Eta Carinae 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 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 detect this
companion at optical wavelengths. The Eta Carinae system is enclosed in the Homunculus
Nebula, in turn a 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 as a supernova or hypernova in the astronomically
near future.
Observational history 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, despite
its enormous distance. at 14.5 meters to another light source about 10 kilometers 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 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 binary star due
to peculiarities in its pattern of brightening and dimming.
Surroundings
Eta Carinae lies near the center of the Trumpler 16 star cluster, which is, together with Trumpler
14 one of the main star clusters of Carina OB1, which is, together with Carina OB2, one
of the main stellar associations of the Carina Nebula. The position of the star within its
inner regions suggests that it may have been formed as the center of mass of the molecular
cloud during the formation of Trumpler 16. Its position, together with its extreme luminosity,
has notable effects to the other stars of the cluster.
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 km/s. 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.
X-ray source
4U 1037–60 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 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 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. Future prospects
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 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. However, 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 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. Cultural significance
In traditional Chinese astronomy, Eta Carinae has the names Tseen She and Foramen. It is
also known as 海山二, 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. This is the only definitive
indigenous record of Eta Carinae's outburst identified in the literature to date.
See also Bipolar outflow
Homunculus Nebula LBV 1806-20
List of largest stars List of most luminous stars
List of most massive stars Pistol Star
SN 2006gy
Notes
References
Cited text Wagman, Morton. 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. External links
NASA image released February 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.. "An Unusual Brightening
Of Eta Carinae". The Astronomical Journal 118: 1777. Bibcode:1999AJ....118.1777D. doi:10.1086/301063.
Nathan, Smith. "The Behemoth Eta Carinae: A Repeat Offender". Astronomical Society of
the Pacific. Retrieved 2006-08-13. Eta Carinae at Constellation Guide
Eta Carinae at SIMBAD