Eta Carinae 2

Jon Morse, Kris Davidson,
STScI, NASA

Larger infrared image.

Only 7,500 light-years away,
Eta Carinae underwent a
violent outburst more than
150 years ago. Like the Pistol
Star, it is a Luminous Blue
Variable (more at Astronomy
Picture of the Day and STScI).

Breaking News

On February 20, 2008, the European Space Agency's Integral Telescope announced the "unambiguous discovery" of non-thermal emission, high-energy (or "hard") X-rays which are thought to be generated by the collision of stellar winds from Eta Carinae and a binary companion that produces massive, x-ray generating shockwaves. Estimated to have a combined mass of 100 to 150 Solar-masses, the two massive stars in the apparent binary system are shedding around one Earth-mass per day of ejected material, which stellar winds that can reach speeds of 900 to 1,200 miles per second (1,500 to 2,000 kilometers per second). As the two stars are in close proximity, their winds collide in massive shockwaves where temperatures reach several thousand million degrees Kelvin. Electrons caught in the magnetic fields around the shockwaves are bounced back and forth and accelerated to tremendous energy levels before colliding with low-frequency photons that create the hard X-rays detected (more from ESA and Leyder et al, 2007).

Leyder et al, 2007;
Integral, ESA

Larger X-ray image and
shockwave animation.

Eta Carinae's binary nature
may have been confirmed by
the detection of hard X-rays
that may be generated by the
close collision of binary
stellar winds (more).

On November 1, 2005, astronomers using NASA's Far Ultraviolet Spectroscopic Explorer (FUSE) satellite claimed confirmation that the star has a hotter companion that is a brighter emitter of ultraviolet light (NASA press release-- more below), but some astronomers believe that only the primary star's dense wind was detected (Francis Reddy, Astronomy, June 2007; Weigelt et al, 2007; and Martin et al, 2006b and 2006a). In late 2003, some astronomers announced that the star may become a supernova in 10,000 to 20,000 years (more below -- ESO press release).

Galactic Region around Eta Carinae

Don Petit, NASA

Larger image.

Eta Carinae lies within the Carina
and Keyhole Nebulae, near the
Southern Cross and Coal Sack
Nebula (more from Space Station
Science Picture of the day).

One of the brightest stars in the Milky Way, Eta Carinae lies about 7,500 light-years (ly) away from Sol. This possible binary system is located in the eastern part (10:45:3.59-59:41:4.26, ICRS 2000.0) of Constellation Carina, the Ship's Keel. It can be found northwest of Lamda Centauri, southwest of Pi Centauri, southwest of Mu Velorum, northeast of Theta Carinae, and southeast of Scutulam (Iota Carinae), Avior (Epsilon Carinae), and Canopus (Alpha Carinae).

N. Smith, Hubble
Heritage team,
STScI, AURA,
NASA, ESA

Larger image.

Eta Carinae lies
in a region with
many interesting
celestial objects.

According to Robert Burnham, Jr. (1931-93), Eta Carinae was first recorded by Sir Edmond Halley (1656-1742) as a 4th magnitude star in 1677. However, it has changed greatly in brightness ever since, reaching 2nd magnitude in 1730 then repeatedly falling and rising in brightness (more on historical variations since 1827). In 1843, Eta Carinae had a "Great Eruption," when it brightened to outshine every star in Earth's night sky except for Sirius, reaching a maximum magnitude of -0.8 and was one of the most luminous stars in the Milky Way Galaxy. Despite releasing as much visible light as a supernova explosion, the star survived. As shown in the top image, that explosion produced two polar lobes and a large thin equatorial disk, all moving outward at about 1.5 million miles (2.4 million km) per hour.

© ESO

Larger composite
image (more).

Eta Carinae lies
in the Carinae
Nebula, on which
the dark Keyhole
Nebula is
superimposed
(more).

Eta Carinae lies in the OB association Trumpler 16, within the Carina Nebula (NGC 3372). The Nebula sprawls across more than 200 ly, around 300 ly if its faint outer filaments are included (see wide field and three color images). Visible to the naked eye of southern observers, its brilliant appearance is caused by intense ultraviolet radiation from many, very young, hot, and massive stars, including spectral type O3 that have surface temperatures 10 times than Sol's as well as masses up to 100 times or more than Sol's; indeed, the first O3 stars were discovered in the Carina Nebula (Nolan R. Walborn, 1971). The Nebula also contains three Wolf-Rayet stars of the nitrogen sequence (WN), immediate evolutionary descendants of the O3 stars with very large rates of mass ejection, in addition to the famous explosive variable star, Eta Carinae itself.

CTIO, NOAO, AURA/ NSF

Larger image.

Location of Eta Carinae
and two other massive
young stars in the
Carina Nebula (more).

Superimposed on the bright Carina Nebula is the dark Keyhole Nebula (NGC 3324). Enhanced by the rich field of bright stars around it, the Keyhole also has been admired for its size and complexity. It was given its name just before the middle of the 19th Century by Sir John Frederick William Herschel (1792-1871).

AURA, STScI, NASA

Large image.

This region of the Carinae
Nebula is dominated by a
seven-light-year-wide ring
of the Keyhole Nebula with
numerous small dark
globules that may be
collapsing to form new
stars (more).

Observing from from South Africa, Herschel was inspired by a dark ring in the upper part of the Carina Nebula which resembled a "keyhole," which he first described and drew when Eta Carinae was at its 19th Century peak brightness. Since then, however, the fading of the binary system has made the Keyhole's appearance less definite. Furthermore, the nature of the ring structure remains a mystery as there are no bright stars within it, not even embedded infrared sources. As its radio signature indicates that it is not a supernova remnant, the Keyhole ring may simply be a large dust cloud within the Carina Nebula that is being shaped by surrounding O3 and WN stars (more discussion at Nolan R. Walborn's Carina Nebula).

Eta Carinae A

Cerro Tololo 4-meter Blanco telescope
© NOAO, AURA, NSF

Larger image taken in 1976.

Currently with light dimmed and
re-radiated by dust, Eta Carinae
is the orange-colored object to
the left center of the Eta Carinae
Nebula, also known as the
Keyhole (more).

According to astronomer James Kaler, analysis of Eta Carinae's spectrum indicates that the primary has a surface temperature between 20,000 and 30,000 °Kelvin and a spectral class between B0 and B1 (James Kaler, 2001). Similar to the Pistol Star in brightness, Eta Carinae is one of the most luminous stars in the Local Group of galaxies surrounding the Milky Way (Figer et al, 1995). With an absolute bolometric magnitude around -12, it appears to be a hypergiant (luminosity class 0) with a total luminosity around five million times of Sol's. After the 1837-56 Great Eruption, Eta Carinae faded to become a dim object, invisible to the naked eye, but it began to brighten after 1940 to again become visible to the naked eye. The binary system is a natural emitter of coherent, ultraviolet LASER light that passes through an enveloping cloud of gas and dust (see illustration and discussion at Astronomy Picture of the Day).

2MASS, U. of Massachusetts,
IPAC (JPL/Caltech), NASA, NSF

Larger infrared field image.

In an infrared image, the star is more
conspicuous because the dark Keyhole
nebula drops out (more images).

Although Eta Carinae A may have started with as much as 200 Solar-masses, it has been violently expelling much of its substance since its birth. As a star with more than 40 Solar-masses with sudden outflows of mass in vast explosions that may take centuries apart, it is classified as a Luminous Blue Variable like the Pistol Star, as an extremely large star that may be related to the smaller but similarly explosive, Wolf-Rayet stars. It probably lost two to three Solar-masses of gas and dust during its Great Eruption, although its current mass loss rate is estimated to be only around 1/1,000th to 1/10,000th of a Solar-mass annually (Roberta M. Humphreys, 1989, page 5).

MSX, IPAC, NASA

Larger, false-color
infrared image.

The star's light has
been dimmed and
reddened by a dusty,
complex, and evolving
nebula (more).

If Eta Carinae was a single star, it would have also been one of the most massive stars known, having around 120 Solar-masses left (Augusto Damineli, 1996). The star may only be around one to three million years old (perhaps 2.6 million years), but some astronomers now estimate that it may become a supernova within 10,000 to 20,000 years (more). Some astronomers, however, suspected that the star has at least one stellar companion, which was confirmed in late 2005. Useful catalogue numbers and designations for this star are: Eta Car, NOVA Car 1843, HR 4210, HD 93308, CD-59 3306, CP(D)-59 2620, and SAO 238429.

Chandra X-ray Observatory, NASA

Larger x-ray image.

Eta Carinae lies in the white area within
the three-light-month-diameter central
blue cloud, that is inside a cooler,
two-light-year-diameter "horse-shoe"
of gas and dust blown off about a
thousand years ago (more).

Eta Carinae A is so large that it would extend beyond the orbit of Jupiter (at around 11 AUs) if it substituted for Sol in the Solar System. This size estimate, however, is somewhat arbitrary as the star's outer layers are constantly being blown into space by the radiative pressure of its photons on its gas atoms. Most stars, including main-sequence stars such as Sol, shed mass through their "stellar winds." Eta Carinae, on the other hand, blows away an astounding 500 Earth-masses a year. Moreover, astronomers have found it difficult to define the transition between the outer layers of the star and its surrounding stellar wind.

YEPUN, NAOS-CONICA, VLT, ESO

Larger image.

Infrared interferometric
observation of the central region
of the Homunculus nebula
surrounding Eta Carinae reveals
a point-like light source and
several luminous blobs (more).

In late 2003, astronomers announced that Eta Carinae's stellar wind appears to be extremely elongated. Already unstable in its late stage of evolution (which was achieved quickly because of its tremendous mass), the star itself is also particularly unstable from its fast rotation. According to recent theories of Eta-Carinae-type stars, such stars are flattened at the poles because of the centrifugal force of rapid rotation, which already exceeds 90 percent of its theoretical break-up speed. Since the polar areas of such a star are closer to the core where nuclear fusion processes take place, however, these areas are hotter. Hence, radiation pressure in Eta Carinae's polar directions are higher and so its outer layers above these regions get more "puffed up" than the outer layers at its equator (more discussion in: ESO's press release; and van Boekel et al, 2003).

VINCI, ESO

Larger illustration.

Eta Carina A may be highly oblate from its
fast rotation, within a larger rugby-ball
region of strong stellar winds with a
bi-polar outflow that is currently opaque
to astronomers (more).

Given the apparent youth of Eta Carinae A, it is likely that any protoplanetary bodies that may have formed around the star are still agglomerating other planetesimals, if they have survived its periodic explosions. If still intact, any developing carbon-based life on a developing Earth-type planet would be subject to tremendous heat on a newly formed planet that is under intense asteroidal and cometary bombardment, in addition to the intense and deadly radiation produced by nearby supernovae and other massive young stars.

Eta Carinae B?

On November 1, 2005, astronomers (including Rosina Iping, George Sonneborn, and Theodore Gull) using NASA's Far Ultraviolet Spectroscopic Explorer (FUSE) satellite claimed confirmation that the star has a hotter, if visually dimmer, companion that is a brighter emitter of ultraviolet light (NASA press release and web page), but some astronomers believe that only the primary star's dense wind was detected (Francis Reddy, Astronomy, June 2007; Weigelt et al, 2007; and Martin et al, 2006b and 2006a). An inferred 5.53-year orbit places Star B at an average of about 11 AUs from Star A, but the Eta Carinae system is too far away for telescopes to distinguish two stars in such a close orbit. While several astronomers have searched for light from Star B using ground-based telescopes, none have succeeded. Because Star B is thought to be much hotter than Eta Carinae, however, astronomers hypothesized that it would be brighter at shorter wavelengths like ultraviolet light, but Star B escaped detection from even the Hubble Space Telescope until the greater far ultraviolet sensitivity of the FUSE satellite was brought to bear.

Some astronomers already believed that Eta Carinae's instability and volatile outbursts may be due to the presence of a stellar companion. According to astronomer James Kaler, analysis of the material in the bipolar flow of gas and dust that was created more than 150 years ago (and is sometimes called the "Homunculus" for its Human-like shape) suggests that it is rich in nitrogen and carbon but low in oxygen. This composition is indicative of a star that is now using carbon as a catalyst in core nuclear fusion, although the star seen through the dust is not so enriched. Thus, Eta Carinae's Great Eruption of the 19th Century may have been produced by a companion star that has been blowing off its outer layers to show its "nuclear-fused innards" but is now hidden by its own dust. As it should be more highly evolved, the companion would have been the more massive star originally despite having now shed enough mass to become the less massive of the pair (James Kaler, 2002).

NASA Observatorium

Star B may have already progressed
beyond core hydrogen burning and
will eventually explode as a
supernova and create a black hole,
followed by the primary star.

See a discussion of "the Burning of
Elements Heavier than Helium" and
"Supernova Explosions" as part of
stellar evolution and death.

In 1996, astronomer Augusto Damineli proposed that Eta Carinae may be a binary system because of an apparent 5.53-year cycle in the star's ultraviolet and x-ray emissions -- "ionization state" (more discussion and recent references; and Augusto Damineli, 1996). On January 7, 2003, a team of astronomers (led by astrophysicist Michael Corcoran of NASA's Goddard Space Flight Center) announced that they had found what appears to be a pattern (but with strong variations) in x-ray emission repeating about every five years. Derived from data collected over 10 years from the Roentgen Satellite (ROSAT) and Rossi X-ray Timing Explorer, the x-ray emission pattern could be interpreted as corresponding to the orbit of a smaller star around a larger one (NASA press release).

© Augusto Damineli
(Artwork used with permission)

Larger and other telescopic
images and animations.

This simulated image depicts
Eta Carinae as a binary system
with high orbital eccentricity
and strong stellar winds from
two very massive stars (more).

Assuming that Eta Carinae is a binary, its x-ray output should vary with the orbital period of a smaller star revolving around a common center of mass with a larger star in a repeating cycle. Peaking of the observed x-rays emissions could be generated by colliding stellar winds when the stars are close, so that the impact of their respective winds would produce shock waves that heat nearby gas and generate x-rays. While many astronomers believe Eta Carinae is a single star that is at least 100 times as massive as Sol, calculations using the ROSAT data suggest it could be composed of a pair of 80 and 30 Solar-mass stars whose hypothesized 5.53-year orbit would bring the stars as close as five AUs and as far apart as 30 AUs (more discussion of x-ray observations). If the pattern of x-ray emissions continues, there should be a large peak of x-rays from Eta Carinae followed by a "blackout" in the summer of 2003, which astronomers hoped to observe at various wavelengths with such telescopes as the Chandra X-ray Observatory and Hubble Space Telescope.

Confirmation of the existence of Eta Carinae B was eventually inferred from a repeating pattern of changes in visual, X-ray, radio, and infrared light over approximately 5.5 years. As previously proposed, astronomers did observe repeated changes in its light from the hypothesized presence of the B star every 5.5 years, as the X-rays coming from the system disappear for about three months. While Eta Carinae A is too cool to generate X-rays, it continuously blasts a flow of gas into space in a stellar wind at about 300 miles per second that collides with a similiar wind from Star B somewhere between the two stars to generate the observed X-rays. As Star A moves in its orbit and passes in front of the region where the winds collide (as viewed from Earth), it eclipses the X-rays once every 5.5 years and blocks their detection from the Solar System, with the last X-ray eclipse beginning on June 29, 2003.

Luminous Blue Variables

To paraphrase an excellent summary by astronomer Stephen White, Luminous Blue Variables (LBVs) are among the most massive stars that astronomers know of. LBVs are more than 40 to more than 100 times as massive as Sol and are presumed to have started out as early O-type, main-sequence dwarf stars. Since the most massive stars tend to also burn hotter and to consume their core hydrogen the quickest, they live just a short while by astronomical standards -- only a few million years at most.

NICMOS, STScI, NASA
(Figer et al, 1995)

Larger infrared image.

Luminous Blue Variables
like the Pistol Star, at left,
appear to eject huge
amounts of their outer
gas envelopes in quick
but violent outbursts.

Once a massive star has fused most of its core hydrogen to helium ash, it becomes very unstable, and they eventually may blow up as spectacular Type-II supernovae. Some of the most massive ones may first pass through an LBV stage when they appear to eject huge amounts of mass from their outer layers of gas in a very short time (even more mass in a shorter period than Wolf-Rayet stars). The ejected gas (mostly hydrogen) moves outwards from the star at speeds of 50 to 500 kilometers per second and may be observed as nebulae that are bright in radio wavelengths. (Radio images of some LBVs and more discussion are available.)

Other Information

More information on this star is available from astronomer Augusto Damineli's web pages on Eta Carinae.

Up-to-date technical summaries on this star are available at: NASA's ADS Abstract Service for the Astrophysics Data System; and the SIMBAD Astronomical Database mirrored from CDS, which may require an account to access.

A large southern constellation, Carina is the keel of the even larger, ancient constellation of Argo Navis, the ship of the Argonauts, which also included Vela's sibling constellations of Puppis, Pyxis, and Vela. For more information about the stars and objects in this constellation and an illustration, go to Christine Kronberg's Carina. For another illustration, see David Haworth's Carina.

For more information about stars including spectral and luminosity class codes, go to ChView's webpage on The Stars of the Milky Way.