the last of which appeared in 1841, at around the time of its maximum brightness. The reason for these outbursts is not known. The most likely explanation is that they are caused by a build-up of radiation pressure caused by the star's enormous luminosity. After 1843, Eta Carinae faded yet again and between about 1900 and 1940 it was only 8th magnitude, invisible to the naked eye. A sudden and unexpected doubling of brightness was observed in 1998–1999. In 2007, at magnitude 5, Eta Carinae could once again be seen with the naked eye.
In 2008, the formerly clockwork regularity of the dimming was upset. Following its 5.52-year cycle, the star would normally have started its next dimming in January 2009, but the pattern was noticed starting early in July 2008 by the southern Gemini Observatory near La Serena, Chile. Spectrographic measurements showed an increase in blue light from superheated helium, which was formerly assumed to occur with the wind shock. However, if the cause is a binary star, it would be located too far away at this point in time for the wind to interact in so significant a fashion. There is some debate about the cause of the recent event.
In 2011, light echoes from the 19th century Great Eruption of Eta Carinae were detected using the U.S. National Optical Astronomy Observatory's Blanco 4-meter telescope at the Cerro Tololo Inter-American Observatory. Analysis of the reflected spectra indicated the light was emitted when Eta Carinae was a 5000 K G2-to-G5 supergiant, some 2000 K cooler than other supernova impostor events.
System and properties
This stellar system is currently one of the most massive that can be studied in great detail. Until recently, Eta Carinae was thought to be the most massive single star, but in 2005 it was proved to be a binary system. The most massive star in the Eta Carinae multiple star system probably has more than 100 times the mass of the Sun. Other known massive stars are more luminous and more massive.
Stars in the mass class of Eta Carinae produce more than a million times as much light as the Sun. They are quite rare — only a few dozen are present in a galaxy the size of 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 impostor event, which was observed 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 possible false supernovae SN 1961v in NGC 1058 and SN 2006jc in UGC 4904, which produced a false supernova, noted in October 2004. Significantly, SN 2006jc was destroyed in a supernova explosion two years later, observed on October 9, 2006. The supernova impostor phenomenon may represent a surface instability or a failed supernova. Eta Carinae's giant eruption was the prototype for this phenomenon.
One remarkable aspect of Eta Carinae is its changing brightness. It is currently classified as a luminous blue variable (LBV) binary star due to peculiarities in its pattern of brightening and dimming.
|This section requires expansion. (December 2012)|
Eta Carinae is surrounded by the Homunculus Nebula, a reflection nebula which is itself a part of the much larger Carina Nebula. The Homunculus Nebula is composed mainly of dust which condensed from the debris ejected during the Great Eruption event in the mid nineteenth century. The nebula consists of two polar lobes and a smaller equatorial "skirt". Surrounding the Homunculus is an outer region of debris which shows an emission spectrum. This region has complex morphology with knots of material and highly collimated jets. The velocity of this material is of the order of 2,000 kms-1. Chandra X-Ray observations show a relatively strong x-ray source in a horse shoe shaped region surrounding Eta Carina indicating strong interactions between the outer ejecta and the ISM.
4U 1037–60 (A 1044–59) is Eta Carinae. Three structures around Eta Carinae are thought to represent shock waves produced by matter rushing away from the superstar at supersonic speeds. The temperature of the shock-heated gas ranges from 60 MK in the central regions to 3 MK on the horseshoe-shaped outer structure. "The Chandra image contains some puzzles for existing ideas of how a star can produce such hot and intense X-rays," says Prof. Kris Davidson of the University of Minnesota.
A "spectroscopic minimum", or "X-ray eclipse", occurred in July and August 2003. Astronomers organized a large observing campaign which included every available ground-based (e.g. CCD optical photometry) 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 and, 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.
There is good agreement between the X-ray light curve and the evolution of a wind-wind collision zone of a binary system. These results were complemented by new tests on radio wavelengths.
Spectrographic monitoring of Eta Carinae showed that some emission lines faded every 5.52 years, and that this period was stable for decades. The star's radio emission, along with its X-ray brightness, also drop precipitously during these "events" as well. These variations, along with ultra-violet variations, suggest a high probability 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.
The ionizing radiation emitted by the secondary star in Eta Carinae is the major radiation source of the system. Much 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 Carinae. Fast variations near periastron passage are attributed to the onset of the accretion phase.
Because of their disproportionately high luminosities, very large stars such as Eta Carinae use up their fuel very quickly. Eta Carinae is expected to explode as a supernova or hypernova some time within the next million years or so. As its current age and evolutionary path are uncertain, however, 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.
More recently, another possible Eta Carinae analogue was observed: SN 2006jc, some 77 million light years away in UGC 4904, in the constellation of Lynx. Its brightened appearance was noted on 20 October 2004, and was reported by amateur astronomer Koichi Itagaki as a supernova. However, although it had indeed exploded, hurling 0.01 solar masses (~20 Jupiters) of material into space, it had survived, before finally exploding nearly two years later as a Mag 13.8 type Ib supernova, seen on 9 October 2006. Its earlier brightening was a supernova impostor event.
The similarity between Eta Carinae and SN 2006jc has led Stefan Immler of NASA's Goddard Space Flight Center to suggest 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 says it is likely that Eta Carinae is at an earlier stage of evolution, and that there are still several stages of nuclear burning to go before the star runs out of fuel.
In NGC 1260, a spiral galaxy in the constellation of Perseus some 238 million light years from earth, another analogue star explosion, supernova SN 2006gy, was observed on September 18, 2006. 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.
Possible effects on Earth
It is possible that the Eta Carinae hypernova or supernova, when it occurs, could affect Earth, which is about 7,500 light years from the star. It is unlikely, however, to affect terrestrial lifeforms directly, as they will be protected from gamma rays by the atmosphere, and 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. (At least one paper has projected that *complete* loss of the Earth's ozone layer is a plausible consequence of a nearby supernova, which would result in a significant increase in surface UV radiation reaching the Earth's surface from our own Sun.) 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". A supernova or hypernova produced by Eta Carinae would probably eject a gamma ray burst (GRB) out from both polar areas of its rotational axis. Calculations show that the deposited energy of such a GRB striking the Earth's atmosphere would be equivalent to one kiloton of TNT per square kilometer over the entire hemisphere facing the star, with ionizing radiation depositing ten times the lethal whole body dose to the surface. This catastrophic burst would probably not hit Earth, though, because the rotation axis does not currently point towards our solar system. If Eta Carinae is a binary system, this may affect the future intensity and orientation of the supernova explosion that it produces, depending on the circumstances.
In traditional Chinese astronomy, Eta Carinae has the names Tseen She (from the Chinese 天社 [Mandarin: tiānshè] "Heaven's altar") and Foramen. It is also known as 海山二 (Hǎi Shān èr, English: the Second Star of Sea and Mountain), referring to Sea and Mountain, an asterism that Eta Carinae forms with s Carinae, λ Centauri and λ Muscae.
In 2010, astronomers Duane Hamacher and David Frew from Macquarie University in Sydney showed that the Boorong Aboriginal people of northwestern Victoria, Australia, witnessed the outburst of Eta Carinae in the 1840s and incorporated it into their oral traditions as Collowgulloric War, the wife of War (Canopus, the Crow – wɑː). This is the only definitive indigenous record of Eta Carinae's outburst identified in the literature to date.
- Homunculus Nebula
- LBV 1806-20
- Pistol Star
- SN 2006gy
- List of most luminous stars
- List of largest stars
- List of most massive stars
- Bipolar outflow
- The mass is so high that there is no clearly defined boundary between the star and the surrounding nebula. The effective temperature and radius correspond to a position where τ(ross) is around unity.
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- Wagman, Morton (2003). Lost Stars: Lost, Missing and Troublesome Stars from the Catalogues of Johannes Bayer, Nicholas Louis de Lacaille, John Flamsteed, and Sundry Others. Blacksburg, VA: The McDonald & Woodward Publishing Company. ISBN 978-0-939923-78-6.
|Wikimedia Commons has media related to Eta Carinae.|
- NASA image released Feb. 24, 2012
- 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
- ESO press release about the possibility of a supernova in 10 to 20 millennia
- The 2003 Observing Campaign
- Davidson, Kris et al. (1999). "An Unusual Brightening Of Eta Carinae". The Astronomical Journal (The Astronomical Journal) 118 (4): 1777. Bibcode:1999AJ....118.1777D. doi:10.1086/301063.
- Nathan, Smith (1998). "The Behemoth Eta Carinae: A Repeat Offender". Astronomical Society of the Pacific. Retrieved 2006-08-13.
- Eta Carinae at SIMBAD