Eta Carinae

from Wikipedia, the free encyclopedia
Double star
η Carinae
Eta carinae IR.jpg
The surroundings of η Carinae,
the Carina Nebula , in infrared light
Location of the Eta Carinae area.
the "corner" below on the right is PP Car .
AladinLite
Observation
dates equinox :  J2000.0 , epoch : J2000.0
Constellation Keel of the ship
Right ascension 10 h 45 m 3.54 s
declination -59 ° 41 ′ 4 ″
Apparent brightness +6.21 (−1.0 to +7.9) mag
Typing
B − V color index +0.61 
U − B color index −0.45 
R − I index +0.49 
Spectral class O var / O
Variable star type SDOR + HB 
Astrometry
Radial velocity −25.0 km / s
distance 7,500  ly
2,300  pc  
Proper movement 
Rec. Share: (−11.0 ± 0.8)  mas / a
Dec. portion: (4.1 ± 0.7)  mas / a
Physical Properties
Dimensions ~ 100–200 / 30–80  M ☉
radius 60-240 / 14.3-23.6  R ☉

Luminosity

5,000,000 / <1,000,000  L ☉

Effective temperature > 9,400-35,200 / ~ 37,200  K
Age <3 million  a
Other names
and catalog entries
Bayer name η Carinae
Cordoba Survey CD −59 ° 3306
Bright Star Catalog HR 4210 [1]
Henry Draper Catalog HD 93308 [2]
SAO catalog SAO 238429 [3]
Tycho catalog TYC 8626-2809-1 [4]
2MASS catalog 2MASS J10450360-5941040 [5]

η Carinae or Eta Carinae is a variable , very massive double star of around 100 to 200 solar masses (primary star) or 30 to 80 solar masses (secondary star), which shines with around four to five million times the luminosity of the sun . Its secondary star can only be detected through fluctuations in the spectrum and cannot be directly observed.

It bears its name because it is located in the southern constellation Carina, the keel of the ship . The double star located at a distance of about 7,500 light years , within the open cluster Tr 16 , which in turn into a giant nebula complex is embedded, the Carina Nebula NGC  3372. It is one of the hypergiants and luminous blue variables .

Impact of mass on the life cycle

The primary star of η Carinae is one of the most massive stars in the Milky Way .

The fusion consumed in such stars because of the high internal generated by the ground pressure and the resulting high temperature of the available hydrogen (and in the course of its further development also heavier elements) at a much higher rate than in the sun, whereby enormous amounts of energy in the form of released by radiation . Compared to a smaller, lower-mass star, η Carinae requires an exponentially higher amount of energy to maintain a hydrostatic balance between radiation and gravitational pressure . Instabilities in the equilibrium position can be accompanied by strong changes in brightness ("outbreaks", see below).

The high rate of fusion means that their nuclear fuel will be used up in a relatively short time, namely within a few million years. These stars will then explode in a supernova or a hypernova and most likely end up as a black hole . In comparison, the sun has an expected lifespan of 10 billion years.

The star η Carinae belongs to a special class of unstable blue giant stars , which in English as Luminous Blue Variable (LBV), so Luminous Blue Variable be called. It is assumed that all stars with an initial mass of more than about 20 solar masses go through the LBV stage, but only stay there for a few tens of thousands of years. There were only discovered six LBVs in the Milky Way, some more are in neighboring galaxies of the local group known.

outbreaks

Eta Carinae is remarkable because of its outbreaks and the resulting change in brightness. When first cataloged by Edmond Halley in 1677, it was a 4th mag star , but increased in brightness and was perceived as one of the brightest stars in the ship's Kiel constellation in 1730. It sank back to its previous brightness by 1782 and then gradually increased it again from 1820 onwards. In 1827 it was ten times as high, corresponding to 2.5 size classes, and between 1837 and 1856 there was a huge eruption, the Great Eruption , in which it finally reached −0.8 magnitudes around 1843 . The eruption had the size of a supernova and, despite its distance, made η Carinae the second brightest star next to Sirius within a very short time . It faded noticeably in the following years. From 1900 to 1940 it was only visible in a telescope or prism binoculars at 7 to 8 magnitudes . In 1940 it gradually became brighter again and was visible to the naked eye again. From 1998 to 1999 the star doubled its brightness within 18 months and in 2002 had reached a brightness of 5 to 6 magnitudes.

Light echoes of the great outbreak

From the great eruption in the middle of the 19th century, only contemporary visual brightness estimates were available so far. With the help of light echoes , the course of brightness during the eruption could be measured in 2011 and several spectra recorded. With a light echo, the electromagnetic radiation is scattered by dust particles and therefore arrives on earth much later. The measured light curve confirms the contemporary reports. The spectra of the big eruption show an unexpectedly low temperature of about 5000 K , characteristic of a G2 – G5 supergiant with blue-shifted absorption lines , from which the speed of the outflowing gas could be determined as about 220 km / s. During the big eruption, about a tenth of the energy of a core collapse supernova was released and the radiation exceeded the Eddington limit for at least 10 years without destroying the star. With the low temperature, the eruption of Eta Carinae is rather untypical for the class of Supernova Impostors . The cause of the large eruption is assumed to be instability in the core zone of the massive star, whose energy production has multiplied. The outer layers of the star expanded and were removed by a strong stellar wind . Part of it fell on the companion, and the released gravitational energy was the main source of the increase in brightness. The mass transfer probably extended the orbital period of the binary star from 5 to the 5.5 years measured today. During the eruption, there was an increase in brightness every 5 years when the two stars came particularly close on their elliptical orbit.

Homunculus Nebula

Image of the Homunculus Nebula through the Hubble Space Telescope

η Carinae is surrounded by a spreading bipolar nebula, which is also called the homunculus nebula because of its appearance on photo plates . The nebula has the shape of two opposing cones, the tips of which have their origin in η Carinae, and with an apparent size of 18 ″ measures a little more than 0.5 light years from end to end. Due to its propagation speed of up to 700 km / s, which was estimated from its own motion with the help of various recordings from 1945 to 1995 , the cloud can be traced back to the eruption in the 1840s; it is probably partly responsible for the decrease in brightness at that time, as it covers the star and swallows most of its light . Visible changes in size can already be seen in recordings made every year.

The cones are oriented in the direction of the axis of rotation of the star. In the direction of the two cones, i.e. at the poles of rotation, the star continues to eject enormous amounts of matter. From the earth η Carinae is seen exactly lengthways through one of the cone walls. This weakens the light to a hundredth - by about 5 magnitudes . Other LBVs also have such bipolar nebulae, but due to the much higher contrast they appear less splendid on images.

Equatorial disc

At right angles to the direction of propagation of the conical clouds, in the so-called equatorial plane, there is a relatively flat disk, which is also made of material that has been thrown away. The speed estimates for it give a higher speed than that of the bipolar cloud and show that it must have been ejected much later than this, in the 1890s. Since η Carinae was observed very closely after its great eruption in the 1840s, a brief increase could also be found in the records of its brightness curve during this period.

In the case of bipolar clouds around other, much less heavy stars (see planetary nebula ), a dense equatorial disk was assumed that only allows the ejecta to escape unhindered at the two poles of the star. Since at η Carinae material escapes at high speed in the plane of the equatorial disc itself, one is not sure which mechanisms actually work here.

Cloud material and energy release

Chandra X-ray image of the Horse-Shoe Nebula

The material of the cloud and disk consists of gas with a high proportion of nitrogen and dust. It is heated by the star so that many chemical compounds can arise in the gas cloud ( cosmochemistry ). As a result, the Homunculus Nebula also radiates in the infrared range and is one of the brightest infrared objects in the Milky Way. Since infrared radiation, in contrast to visible light, is able to penetrate the dust, it is possible to observe the mostly covered half of the cloud facing away from us in this wavelength range . This made it possible to estimate the mass of the two clouds at about one and that of the equatorial disk at about half the mass of the sun. The existence of dust in the star's ejection material is attributed to the fact that it cooled with increasing distance, thus allowing the formation of dust particles.

The kinetic energy of the bipolar clouds was calculated from the mass and speed of propagation , which provides information about the extent of the eruptions. Thus, it corresponds to the amount of energy that liberates our sun in 200 million years ago and is in the order of 2 x 10 42  J . For the equatorial disk there is about half the value, because it has a higher velocity of propagation, but contains less mass.

Older outbreaks

Somewhat away from the Homunculus Nebula is older ejecta that may have been thrown away in a similar eruption in the 15th century. Images from 1999 by the Chandra X-ray satellite also reveal a horseshoe-shaped ring about 2 light - years in diameter, suggesting another major eruption more than a thousand years ago. The X-ray range also shows that the gas in the immediate vicinity of the central star has a temperature of around 60 million Kelvin and in the outer area of ​​the ring, where the gas collides with the interstellar matter and is slowed down, around 3 million Kelvin.

The cause of such outbreaks is not yet understood. A likely assumption is that they are caused by pent-up radiation pressure of tremendous luminosity; This means that the pressure of the outward directed radiation will eventually outweigh the inward directed gravity , causing the hydrostatic equilibrium to collapse for a short time and the star to explosively repel huge amounts of matter from its outer shells.

In any case, they show that the star is extremely unstable and has reached the end of its life cycle. It is believed to have a major eruption at least once in a thousand years and that it is likely to explode as a supernova within the next 100,000 years . This makes it a very interesting research object, as the last stages of stellar evolution and their transitions can be observed on it.

Radiation fluctuations

The observations of the last few years have shown that the star's brightness is continuously increasing. The cause is not known. The bolometric brightness of Eta Carinae probably does not change, but a change in the density of the absorbing material in the immediate vicinity leads to an increase in the optical brightness. Several periodic fluctuations are superimposed on the increase:

  • The X-rays emitted gradually increase over a period of 5.5 years. Towards the end it increases dramatically and then suddenly drops by a factor of 100 to a three-month minimum until a new cycle begins.
  • There is also an 85.1-day fluctuation during which brief bursts of radiation occur. This could be caused by a pulsation of the star, i. H. by periodic expansion and contraction of the star shells.

Theories

In the range of η Carinae periodic changes were found to hindeuteten that it was at η Carinae is a binary star system is, the two components in the orbiting approximately 5.54 years time. The minima of the X-ray radiation from the central area also occur with this same period, which could thus be explained as the obscuration of one binary star component by the other. The X-ray radiation could be generated by the collision of the stellar winds of the two components, and occlusion processes could also play a role. It has not yet been possible to set up a conclusive model of this system that explains all observed phenomena at the same time, but recently the contribution of the companion to the total light in the ultraviolet wavelength range has been demonstrated, so that there is hardly any doubt about the binary star thesis itself.

There are several theories to describe the mechanism that caused the formation of the bipolar clouds of the Homunculus Nebula: One says that the star's magnetic field focused the ejected plasma in two preferred directions. Another attributes the clouds to the influence of the gravity of the companion star, while a third blames the rotation of the star in conjunction with the extremely high luminosity in the area of ​​the Eddington boundary . The latter is favored by the latest data; but there is still no unanimous doctrine.

In addition, based on spectrographic investigations on η Carinae from 1996 , the astronomer Sveneric Johansson put forward the theory that ultraviolet laser light is generated directly around the star . Such laser phenomena had not yet been observed in nature, but in the energetically weaker microwave range radiant cosmic grains were .

According to a more recent, less widespread hypothesis, η Carinae may also be a triple star system, consisting of two “normal” stars with less than 60 solar masses and a neutron star with a heavy accretion disk that orbits the secondary component closely.

The uniqueness of η Carinae

The sight that η Carinae offers is unique. This is due to the relative proximity to Earth compared to other LBVs and the fact that the light from the central star is strongly weakened against the light from the nebula. This not only makes the nebula clearer in images, but also the spectral lines of the nebula appear a factor of a hundred stronger than without this attenuation. That is why η Carinae itself has long been considered a unique object. However, there are increasing signs that η Carinae, if we saw it from a different angle, would differ only slightly from other LBVs in the upper mass range. For example, all LBVs examined in sufficient detail show bipolar nebulae like the homunculus.

Web links

Commons : η Carinae  - collection of images, videos and audio files
Video

Individual evidence

  1. ↑ a b c eta Car. In: VSX. AAVSO, accessed November 1, 2018 .
  2. ↑ a b c d e f eta Car. In: SIMBAD . Center de Données astronomiques de Strasbourg , accessed on November 1, 2018 .
  3. ^ Nolan R. Walborn: The Company Eta Carinae Keeps: Stellar and Interstellar Content of the Carina Nebula . In: Eta Carinae and the Supernova Impostors  (= Astrophysics and Space Science Library), Volume 384 2012, ISBN 978-1-4614-2274-7 , pp. 25-27, doi : 10.1007 / 978-1-4614-2275- 4_2 .
  4. ↑ N. Clementel, TI Madura, CJH Kruip, J.-P. Paardekooper, TR Gull: 3D radiative transfer simulations of Eta Carinae's inner colliding winds - I. Ionization structure of helium at apastron . In: Monthly Notices of the Royal Astronomical Society . 447, No. 3, 2015, p. 2445. arxiv : 1412.7569 . bibcode : 2015MNRAS.447.2445C . doi : 10.1093 / mnras / stu2614 .
  5. ↑ a b A. Kashi, N. Soker: Periastron Passage Triggering of the 19th Century Eruptions of Eta Carinae . In: The Astrophysical Journal . 723, 2010, p. 602. arxiv : 0912.1439 . bibcode : 2010ApJ ... 723..602K . doi : 10.1088 / 0004-637X / 723/1/602 .
  6. ↑ TR Gull, A. Damineli: JD13 - Eta Carinae in the context of the Most Massive Stars . In: Proceedings of the International Astronomical Union . 5, 2010, p. 373. arxiv : 0910.3158 . bibcode : 2010HiA .... 15..373G . doi : 10.1017 / S1743921310009890 .
  7. ^ Eta Car. In: STARS. Jim Kaler, accessed November 1, 2018 .
  8. ↑ a b E. Verner, F. Bruhweiler, T. Gull: The Binarity of η Carinae Revealed from Photoionization Modeling of the spectral variability of the blobs Weigelt B and D . In: The Astrophysical Journal . 624, No. 2, 2005, p. 973. arxiv : astro-ph / 0502106 . bibcode : 2005ApJ ... 624..973V . doi : 10.1086 / 429400 .
  9. ^ A b Andrea Mehner, Kris Davidson, Gary J. Ferland, Roberta M. Humphreys: High-excitation Emission Lines near Eta Carinae, and Its Likely Companion Star . In: The Astrophysical Journal . 710, 2010, p. 729. arxiv : 0912.1067 . bibcode : 2010ApJ ... 710..729M . doi : 10.1088 / 0004-637X / 710/1/729 .
  10. ↑ ESO source First Astronomical Images from the VLT UT1 ( Memento of August 3, 2003 in the Internet Archive ) speaks of 1841
  11. ^ A. Rest, JL Prieto, NR Walborn, N. Smith, FB Bianco, R. Chornock, DL Welch, A. Howell, ME Huber, RJ Foley, W. Fong, B. Sinnott, HE Bond, RC Smith, I. Toledo, D. Minniti & K. Mandel: Light echoes reveal an unexpectedly cool η Carinae during its nineteenth-century Great Eruption . In: Nature . tape 482 , 2012, p. 375–378 , doi : 10.1038 / nature10775 .
  12. ↑ http://science.nasa.gov/science-news/science-at-nasa/1999/ast08oct99_1/ (1999) speaks of 600,000 km / h = 170 km / s; the older site http://hubblesite.org/newscenter/archive/releases/1996/23/image/a/ (1996) speaks of 1.5 million mph = 670 km / h; this coincides with the report by an astronomer on The Behemoth Eta Carinae: A Repeat Offender ( January 1, 2004 memento in the Internet Archive ) (1998) who was involved in speed estimates.
  13. ↑ In the absence of a specific numerical value, calculated by: Energy = luminosity of the sun * 200 million years = 3.85 * 10 26 W * 200 million years.
  14. ↑ See First Astronomical Images from the VLT UT1 ( Memento from August 3, 2003 in the Internet Archive )
  15. ↑ A. Damineli, M. Teodoro, M. Corcoran, JH Groh: Eta Carinae long-term variability . In: Astrophysics. Solar and Stellar Astrophysics . 2010, arxiv : 1009.4399v1 .
  16. ^ Jean-Christophe Leyder, Roland Walter, Gregor Rauw: Hard X-ray identification of Eta Carinae and steadiness close to periastron . In: Astrophysics. Solar and Stellar Astrophysics . 2010, doi : 10.1051 / 0004-6361 / 201014316 , arxiv : 1008.5366v1 .
  17. ↑ Iping et al. 2005, ApJL 633, L37.
  18. ↑ Chandra Takes X-ray Image of Repeat Offender - NASA Science
  19. ↑ Wolfgang Kundt, Christoph Hillemanns: Eta Carinae - an evolved triple star system? (PDF; 2 MB) , Chin. J. Astron. Astrophys. Vol. 3 (2003), Suppl., Pp. 349-360.
This version was added to the list of articles worth reading on September 16, 2009 .