Eta Carinae

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Eta Carinae
EtaCarinae.jpg
Hubble Space Telescope image showing the bipolar Homunculus Nebula which surrounds Eta Carinae.
Observation data
Epoch J2000      Equinox J2000
Constellation Carina
Right ascension 10h 45m 03.591s[1]
Declination −59° 41′ 04.26″[1]
Apparent magnitude (V) −0.8 to 7.9[2]
Characteristics
Spectral type F:I_pec_e[3] / O[4][5]
U−B color index -0.45
B−V color index 0.61
Variable type LBV[2] & binary
Astrometry
Radial velocity (Rv) −25.0[6] km/s
Proper motion (μ) RA: −7.6[1] mas/yr
Dec.: 1.0[1] mas/yr
Absolute magnitude (MV) -7 (current)
Details
Mass 120 / 30[7] M
Radius ~240[8] / 24[4] R
Luminosity 5,000,000 / <1,000,000[4][5] L
Temperature 9,400[9] / 37,200[4] K
Age <3 × 106[5] years
Orbit
Primary Eta Carinae A
Companion Eta Carinae B
Period (P) 2,022.7 days[10]
(5.54 yr)
Semi-major axis (a) 15.4[11] AU
Eccentricity (e) 0.9[12]
Inclination (i) ~45[11]°
Other designations
Foramen, Tseen She, 231G Carinae,[13] HR 4210, CD−59°2620, HD 93308, SAO 238429, WDS 10451-5941, IRAS 10431-5925, GC 14799, CCDM J10451-5941
Database references
SIMBAD data

Eta Carinae (η Carinae or η Car) is a stellar system containing at least two stars, about 7,500 light-years from the Sun in the direction of the constellation Carina. It is a member of the Trumpler 16 open cluster within the much larger Carina Nebula and currently has a combined bolometric luminosity of over five million times the Sun's.[9]

Eta Carinae is circumpolar south of latitude 30°S, but is never visible north of latitude 30°N. It was first recorded as a 4th magnitude star, became the second brightest star in the sky before fading well below naked eye visibility, then brightened again and is now back at 4th magnitude.

The two main stars of the Eta Carinae system revolve in an eccentric orbit every 5.54 years. The primary is a peculiar star similar to a luminous blue variable (LBV) that initially had around 150 solar masses and has lost at least 30. 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. This is currently the only star known to emit natural LASER light in ultraviolet wavelengths.[14] The secondary is a hot star, probably class O, of approximately 30 solar masses and is itself a highly luminous star. The system is heavily obscured by the Homunculus Nebula, material ejected from the primary during its Great Eruption in the 19th century.

Observations[edit]

Eta Carinae is currently a 4th magnitude star, comfortably visible to the naked eye in dark skies.

Discovery[edit]

The earliest reliable record of Eta Carinae was made by Edmond Halley in 1677 when he recorded the star simply as "Sequens" (i.e. "following" relative to another star) within a new constellation Robur Carolinum. His Catalogus Stellarum Australium was published in 1670.[15]

There are some possible earlier observations of Eta Carinae. There are speculative reports from antiquity that may relate to Eta Carinae, but no reliable observations. Most observations of bright stars in the southern constellations in the 16th century fail to record Eta Carinae. Pieter Keyser described a fourth magnitude star at approximately the correct position in the late 16th century, but Frederick de Houtman's catalogue from 1603 does not include Eta Carinae among the other fourth magnitude stars in the region.[15]

In 1751 Nicolas Louis de Lacaille mapped the stars of Argo Navis and Robur Carolinum into separate smaller constellations and gave the brighter members Greek alphabet Bayer designations. Eta was placed within the keel portion of the ship named as the new constellation Carina.[16]

Surroundings[edit]

The Carina Nebula. Eta Carinae is the brightest star, just left of centre.
Annotated image of Carina Nebula

Eta Carinae lies within the scattered stars of the Trumpler 16 open cluster. All the other members are well below naked eye visibility, although WR 25 is another extremely massive luminous star. Trumpler 16 and its neighbour Trumpler 14 are the main star clusters of the Carina OB1 association, one of the two main stellar associations of the Carina Nebula, together with Carina OB2.

Eta Carinae is enclosed by the Homunculus Nebula, a reflection nebula lit mainly by Eta Carinae itself.[17] 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 primarily of two polar lobes aligned with the rotation axis of the star, plus an equatorial "skirt". Closer studies show many fine details: a Little Homunculus within the main nebula, probably formed by the 1890 eurption; a jet; fine streams and knots of material, especially noticeable in the skirt region; and three Weigelt Blobs, dense gas condensations very close to the star itself.[14][18]

Brightness[edit]

Combined UV and visual light image of Eta Carinae

Halley gave an approximate apparent magnitude of "4" at the time of discovery, which has been calculated as magnitude 3.3 on the modern scale. The handful of possible earlier sightings suggest that Eta Carinae was not significantly brighter than this for much of the 17th century.[15]

There are further sporadic observations over the next 70 years showing that Eta Carinae was probably around or below 4th magnitude, until Lacaille's recorded it at second magnitude in 1751.[16] It is unclear whether Eta Carinae varied significantly in brightness over the next 50 years, with occasional observations such as William Burchell at 4th magnitude in 1815, but it is uncertain whether these are just re-recordings of earlier observations. In 1827 Burchell specifically noted its unusual brightness at 1st magnitude.[15]

Then in the 1830s John Herschel made a detailed series of accurate measurements showing Eta Carinae consistently around magnitude 1.2. However at the end of 1837, it brightened suddenly to magnitude 0 before dropping slightly to magnitude 0.6.[19]

In summary, the brightness of Eta Carinae increased from around 4th magnitude to 1st magnitude over about 150 years, possibly erratically, before brightening dramatically. This was the start of the Great Eruption, which by 1843 saw Eta Carinae become the second brightest star in the sky after Sirius. To put the relationship in perspective, Sirus is nearly a thousand times closer, but only appears 40% brighter than Eta Carinae at its peak. Particular peaks in 1827, 1838, and 1843 may have been related to the periastron passage of the binary orbit.[20] From 1845 to 1856, the brightness decreased by around 0.1 magnitudes per year, but with possible rapid and large fluctuations.[15]

From 1857 the brightness decreased rapidly until it faded below naked eye visibility by 1886. This has been calculated to be due to the condensation of dust in the ejected material surrounding the star rather than an intrinsic change in luminosity.[21] There was a brightening from 1887 - 1895, peaking at about magnitude 6.2 then dimming rapidly to about magnitude 7.5. This appeared to be a smaller copy of the Great Eruption, expelling material that formed the Little Homunculus and Weigelt Blobs.[22][23]

For the first half of the 20th century, Eta Carinae appeared to have settled at a constant brightness at 8th magnitude, but in 1953 it was noted to have brightened again to magnitude 6.5.[24] The brightening continued steadily, but with fairly regular variations of a few tenths of a magnitude that were later identified as having a 5.54 year period.[20] A sudden doubling of brightness was observed in 1998–1999 bringing it back to naked eye visibility. As of 2012, the visual magnitude is 4.6.[25]

The brightness doesn't always vary consistently at different wavelengths, and does not always consistently follow the 5.5 year cycle.[5][26]

Spectrum[edit]

Hubble composite of Eta Carinae showing the unusual emission spectrum taken using STIS

The spectrum of Eta Carinae is peculiar and variable. The earliest observations of the spectrum in 1893 are described as similar to an F5 star, but with a few emission lines. Analysis to modern spectral standards suggests an early F spectral type.[27] By 1895, the spectrum consisted mostly of strong emission lines, with the absorption lines present but largely obscured by emission. The lines vary greatly in width and profile.[28][29]

Direct spectral observations do not begin until after the Great Eruption, but light echoes from the eruption 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 had the appearance of a 5,000 K G2-to-G5 supergiant, some 2,000 K cooler than expected from other supernova impostor events.[30] Further light echo observations show that following the peak brightness of the Great Eruption the spectrum developed prominent P Cygni profiles and CN molecular bands. These indicate that the star, or the expanding cloud of ejected material, had cooled further and may have been colliding with circumstellar material in a similar way to a type IIn supernova.[31]

In the second half of the 20th century, infra-red and ultra-violet spectra became available, as well as much higher resolution visual spectra. The spectrum continued to show complex and baffling features, with much of the energy from the central star being recycled into the infra-red by surrounding dust, some reflection of light from the star from dense localised objects in the circumstellar material, but with obvious high ionisation features indicative of very high temperatures. The line profiles are complex and variable, indicating a number of absorption and emission features at various velocities relative to the central star.[32][33]

The 5.5 year orbital cycle produces strong spectral changes at periastron that are known as "spectroscopic event"s. Certain wavelengths of radiation suffer eclipses, either due to actual occultation by one of the stars, or due to passage within opaque portions of the complex stellar winds. Despite being ascribed to orbital rotation, these events vary significantly from cycle to cycle. These changes have become stronger since 2003 and it is generally believed that longterm secular changes in the stellar winds or previously ejected material may be the culmination of a return to the state of the star prior to its Great Eruption.[26][34][35]

High energy radiation[edit]

X-Rays around Eta Carinae (red is low energy, blue higher)

Several x-ray and gamma-ray sources have been detected around Eta Carinae, for example 4U 1037–60 in the 4th Uhuru catalogue and 1044–59 in the HEAO-2 catalog. The earliest detection of x-rays in the Eta Carinae region was from Terrier-Sandhawk rocket, [36] followed by Ariel 5,[37] OSO 8,[38] and Uhuru[39] sightings.

More detailed observations were made with Einstein,[40] ROSAT,[41] ASCA,[42] and Chandra. There are multiple sources at various wavelengths right across the high energy electromagnetic spectrum: hard x-rays and gamma rays within 1 light month of the Eta Carinae; hard x-rays from a central region about 3 light months wide; a distinct partial ring "horse-shoe" structure in low energy x-rays 0.67 pc across corresponding to the main shockfront from the Great Eruption; diffuse x-ray emission across the whole area of the Homunculus; and numerous condensations and arcs outside the main ring.[43][43][44][45][46]

All the high energy emission associated with Eta Carinae varies during the orbital cycle. A spectroscopic minimum, or X-ray eclipse, occurred in July and August 2003 and similar events in 2009 and 2014 have been intensively observed.[47] The highest energy gamma-rays above 100MeV detected by AGILE show strong variability, while lower energy gamma-rays observed by Fermi show little variability.[43][48]

Radio emission[edit]

Eta Carinae has been detected at various radio wavelengths including EHF (millimeter wave), SHF, and UHF. The detected radiation appears to be both thermal emission from warm gas and free-free emission from ionised gas. The free-free emission in particular varies during the orbital cycle, with significant dips during the periastron passage Spectroscopic events.[49][50]

Properties[edit]

X-ray, optical and infrared images of Eta Carinae (August 26, 2014).

The Eta Carinae 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 the system's binary nature was confirmed.[51] Unfortunately, both component stars are largely obscured by circumstellar material ejected from Eta Carinae A and basic properties such as their temperatures and luminosities can only be inferred. Rapid changes to the stellar wind in the 21st century suggest that the star itself may be revealed as dust from the great eruption finally clears.[52]

Classification[edit]

Eta Carinae A is classified as a luminous blue variable (LBV) due to peculiarities in its pattern of brightening and dimming. This type of variable star is characterised by irregular changes from a high temperature quiescent state to a low temperature eruptive state at roughly constant luminosity. LBVs in the quiescent state lie on a narrow S Doradus instability strip, with more luminous stars being hotter. In eruption all LBVs have about the same temperature near 8,000K. LBVs in eruption are visually brighter than when quiescent although the bolometric luminosity is unchanged.

Eta Carinae A is not a typical LBV. It is more luminous than any other LBV in the Milky Way although possibly comparable to other Supernova Imposters detected in external galaxies. It doesn't currently lie on the S Doradus instability strip, although it is unclear what the temperature or spectral type of the underlying star actually is. The 1890 eruption may have been fairly typical of LBV eruptions, with an early F spectral type, and it has been estimated that the star may currently have an opaque stellar wind forming a pseudo-photosphere with a temperature of 9,000K - 14,000K which would be typical for an LBV in eruption.[21]

The Great Eruption event of Eta Carinae A has only been observed in a handful of other possible LBVs in external galaxies. It isn't clear if this is something that only a very few of the most massive LBVs undergo, something that is caused by a close companion star, or a very brief but common phase for massive stars.

Eta Carinae B is a massive luminous hot star, but little else is known. From certain high excitation spectral lines that ought not to be produced by the primary, it is thought that Eta Carinae B is a young O-type star. Most authors suggest it is a somewhat evolved star such as a supergiant or giant, although a Wolf-Rayet star cannot be ruled out.[51]

Mass[edit]

The masses of stars are difficult to measure except by determination of a binary orbit. Eta Carinae is a binary system, but certain key information about the orbit is not known accurately. Several models of the system use masses of 120-160 M and 30-60 M for the primary and secondary respectively. Eta Carinae A has clearly lost a great deal of mass since it formed and was initially 150-180 M.[7]

Mass loss[edit]

HST image of the Homunculus Nebula, inset is a VLT NACO infra-red image of Eta Carinae

Mass loss is one of the most intensively studied aspects of massive star research. Put simply, using observed mass loss rates in the best models of stellar evolution do not reproduce the observed distribution of evolved massive stars such as Wolf-Rayets, the number and types of core collapse supernovae, or their progenitors. To match those observations, the models require much higher mass loss rates. Eta Carinae A has one of the highest known mass loss rates, currently around 10-3 M/year, and is an obvious candidate for study.[7]

Eta Carinae A is losing so much mass due to its extreme luminosity and relatively low surface gravity that the stellar wind is entirely opaque and appears as a pseudo-photosphere. This optically dense surface hides the true physical surface of the star. During the Great Eruption the mass loss rate was a thousand times higher, around 1 M/year sustained for ten years or more. The total mass loss during the eruption was 10-20 M with much of it now forming the Homunculus Nebula. The smaller 1890 eruption produced the Little Homunculus Nebula, much smaller and only about 0.1 M.[8] The bulk of the mass loss occurs in a wind with a terminal velocity of about 400 km/s, but some material is seen at higher velocities, up to 3,200 km/s, possibly material blown from the accretion disk by the secondary star.[53]

Eta Carinae B is presumably also losing mass via a thin fast stellar wind, but this cannot be detected directly. Models of the radiation observed from interactions between the winds of the two stars show a mass loss rate of the order of 10-6 M/year, typical of a hot O class star. For a portion of the highly eccentric orbit, it actually gains material from the primary via an accretion disk. During the Great Eruption of the primary, the secondary accreted several M, producing strong jets which formed the bipolar shape of the Homunculus Nebula.[7]

Luminosity[edit]

The stars of the Eta Carinae system are completely obscured by dust and opaque stellar winds. The total electromagnetic radiation across all wavelengths for both stars combined is several million L. The best estimate for the luminosity of the primary is 5 million L. The luminosity of Eta Carinae B is particularly uncertain, probably several hundred thousand L and almost certainly no more than 1 million L. Due to the surrounding dust, 90% of the radiation from the stars reaches us as infra-red and Eta Carinae is the brightest IR source outside the solar system.[21]

The most notable feature of Eta Carinae is its giant eruption or supernova impostor event, which originated in the primary star and was observed around 1843. In a few years, it produced almost as much visible light as a faint supernova explosion, but the star survived. It is estimated that at peak brightness it was around 20 million times more luminous than the sun. Other supernova impostors have been seen in other galaxies, for example the possible false supernova SN 1961v in NGC 1058[54] and SN 2006jc in UGC 4904,[55] which produced a false supernova noted in October 2004. Significantly SN 2006jc was destroyed in a supernova explosion two years later on October 9, 2006.

Temperature[edit]

Eta Carinae taken with the NACO near-IR adaptive optics instrument of ESO's Very Large Telescope.

The temperature of Eta Carinae B can be estimated with some accuracy due to spectral lines that are only likely to be produced by a star around 37,000K.[5]

The temperature of the primary star is more uncertain. For many years it was expected to be over 30,000K due to the presence of the high temperature spectral lines now attributed to the secondary star, although this conflicted with other spectral characteristics that ought only to be found in cooler stars. That conflict is now resolved, and Eta Carinae A, or at least what we can see of it, is accepted to be considerably cooler than Eta Carinae B. The star is likely to have been an early B hypergiant with a temperature of 20,000K - 25,000K at the time of its dscovery by Halley. An effective temperature based on its luminosity today would also be around 20,000K - 25,000K, but the hints of light directly from the star itself via some dense nebular features suggest a much cooler star at 9,000K - 14,000K. This cooler temperature may be a pseudo-photosphere formed where the opaque stellar wind starts to become transparent. Very recent observations show dramatic changes in the stellar wind and a possible increase in the temperature of any pseudo-photosphere, but it is still largely shrouded in dust shifting most of the light output into the infra-red.[25][34][52]

The powerful stellar winds from the two stars collide and produce temperatures as high as 60 MK, which is the source of the hard x-rays and gamma-rays close to the stars. Further out, expanding gases from the Great Eruption collides with interstellar material and is heated to around 60 MK, producing less energetic x-rays seen in a ring shape.

Size[edit]

The size of the two main stars in the Eta Carinae system is difficult to determine precisely because neither star can be seen directly. Eta Carinae B is likely to have a well-defined photosphere and its radius can be estimated from the assumed type of star. An O supergiant of 933,000 L at temperature 37,200K has an effective radius of 23.6 R.[4]

The size of Eta Carinae A is not even well defined. It has an optically dense stellar wind so the typical definition of a star's surface being approximately where it becomes opaque gives a very different result to where a more traditional definition of a surface might be. One study calculated a radius of 60 R for a hot "core" of 35,000K at optical depth 150, near the sonic point or very approximately what might be called a physical surface, but over 800 R for optical depth 0.67, the visible surface of the stellar wind.[56]

Rotation[edit]

Rotation rates of massive stars have a critical influence on their evolution and eventual death. The rotation rate of the Eta Carinae stars cannot be measured directly because their surfaces cannot be seen. Single massive stars spin down quickly due to braking from their string winds, but there are hints that both Eta Carinae A and B are fast rotators. One or both could have been spun up by binary interaction, for example accretion onto the secondary, and orbital dragging on the primary.[57]

Future prospects[edit]

With 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.[58]

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.[55] 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.[55] 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. When it does occur, the supernova will be brighter than Venus but not as bright as a full moon.[59]

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.[citation needed]

Possible effects on Earth[edit]

One theory of Eta Carinae's ultimate fate is collapsing to form a black hole - energy released as jets along the axis of rotation forms gamma ray bursts.

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.[60] 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.[61] 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.[51]

Cultural significance[edit]

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),[62] referring to Sea and Mountain, an asterism that Eta Carinae forms with s Carinae, λ Centauri and λ Muscae.[63]

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).[64] This is the only definitive indigenous record of Eta Carinae's outburst identified in the literature to date.

See also[edit]

References[edit]

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