Definitions

Eta Carinae

Eta Carinae

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 solar masses, it is one of the most massive stars yet discovered. Because of its mass and the stage of life, it is expected to explode in a supernova in the "near" future.

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 and SN 2006jc in NGC 4904, which produced a false supernova in October 2004. Significantly, SN 2006jc was destroyed in a supernova explosion two years later, 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, 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:

  1. "Eta Carinae" means the star itself.
  2. 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.
  3. "The Keyhole Nebula" is a much larger, nearby diffuse structure.
  4. "The Carina Nebula," NGC 3372, is a large, bright star-formation region that produced a number of very massive stars including Eta Carinae.
  5. "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.

Brightness variations

One remarkable aspect of Eta Carinae is its changing brightness. It is currently classified as a luminous blue variable (LBV) double star due to peculiarities in its pattern of brightening and dimming.

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 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. Eta Carinae suddenly and unexpectedly doubled its brightness in 1998–1999. As of 2007 it can be easily seen with the naked eye, because it is brighter than magnitude 5.

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) 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.

There is good agreement between the X-rays 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 precisely 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 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.

Kashi and Soker 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.

The formerly clockwork regularity of the dimming was upset in 2008. Following the 5.52 year cycle, the star would have started its next dimming in January 2009, but the pattern was noticed starting early in July 2008 by the southern Gemini Observatory in 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.

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 some time within the next million years or so. 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.

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. 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 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". A supernova or hypernova produced by Eta Carinae would probably eject a gamma ray burst (GRB) out on both polar areas of its rotational axis. Calculations show that the deposited energy of such a GRB impacting the Earths 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 at us. 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.

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References

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