Eta Carinae ( just the facts - a article written in 2007)
Eta Carinae is a highly luminous hypergiant star. Estimates of its mass range from 100–150 times the mass of the Sun, & its luminosity is about four million times that of the Sun.
This object is currently the most massive star that can be studied in great detail. Several other known stars may be more luminous & more massive, but data on them is far less robust. (Caveat: Since examples such as the Pistol Star have been demoted by improved data, one should be skeptical of most available lists of "most massive stars." As of late 2006, Eta Car still has the highest confirmed luminosity, based on data across a broad range of wavelengths.) Stars with more than 80 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 ours -- & they flirt with disaster near 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, & their gravity is barely strong enough to hold in its radiation & 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, this star produced almost as much visible light as a supernova explosion, but it survived. Other supernova impostors have been seen in other galaxies, for instance SN 1961v in NGC 1058, but information on them is relatively sparse because they are far more distant than Eta Carinae. The supernova impostor phenomenon is thought to represent a surface instability; it is basically more mysterious than most real supernovae, because the mechanism is not yet understood. Eta Carinae's giant eruption was the prototype for this phenomenon, & 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 visible north of, typically, 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 Car. The term "Eta Carinae Nebula" should be deprecated because sometimes it means the Homunculus, & sometimes NGC 3372.
Brightness variations
One
remarkable aspect of Eta Carinae is its changing brightness. It is currently classified
as a luminous blue variable (LBV) 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, & was at that point one of the brightest stars in Carina. From there it dimmed again, & 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, & 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), despite its enormous distance (7,000–8,000 light-years).
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, & between about 1900 & 1940 it was only of the 8th magnitude: invisible to the naked eye (see the light curve).
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) & space observatory, including major observations with the Hubble Space Telescope, the Chandra X-ray Observatory, the INTEGRAL Gamma-ray space observatory, & 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"; & to understand their relation (if any) to the large scale eruptions of the 19th century.
Falceta-Gonçalves & co-workers have found good agreements between the X-rays light curve & 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 showed that some emission lines faded precisely every 5.52 years, & 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 probabilityfor the scenario that Eta Carinae is actually a binary star, in which a hot, lower mass star revolves around ? Car in a 5.52-year, highly eccentric elliptical orbit.
Kashi & 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 & 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 & 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 & unexpectedly doubled its brightness in 1998–1999. Currently (2006) it can be seen with the naked eye, because it is brighter than magnitude V=4.7 (see its brightness up to date).
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, but as its current age is uncertain,
it could "go any time now." 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 & become Wolf-Rayet Stars before
they go supernova, if they are unable to hold their mass to explode as a hypernova.
The
possible Eta Carinae Hypernova could affect Earth nearly 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, & any astronauts
in space.
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