 The Homunculus Nebula, surrounding Eta Carinae, imaged by WFPC2 at red and near-ultraviolet wavelengths Credit: Jon Morse (University of Colorado) & NASA Hubble Space Telescope | |
| Observation data Epoch J2000 Equinox J2000 | |
|---|---|
| Constellation | Carina |
| Right ascension | 10h 45m 03.591s |
| Declination | −59° 41′ 04.26″ |
| Apparent magnitude (V) | −1.0 to ~7.6 4.8 (2011) 4.6 (2013) 4.3 (2018) |
| Characteristics | |
| Spectral type | variable + O (WR?) |
| Apparent magnitude (U) | 6.37 |
| Apparent magnitude (B) | 6.82 |
| Apparent magnitude (R) | 4.90 |
| Apparent magnitude (J) | 3.39 |
| Apparent magnitude (H) | 2.51 |
| Apparent magnitude (K) | 0.94 |
| U−B color index | −0.45 |
| B−V color index | +0.61 |
| Variable type | LBV & binary |
| Astrometry | |
| Radial velocity (Rv) | −25.0 km/s |
| Proper motion (μ) | RA: −7.6 mas/yr Dec.: 1.0 mas/yr |
| Distance | 7,500 ly (2,300 pc) |
| Absolute magnitude (MV) | −8.6 (2012) |
| Orbit | |
| Primary | η Car A |
| Companion | η Car B |
| Period (P) | 2,022.7±1.3 days (5.54 yr) |
| Semi-major axis (a) | 15.4 AU |
| Eccentricity (e) | 0.9 |
| Inclination (i) | 130–145° |
| Periastron epoch (T) | 2009.03 |
| Details | |
| η Car A | |
| Mass | ~100, 120-200 M☉ |
| Radius | ~240 60-881 R☉ |
| Luminosity | 5,000,000 L☉ |
| Temperature | 9,400–35,200 K |
| Age | l.t. 3 Myr |
| η Car B | |
| Mass | 30–80 M☉ |
| Radius | 14.3–23.6 R☉ |
| Luminosity | <1 a="" href="https://en.wikipedia.org/wiki/Solar_luminosity" title="Solar luminosity">L☉ |
Eta Carinae is a stellar system containing at least two stars with a combined luminosity greater than five million times that of the Sun, located around 7,500 light-years (2,300 parsecs) distant in the constellation Carina. Previously a 4th-magnitude star, it brightened in 1837 to become brighter than Rigel marking the start of the Great Eruption. Eta Carinae became the second-brightest star in the sky between 11 and 14 March 1843 before fading well below naked eye visibility after 1856. In a smaller eruption, it reached 6th magnitude in 1892 before fading again. It has brightened consistently since about 1940, becoming brighter than magnitude 4.5 by 2014. Eta Carinae is circumpolar from latitudes south of latitude 30°S, and it is never visible north of about latitude 30°N.
The two main stars of the Eta Carinae system have an eccentric orbit with a period of 5.54 years. The primary is a peculiar star similar to a luminous blue variable (LBV) that was initially 150–250 M☉ of which it has lost at least 30 M☉ already, and is expected to explode as a supernova in the astronomically near future. This is the only star known to produce ultraviolet laser emission. The secondary star is hot and also highly luminous, probably of spectral class O, around 30–80 times as massive as the Sun. The system is heavily obscured by the Homunculus Nebula, material ejected from the primary during the Great Eruption. It is a member of the Trumpler 16 open cluster within the much larger Carina Nebula.
Although unrelated to the star and nebula, the weak Eta Carinids meteor shower has a radiant very close to Eta Carinae.
Observational history
Discovery and naming
There is no reliable evidence of Eta Carinae being observed or recorded before the 17th century, although Dutch navigator Pieter Keyser
described a fourth-magnitude star at approximately the correct position
around 1595–1596, which was copied onto the celestial globes of Petrus Plancius and Jodocus Hondius and the 1603 Uranometria of Johann Bayer. Frederick de Houtman's
independent star catalogue from 1603 does not include Eta Carinae among
the other 4th magnitude stars in the region. The earliest firm record
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 1679. The star was also known by the Bayer designations Eta Roboris Caroli, Eta Argus or Eta Navis. In 1751 Nicolas Louis de Lacaille gave the stars of Argo Navis and Robur Carolinum
a single set of Greek letter Bayer designations within his
constellation Argo, and designated three areas within Argo for the
purposes of using Latin letter designations three times over. Eta fell
within the keel portion of the ship which was later to become the
constellation Carina.
It was not generally known as Eta Carinae until 1879, when the stars of
Argo Navis were finally given the epithets of the daughter
constellations in the Uranometria Argentina of Gould.
Eta Carinae is too far south to be part of the mansion-based traditional Chinese astronomy, but it was mapped when the Southern Asterisms were created at the start of the 17th century. Together with s Carinae, λ Centauri, and λ Muscae, Eta Carinae forms the asterism 海山 (Sea and Mountain). 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).
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The lightcurve of Eta Carinae from some of the earliest observations to the current day
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.
Further sporadic observations over the next 70 years show that Eta
Carinae was probably around 3rd magnitude or fainter, until Lacaille
reliably recorded it at 2nd magnitude in 1751.
It is unclear whether Eta Carinae varied significantly in brightness
over the next 50 years; there are occasional observations such as William Burchell's at 4th magnitude in 1815, but it is uncertain whether these are just re-recordings of earlier observations.
Great Eruption
In
1827 Burchell specifically noted Eta Carinae's unusual brightness at
1st magnitude, and was the first to suspect that it varied in
brightness. John Herschel
made a detailed series of accurate measurements in the 1830s showing
that Eta Carinae consistently shone around magnitude 1.4 until November
1837. On the evening of December 16, 1837, Herschel was astonished to
see that it had brightened to slightly outshine Rigel. This event marked the beginning of a roughly 18-year period known as the Great Eruption.
Eta Carinae was brighter still on January 2, 1838, equivalent to Alpha Centauri,
before fading slightly over the following three months. Herschel did
not observe the star after this, but received correspondence from the
Reverend W.S. Mackay in Calcutta, who wrote in 1843, "To my great
surprise I observed this March last (1843), that the star Eta Argus had
become a star of the first magnitude fully as bright as Canopus, and in color and size very like Arcturus."
Observations at the Cape of Good Hope indicated it peaked in
brightness, surpassing Canopus, over March 11 to 14, 1843 before
beginning to fade, then brightened to between the brightness of Alpha
Centauri and Canopus between March 24 and 28 before fading once again. For much of 1844 the brightness was midway between Alpha Centauri and Beta Centauri,
around magnitude +0.2, before brightening again at the end of the year.
At its brightest in 1843 it likely reached an apparent magnitude of
−0.8, then −1.0 in 1845. The peaks in 1827, 1838, and 1843 are likely to have occurred at the periastron passage—the point the two stars are closest together—of the binary orbit. From 1845 to 1856, the brightness decreased by around 0.1 magnitudes per year, but with possible rapid and large fluctuations.
In their oral traditions, the Boorong clan of the Wergaia people of Lake Tyrrell, north-western Victoria, Australia told of a reddish star they knew as Collowgulloric War, the wife of War (Canopus, the Crow — wɑː). In 2010, astronomers Duane Hamacher and David Frew from Macquarie University in Sydney showed that this was Eta Carinae during its Great Eruption in the 1840s. 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.
Lesser Eruption
A
new brightening started in 1887, peaked at about magnitude 6.2 in 1892,
then at the end of March 1895 faded rapidly to about magnitude 7.5.
Although there are only visual records of the 1890 eruption, it has
been calculated that Eta Carinae was suffering 4.3 magnitudes of visual
extinction due to the gas and dust ejected in the Great Eruption. An
unobscured brightness would have been magnitude 1.5–1.9, significantly
brighter than the historical magnitude. This appeared to be a smaller copy of the Great Eruption, expelling much less material.
Twentieth century
Between 1900 and at least 1940, Eta Carinae appeared to have settled at a constant brightness at around magnitude 7.6, but in 1953 it was noted to have brightened again to magnitude 6.5. The brightening continued steadily, but with fairly regular variations of a few tenths of a magnitude.
In 1996 the variations were first identified as having a 5.52-year period,
later measured more accurately at 5.54 years, leading to the idea of a
binary system. The binary theory was confirmed by observations of radio,
optical, and near infrared radial velocity and line profile changes, referred to collectively as a spectroscopic event, at the predicted time of periastron passage in late 1997 and early 1998. At the same time there was a complete collapse of the X-ray emission presumed to originate in a colliding wind zone.
The confirmation of a luminous binary companion greatly modified the
understanding of the physical properties of the Eta Carinae system and
its variability.
A sudden doubling of brightness was observed in 1998–99 bringing
it back to naked eye visibility. During the 2014 spectroscopic event,
the apparent visual magnitude became brighter than magnitude 4.5.
The brightness does not always vary consistently at different
wavelengths, and does not always exactly follow the 5.5 year cycle.
Radio, infrared, and space-based observations have expanded coverage of
Eta Carinae across all wavelengths and revealed ongoing changes in the spectral energy distribution.
In July 2018, Eta Carinae was reported to have the strongest
colliding wind shock in the solar neighborhood. Observations with the NuSTAR satellite gave much higher resolution data than the earlier Fermi Gamma-ray Space Telescope.
Using direct focusing observations of the non-thermal source in the
extremely hard X-ray band that is spatially coincident with the star,
they showed that the source of non-thermal X-rays varies with the
orbital phase of the binary star system and that the photon index of the
emission is similar to that derived through analysis of the γ-ray
(gamma) spectrum.
| Approx. magnitude |
Year reached | Mag/yr |
|---|---|---|
| 7.6 | 1911 | - |
| 6.8 | 1951 | g.t. 0.1 |
| 6.5 | 1970 | 0.016 |
| 6.0 | 1989 | 0.026 |
| 5.5 | 1999 | 0.050 |
| 5.0 | 2008 | 0.056 |
| 4.5 | 2015 | 0.071 |
| 4.3 | 2018 | 0.067 |
Visibility
Eta Carinae and Carina Nebula in the constellation of Carina
As a 4th-magnitude star, Eta Carinae is comfortably visible to the naked eye in all but the most light-polluted skies in inner city areas according to the Bortle scale.
Its brightness has varied over a wide range, from the second-brightest
star in the sky at one point in the 19th century to well below naked eye
visibility. Its location at around 60°S in the far Southern Celestial Hemisphere means it cannot be seen by observers in Europe and much of North America.
Located between Canopus and the Southern Cross,
Eta Carinae is easily pinpointed as the brightest star within the large
naked eye Carina Nebula. In a telescope the "star" is framed within the
dark "V" dust lane of the nebula and appears distinctly orange and clearly non-stellar. High magnification will show the two orange lobes of a surrounding reflection nebula known as the Homunculus Nebula
on either side of a bright central core. Variable star observers can
compare its brightness with several 4th- and 5th-magnitude stars closely
surrounding the nebula.
Discovered in 1961, the weak Eta Carinids meteor shower has a radiant
very close to Eta Carinae. Occurring from 14 to 28 January, the shower
peaks around 21 January. Meteor showers are not associated with bodies
outside the Solar System, making the proximity to Eta Carinae merely a
coincidence.
Visual spectrum
The strength and profile of the lines in the Eta Carinae spectrum are highly variable, but there are a number of consistent distinctive features. The spectrum is dominated by emission lines, usually broad although the higher excitation lines are overlaid by a narrow central component from dense ionized nebulosity, especially the Weigelt Blobs. Most lines show a P Cygni profile but with the absorption wing much weaker than the emission. The broad P Cygni lines are typical of strong stellar winds, with very weak absorption in this case because the central star is so heavily obscured. Electron scattering wings are present but relatively weak, indicating a clumpy wind. Hydrogen lines are present and strong, showing that Eta Carinae still retains much of its hydrogen envelope. HeI lines are much weaker than the hydrogen lines, and the absence of HeII lines provides an upper limit to the possible temperature of the primary star. NII
lines can be identified but are not strong, while carbon lines cannot
be detected and oxygen lines are at best very weak, indicating core hydrogen burning via the CNO cycle with some mixing to the surface. Perhaps the most striking feature is the rich FeII emission in both permitted and forbidden lines, with the forbidden lines arising from excitation of low density nebulosity around the star.
The earliest analyses of the star's spectrum are descriptions of
visual observations from 1869, of prominent emission lines "C, D, b, F,
and the principal green nitrogen line". Absorption lines are explicitly
described as not being visible. The letters refer to Fraunhofer's spectral notation and correspond to Hα, HeI ("D" usually refers to the sodium doublet, but "d" or "D3" was used for the nearby helium line), FeII, and Hβ. It is assumed that the final line is from FeII very close to the green nebulium line now known to be from OIII.
Photographic spectra from 1893 were described as similar to an F5
star, but with a few weak emission lines. Analysis to modern spectral
standards suggests an early F spectral type.
By 1895 the spectrum again consisted mostly of strong emission lines,
with the absorption lines present but largely obscured by emission. This
spectral transition from F supergiant to strong emission is characteristic of novae, where ejected material initially radiates like a pseudo-photosphere and then the emission spectrum develops as it expands and thins.
The emission line spectrum associated with dense stellar winds
has persisted ever since the late 19th century. Individual lines show
widely varying widths, profiles, and Doppler shifts,
often multiple velocity components within the same line. The spectral
lines also show variation over time, most strongly with a 5.5-year
period but also less dramatic changes over shorter and longer periods,
as well as ongoing secular development of the entire spectrum. The spectrum of light reflected from the Weigelt Blobs, and assumed to originate mainly with the primary, is similar to the extreme P Cygni-type star HDE 316285 which has a spectral type of B0Ieq.
Direct spectral observations did not begin until after the Great Eruption, but light echoes from the eruption reflected from other parts of the Carina Nebula 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.
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 shell
of ejected material, had cooled further and may have been colliding with
circumstellar material in a similar way to a type IIn supernova.
In the second half of the 20th century, much higher resolution
visual spectra became available. The spectrum continued to show complex
and baffling features, with much of the energy from the central star
being recycled into the infrared 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.
The 5.5 year orbital cycle produces strong spectral changes at
periastron that are known as spectroscopic events. 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 long-term
secular changes in the stellar winds or previously ejected material may
be the culmination of a return to the state of the star before its Great
Eruption.
Ultraviolet
The ultraviolet spectrum of the Eta Carinae system shows many emission lines of ionised metals such as FeII and CrII, as well as Lymanα (Lyα)
and a continuum from a hot central source. The ionisation levels and
continuum require the existence of a source with a temperature at least
37,000 K.
Certain FeII UV lines are unusually strong. These originate in the Weigelt Blobs and are caused by a low-gain lasing effect. Ionized hydrogen between a blob and the central star generates intense Lyα emission which penetrates the blob. The blob contains atomic hydrogen with a small admixture of other elements, including iron photo-ionized by radiation from the central stars. An accidental resonance (where emission coincidentally has a suitable energy to pump the excited state) allows the Lyα emission to pump the Fe+ ions to certain pseudo-metastable states, creating a population inversion that allows the stimulated emission to take place. This effect is similar to the maser
emission from dense pockets surrounding many cool supergiant stars, but
the latter effect is much weaker at optical and UV wavelengths and Eta
Carinae is the only clear instance detected of an ultraviolet astrophysical laser. A similar effect from pumping of metastable OI states by Lyβ emission has also been confirmed as an astrophysical UV laser.
Infrared
Stars similar to Eta Carinae in nearby galaxies
Infrared observations of Eta Carinae have become increasingly
important. The vast majority of the electromagnetic radiation from the
central stars is absorbed by surrounding dust, then emitted as mid- and far infrared
appropriate to the temperature of the dust. This allows almost the
entire energy output of the system to be observed at wavelengths that
are not strongly affected by interstellar extinction, leading to estimates of the luminosity that are more accurate than for other extremely luminous stars. Eta Carinae is the brightest source in the night sky at mid-infrared wavelengths.
Far infrared observations show a large mass of dust at 100–150 K, suggesting a total mass for the Homunculus of 20 solar masses (M☉)
or more. This is much larger than previous estimates, and is all
thought to have been ejected in a few years during the Great Eruption.
Near-infrared
observations can penetrate the dust at high resolution to observe
features that are completely obscured at visual wavelengths, although
not the central stars themselves. The central region of the Homunculus
contains a smaller Little Homunculus from the 1890 eruption, a butterfly of separate clumps and filaments from the two eruptions, and an elongated stellar wind region.
High energy radiation
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 the Terrier-Sandhawk rocket, followed by Ariel 5, OSO 8, and Uhuru sightings.
More detailed observations were made with the Einstein Observatory, ROSAT X-ray telescope, Advanced Satellite for Cosmology and Astrophysics (ASCA), and Chandra X-ray Observatory.
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 parsec (2.2 light-years) 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.
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. The highest energy gamma-rays above 100 MeV detected by AGILE show strong variability, while lower energy gamma-rays observed by Fermi show little variability.
Radio emission
Radio emissions have been observed from Eta Carinae across the microwave band. It has been detected in the 21 cm HI line, but has been particularly closely studied in the millimetre and centimetre bands. Masing hydrogen recombination lines
(from the combining of an electron and proton to form a hydrogen atom)
have been detected in this range. The emission is concentrated in a
small non-point source less than 4 arcseconds across and appears to be mainly free-free emission (thermal bremsstrahlung) from ionised gas, consistent with a compact HII region at around 10,000 K. High resolution imaging shows the radio frequencies originating from a disk a few arcseconds in diameter, 10,000 astronomical units (AU) wide at the distance of Eta Carinae.
The radio emission from Eta Carinae shows continuous variation in strength and distribution over a 5.5 year cycle. The HII
and recombination lines vary very strongly, with continuum emission
(electromagnetic radiation across a broad band of wavelengths) less
affected. This shows a dramatic reduction in the ionisation level of the
hydrogen for a short period in each cycle, coinciding with the
spectroscopic events at other wavelengths.
Surroundings
Annotated image of Carina Nebula
Eta Carinae is found within the Carina Nebula, a giant star-forming region in the Carina–Sagittarius Arm of the Milky Way.
The nebula is a prominent naked eye object in the southern skies
showing a complex mix of emission, reflection, and dark nebulosity. Eta
Carinae is known to be at the same distance as the Carina Nebula and its
spectrum can be seen reflected off various star clouds in the nebula.
The appearance of the Carina Nebula, and particularly of the Keyhole
region, has changed significantly since it was described by John Herschel over 150 years ago. This is thought to be due to the reduction in ionizing radiation from Eta Carinae since the Great Eruption.
Prior to the Great Eruption the Eta Carinae system contributed up to
20% of the total ionizing flux for the whole Carina Nebula, but that is
now mostly blocked by the surrounding gas and dust.
Trumpler 16
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 neighbor Trumpler 14 are the two dominant star clusters of the Carina OB1 association, an extended grouping of young luminous stars with a common motion through space.
Homunculus
A 3D model of the Homunculus Nebula
Eta Carinae is enclosed by, and lights up, the Homunculus Nebula,
a small emission and reflection nebula composed mainly of gas ejected
during the Great Eruption event in the mid-19th century, as well as dust
that condensed from the debris. The nebula consists of two polar lobes aligned with the rotation axis of the star, plus an equatorial "skirt", the whole being around 18″ long. Closer studies show many fine details: a Little Homunculus
within the main nebula, probably formed by the 1890 eruption; 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.
The lobes of the Homunculus are considered to be formed almost
entirely due to the initial eruption, rather than shaped by or including
previously ejected or interstellar material, although the scarcity of
material near the equatorial plane allows some later stellar wind and
ejected material to mix. Therefore, the mass of the lobes gives an
accurate measure of the scale of the Great Eruption, with estimates
ranging from 12–15 M☉ up to as high as 40 M☉.
The results show that the material from the Great Eruption is strongly
concentrated towards the poles; 75% of the mass and 90% of the kinetic
energy were released above latitude 45°.
A unique feature of the Homunculus is the ability to measure the
spectrum of the central object at different latitudes by the reflected
spectrum from different portions of the lobes. These clearly show a polar wind where the stellar wind is faster and stronger at high latitudes thought to be due to rapid rotation causing gravity brightening towards the poles. In contrast the spectrum shows a higher excitation temperature closer to the equatorial plane. By implication the outer envelope of Eta Carinae A is not strongly convective as that would prevent the gravity darkening.
The current axis of rotation of the star does not appear to exactly
match the alignment of the Homunculus. This may be due to interaction
with Eta Carinae B which also modifies the observed stellar winds.
Distance
The
distance to Eta Carinae has been determined by several different
methods, resulting in a widely accepted value of 2,300 parsecs
(7,800 light-years), with a margin of error around 100 parsecs
(330 light-years). The distance to Eta Carinae itself cannot be measured using parallax
due to its surrounding nebulosity, but other stars in the Trumpler 16
cluster are expected to be at a similar distance and are acessible to
parallax. Gaia Data Release 2
has provided the parallax for many stars considered to be members of
Trumpler 16, finding that the four hottest O-class stars in the region
have very similar parallaxes with a mean value of 0.383 ± 0.017
milli-arcseconds (mas), which translates to a distance of 2,600 ±
100 parsecs. This implies that Eta Carinae may be more distant than
previously thought, and also more luminous, although it is still
possible that it is not at the same distance as the cluster or that the
parallax measurements have large systematic errors.
The distances to star clusters can be estimated by using a Hertzsprung–Russell diagram or color–color diagram to calibrate the absolute magnitudes of the stars, for example fitting the main sequence or identifying features such as a horizontal branch,
and hence their distance from Earth. It is also necessary to know the
amount of interstellar extinction to the cluster and this can be
difficult in regions such as the Carina Nebula.
A distance of 7,330 light-years (2,250 parsecs) has been determined
from the calibration of O-type star luminosities in Trumpler 16.
After determining an abnormal reddening correction to the extinction,
the distance to both Trumpler 14 and Trumpler 16 has been measured at
9,500±1000 light-years (2,900±300 parsecs).
The known expansion rate of the Homunculus Nebula provides an
unusual geometric method for measuring its distance. Assuming that the
two lobes of the nebula are symmetrical, the projection of the nebula
onto the sky depends on its distance. Values of 2,300, 2,250 and
2,300 parsecs have been derived for the Homunculus, and Eta Carinae is
clearly at the same distance.
Properties
X-ray, optical and infrared images of Eta Carinae (August 26, 2014)
The Eta Carinae star 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 the system's binary
nature was proposed by the Brazilian astronomer Augusto Damineli in 1996 and confirmed in 2005.
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.
Orbit
Eta Carinae orbit
The binary nature of Eta Carinae is clearly established, although the
components have not been directly observed and cannot even be clearly
resolved spectroscopically due to scattering and re-excitation in the
surrounding nebulosity. Periodic photometric and spectroscopic
variations prompted the search for a companion, and modelling of the
colliding winds and partial "eclipses" of some spectroscopic features
have constrained the possible orbits.
The period of the orbit is accurately known at 5.539 years,
although this has changed over time due to mass loss and accretion. The
period between the Great Eruption and the smaller 1890 eruption was
apparently 5.52 years, while before the Great Eruption it may have been
lower still, possibly between 4.8 and 5.4 years.
The orbital separation is only known approximately, with a semi-major
axis of 15–16 AU. The orbit is highly eccentric, e = 0.9. This means
that the separation of the stars varies from around 1.6 AU, similar to
the distance of Mars from the Sun, to 30 AU, similar to the distance of
Neptune.
Perhaps the most valuable use of an accurate orbit for a binary
star system is to directly calculate the masses of the stars. This
requires the dimensions and inclination of the orbit to be accurately
known. The dimensions of Eta Carinae's orbit are only known
approximately as the stars cannot be directly and separately observed.
The inclination has been modeled at 130–145 degrees, but the orbit is
still not known accurately enough to provide the masses of the two
components.
Classification
Eta Carinae A is classified as a luminous blue variable (LBV) due to the distinctive spectral and brightness variations. This type of variable star
is characterized by irregular changes from a high temperature quiescent
state to a low temperature outburst 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 outburst
all LBVs have about the same temperature, which is near 8,000 K. LBVs in
a normal outburst are visually brighter than when quiescent although
the bolometric luminosity is unchanged.
A Great Eruption event similar to Eta Carinae A's has only been observed in one other star in the Milky Way—P Cygni—and
in a handful of other possible LBVs in external galaxies. None of them
seem to be quite as violent as Eta Carinae's. It is unclear 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. Some similar events in external galaxies
have been mistaken for supernovae and have been called supernova impostors, although this grouping may also include other types of non-terminal transients that approach the brightness of a supernova.
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 impostors detected in external galaxies. It does not currently
lie on the S Doradus instability strip, although it is unclear what the
temperature or spectral type of the underlying star actually is, and
during its Great Eruption it was much cooler than a typical LBV
outburst. 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 9,000 K–14,000 K which would be
typical for an LBV in eruption.
Eta Carinae B is a massive luminous hot star, about which little
else is known. From certain high excitation spectral lines that ought
not to be produced by the primary, Eta Carinae B is thought to be 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.
Mass
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. The mass can be strongly constrained
to be greater than 90 M☉, due to the high luminosity. Standard models of the system assume masses of 100–120 M☉ and 30–60 M☉
for the primary and secondary respectively. Higher masses have been
suggested, to model the energy output and mass transfer of the Great
Eruption, with a combined system mass of over 250 M☉ before the Great Eruption. Eta Carinae A has clearly lost a great deal of mass since it formed and it is expected that it was initially 150–250 M☉, although it may have formed through binary merger.
Mass loss
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The Carina Nebula. Eta Carinae is the brightest star, on the left side.
Mass loss is one of the most intensively studied aspects of massive
star research. Put simply, observed mass loss rates in the best models
of stellar evolution do not reproduce the observed properties 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.
Eta Carinae A is losing a lot of mass due to its extreme
luminosity and relatively low surface gravity. Its 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 at least 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☉.
The bulk of the mass loss occurs in a wind with a terminal velocity of
about 420 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.
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 10−5 M☉/year at speeds of 3,000 km/s, typical of a hot O class star. For a portion of the highly eccentric orbit, it may actually gain material from the primary via an accretion disk. During the Great Eruption of the primary, the secondary could have accreted several M☉, producing strong jets which formed the bipolar shape of the Homunculus Nebula.
Luminosity
The
stars of the Eta Carinae system are completely obscured by dust and
opaque stellar winds, with much of the ultraviolet and visual radiation
shifted to infrared. The total electromagnetic radiation across all
wavelengths for both stars combined is several million solar
luminosities (L☉). The best estimate for the luminosity of the primary is 5 million L☉
making it one of the most luminous stars in the galaxy. The luminosity
of Eta Carinae B is particularly uncertain, probably several hundred
thousand L☉ and almost certainly no more than 1 million L☉.
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 the luminosity was as high as 50
million L☉. Other supernova impostors have been seen in other galaxies, for example the possible false supernova SN 1961v in NGC 1058 and SN 2006jc's pre-explosion outburst in UGC 4904.
Following the Great Eruption, Eta Carinae became self-obscured by
the ejected material, resulting in dramatic reddening. This has been
estimated at four magnitudes at visual wavelengths, meaning the
post-eruption luminosity was comparable to the luminosity when first
identified.
Eta Carinae is still much brighter at infrared wavelengths, despite the
presumed hot stars behind the nebulosity. The recent visual brightening
is considered to be largely caused by a decrease in the extinction, due
to thinning dust or a reduction in mass loss, rather than an underlying
change in the luminosity.
Temperature
HST image of the Homunculus Nebula; inset is a VLT NACO infrared image of Eta Carinae.
Until late in the 20th century, the temperature of Eta Carinae was
assumed to be over 30,000 K because of the presence of high excitation
spectral lines, but other aspects of the spectrum suggested much lower
temperatures and complex models were created to account for this.
It is now known that the Eta Carinae system consists of at least two
stars, both with strong stellar winds and a shocked colliding wind
(wind-wind collision or WWC) zone, embedded within a dusty nebula that
reprocesses 90% of the electromagnetic radiation into the mid and far
infrared. All of these features have different temperatures.
The powerful stellar winds from the two stars collide in a roughly conical WWC zone and produce temperatures as high as 100 MK
at the apex between the two stars. This zone is the source of the hard
x-rays and gamma-rays close to the stars. Near periastron, as the
secondary ploughs through ever denser regions of the primary wind, the
colliding wind zone becomes distorted into a spiral trailing behind Eta
Carinae B.
The wind-wind collision cone separates the winds of the two
stars. For 55–75° behind the secondary, there is a thin hot wind typical
of O or Wolf–Rayet stars. This allows some radiation from Eta Carinae B
to be detected and its temperature can be estimated with some accuracy
due to spectral lines that are unlikely to be produced by any other
source. Although the secondary star has never been directly observed,
there is widespread agreement on models where it has a temperature
between 37,000 K and 41,000 K.
In all other directions on the other side of the wind-wind
collision zone, there is the wind from Eta Carinae A, cooler and around
100 times denser than Eta Carinae B's wind. It is also optically dense,
completely obscuring anything resembling a true photosphere and
rendering any definition of its temperature moot. The observable
radiation originates from a pseudo-photosphere where the optical density of the wind drops to near zero, typically measured at a particular Rossland opacity value such as 2⁄3. This pseudo-photosphere is observed to be elongated and hotter along the presumed axis of rotation.
Eta Carinae A is likely to have appeared as an early B hypergiant with a temperature of between 20,000 K and 25,000 K at the time of its discovery by Halley. An effective temperature determined for the surface of a spherical optically thick wind at several hundred R☉ would be 9,400–15,000 K, while the temperature of a theoretical 60 R☉ hydrostatic "core" at optical depth 150 would be 35,200 K.
The effective temperature of the visible outer edge of the opaque
primary wind is generally treated as being 15,000 K–25,000 K on the
basis of visual and ultraviolet spectral features assumed to be directly
from the wind or reflected via the Weigelt Blobs.
The Homunculus contains dust at temperatures varying from 150 K
to 400 K. This is the source of almost all the infrared radiation that
makes Eta Carinae such a bright object at those wavelengths.
Further out, expanding gases from the Great Eruption collide with interstellar material and are heated to around 5 MK, producing less energetic X-rays seen in a horseshoe or ring shape.
Size
Hubble Space Telescope image showing the bipolar Homunculus Nebula which surrounds Eta Carinae
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☉ with a temperature of 37,200 K has an effective radius of 23.6 R☉.
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 35,000 K at optical depth
150, near the sonic point or very approximately what might be called a
physical surface. At optical depth 0.67 the radius would be over 800 R☉, indicating an extended optically thick stellar wind.
At the peak of the Great Eruption the radius, so far as such a thing is
meaningful during such a violent expulsion of material, would have been
around 1,400 R☉, comparable to the largest known stars, including VY Canis Majoris.
The stellar sizes should be compared with their orbital separation, which is only around 250 R☉ at periastron. The accretion radius of the secondary is around 60 R☉, suggesting strong accretion near periastron leading to a collapse of the secondary wind.
It has been proposed that the initial brightening from 4th magnitude to
1st at relatively constant bolometric luminosity was a normal LBV
outburst, albeit from an extreme example of the class. Then the
companion star passing through the expanded photosphere of the primary
at periastron triggered the further brightening, increase in luminosity,
and extreme mass loss of the Great Eruption.
Rotation
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 strong winds, but
there are hints that both Eta Carinae A and B are fast rotators, up to
90% of critical velocity. One or both could have been spun up by binary
interaction, for example accretion onto the secondary and orbital
dragging on the primary.
Evolution
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The recent lightcurve of Eta Carinae, with observations at standard wavelengths marked
Eta Carinae is a unique object, with no very close analogues
currently known in any galaxy. Therefore, its future evolution is highly
uncertain, but almost certainly involves further mass loss and an
eventual supernova.
Eta Carinae A would have begun life as an extremely hot star on
the main sequence, already a highly luminous object over a million L☉. The exact properties would depend on the initial mass, which is expected to have been at least 150 M☉ and possibly much higher. A typical spectrum when first formed would be O2If and the star would be mostly or fully convective due to CNO cycle fusion at the very high core temperatures. Sufficiently massive or differentially rotating stars undergo such strong mixing that they remain chemically homogeneous during core hydrogen burning.
As core hydrogen burning progresses, a very massive star would
slowly expand and become more luminous, becoming a blue hypergiant and
eventually an LBV while still fusing hydrogen in the core. When hydrogen
at the core is depleted after 2–2.5 million years, hydrogen shell burning
continues with further increases in size and luminosity, although
hydrogen shell burning in chemically homogeneous stars may be very brief
or absent since the entire star would become depleted of hydrogen. In
the late stages of hydrogen burning, mass loss is extremely high due to
the high luminosity and enhanced surface abundances of helium and
nitrogen. As hydrogen burning ends and core helium burning
begins, massive stars transition very rapidly to the Wolf–Rayet stage
with little or no hydrogen, increased temperatures, and decreased
luminosity. They are likely to have lost over half their initial mass
at this point.
It is unclear whether triple-alpha
helium fusion has started at the core of Eta Carinae A. The elemental
abundances at the surface cannot be accurately measured, but ejecta
within the Homunculus are around 60% hydrogen and 40% helium, with
nitrogen enhanced to ten times solar levels. This is indicative of
ongoing CNO cycle hydrogen fusion.
Models of the evolution and death of single very massive stars
predict an increase in temperature during helium core burning, with the
outer layers of the star being lost. It becomes a Wolf–Rayet star on
the nitrogen sequence,
moving from WNL to WNE as more of the outer layers are lost, possibly
reaching the WC or WO spectral class as carbon and oxygen from the
triple alpha process reach the surface. This process would continue with
heavier elements being fused until an iron core develops, at which
point the core collapses and the star is destroyed. Subtle differences
in initial conditions, in the models themselves, and most especially in
the rates of mass loss, produce different predictions for the final
state of the most massive stars. They may survive to become a
helium-stripped star or they may collapse at an earlier stage while they
retain more of their outer layers.
The lack of sufficiently luminous WN stars and the discovery of
apparent LBV supernova progenitors has also prompted the suggestion that
certain types of LBVs explode as a supernova without evolving further.
Eta Carinae is a close binary and this complicates the evolution
of both stars. Compact massive companions can strip mass from larger
primary stars much more quickly than would occur in a single star, so
the properties at core collapse can be very different. In some
scenarios, the secondary can accrue significant mass, accelerating its
evolution, and in turn be stripped by the now compact Wolf–Rayet
primary.
In the case of Eta Carinae, the secondary is clearly causing additional
instability in the primary, making it difficult to predict future
developments.
Potential supernova
Supernovae types depending on initial mass and metallicity
The overwhelming probability is that the next supernova observed in
the Milky Way will originate from an unknown white dwarf or anonymous
red supergiant, very likely not even visible to the naked eye.
Nevertheless, the prospect of a supernova originating from an object as
extreme, nearby, and well-studied as Eta Carinae arouses great
interest.
As a single star, a star originally around 150 times as massive
as the Sun would typically reach core collapse as a Wolf–Rayet star
within 3 million years. At low metallicity, many massive stars will collapse directly to a black hole with no visible explosion or a sub-luminous supernova, and a small fraction will produce a pair-instability supernova,
but at solar metallicity and above there is expected to be sufficient
mass loss before collapse to allow a visible supernova of type Ib or Ic.
If there is still a large amount of expelled material close to the
star, the shock formed by the supernova explosion impacting the
circumstellar material can efficiently convert kinetic energy to radiation, resulting in a superluminous supernova (SLSN) or hypernova,
several times more luminous than a typical core collapse supernova and
much longer-lasting. Highly massive progenitors may also eject
sufficient nickel to cause a SLSN simply from the radioactive decay.
The resulting remnant would be a black hole since it is highly unlikely
such a massive star could ever lose sufficient mass for its core not to
exceed the limit for a neutron star.
The existence of a massive companion brings many other
possibilities. If Eta Carinae A was rapidly stripped of its outer
layers, it might be a less massive WC- or WO-type star when core
collapse was reached. This would result in a type Ib or type Ic
supernova due to the lack of hydrogen and possibly helium. This
supernova type is thought to be the originator of certain classes of
gamma ray bursts, but models predict they occur only normally in less
massive stars.
Several unusual supernovae and impostors have been compared to
Eta Carinae as examples of its possible fate. One of the most compelling
is SN 2009ip,
a blue supergiant which underwent a supernova impostor event in 2009
with similarities to Eta Carinae's Great Eruption, then an even brighter
outburst in 2012 which is likely to have been a true supernova. SN 2006jc, some 77 million light years away in UGC 4904, in the constellation Lynx,
also underwent a supernova impostor brightening in 2004, followed by a
magnitude 13.8 type Ib supernova, first seen on 9 October 2006. Eta
Carinae has also been compared to other possible supernova impostors
such as SN 1961V, and to superluminous supernovae such as SN 2006gy.
Possible effects on Earth
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.
A typical core collapse supernova at the distance of Eta Carinae would peak at an apparent magnitude around −4, similar to Venus. A SLSN could be five magnitudes brighter, potentially the brightest supernova in recorded history (currently SN 1006). At 7,500 light-years from the star it is unlikely to directly affect terrestrial lifeforms, as they will be protected from gamma rays by the atmosphere and from some other cosmic rays by the magnetosphere. The main damage would 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
supernova, which would result in a significant increase in UV radiation
reaching Earth's surface from the Sun. This would require a typical
supernova to be closer than 50 light-years from Earth, and even a
potential hypernova would need to be closer than Eta Carinae. Another analysis of the possible impact discusses more subtle effects from the unusual illumination, such as possible melatonin suppression with resulting insomnia
and increased risk of cancer and depression. It concludes that a
supernova of this magnitude would have to be much closer than Eta
Carinae to have any type of major impact on Earth.
Eta Carinae is not expected to produce a gamma-ray burst and its axis is not currently aimed near Earth.
A gamma-ray burst in any case would need to be within a few light years
of Earth to have significant effects. The Earth's atmosphere protects
us from all the radiation apart from UV light (it is opaque to gamma
rays which have to be observed using space telescopes). The main effect
would be due to damage to the ozone layer. Eta Carinae is too far away
to do that even if it did produce a gamma ray burst.
