Eta Carinae

4,000,000 L ☉

η Carinae, or Eta Carinae is a variable, very massive star of 100 to 120 solar masses, which shines with about four to five million times the sun's luminosity. It takes its name because it is located in the southern constellation of Carina, the keel of the ship. The star is located at a distance of about 7,000 to 10,000 light- years, within the open cluster Tr 16, which in turn is embedded in a giant nebula complex, the Carina nebula NGC 3372nd He is one of the hyper giants and the luminous blue variables.

Effect of composition on the life cycle

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

Nuclear fusion consumed in such stars due to the generated by the mass of high internal pressure and the resulting high temperature the hydrogen present ( and in the course of their further development and heavier elements ) at a much higher rate than in the sun, with enormous amounts of energy in the form released by radiation. The reason is that a star to agree to an increasing mass with exponentially increasing amount of energy to counteract the inward pressure of its own weight and so maintain the hydrostatic equilibrium.

The high fusion rate means that their nuclear fuel will be in a relatively short time, ie within a few million years, consumed. These stars are then explode in a supernova or a (so far hypothetical ) Hypernova and most likely as a black hole. The sun in comparison, has an expected life of 10 billion years.

The star η Carinae belongs to a special class of unstable blue giant stars, the so Luminous Blue Variables are referred to in English as a Luminous Blue Variable ( LBV). It is assumed that all stars go through with an initial mass of more than about 20 solar masses the LBV stage, however, linger only a few there ten thousand years. It was discovered only six LBVs in the Milky Way, some more are known in the neighboring galaxies of the local group.

Outbreaks

Remarkably Eta Carinae is because of its eruptions and the hand changing brightness. In his first cataloging by Edmond Halley in 1677, he was a star 4 size, but increased its brightness and 1730 it was perceived as one of the brightest stars in the constellation keel of the ship. Until 1782, he fell back to its former brightness back, and then it increased gradually since 1820. In 1827 she was already ten times as high, corresponding to 2.5 magnitudes, and 1837-1856, there was a violent eruption, the Great Eruption, in which he finally reached around 1843 -0.8 mag. The outbreak was the extent of a supernova, making η Carinae, despite its distance in the shortest possible time to second brightest star next to Sirius. He faded rapidly in the following years. From 1900 to 1940 he was with 7-8 magnitudes visible only through the telescope or prism binoculars. In 1940 he was then gradually brighter again and again visible with the naked eye. From 1998 to 1999 doubled the star 's brightness within 18 months and had reached a brightness of 2002 5-6 mag.

Light echoes of the great eruption

From the great escape in the middle of the 19th century have been available only contemporary visual brightness estimates. With the help of light echoes in the year 2011, the brightness curve was measured during the eruption as well as several spectra are recorded. In a light echo, the electromagnetic radiation is scattered by dust particles and, therefore, applies significantly later on Earth. The measured light curve confirms the contemporary reports. The spectra of the big eruption show an unexpected low temperature of about 5000 K, characteristic of a G2 -G5 supergiant with blue-shifted absorption lines, from which the velocity of the outflowing gas could be determined to about 220 km / s. In the great eruption one tenth of the energy of a core-collapse supernova, approximately freely and the radiation exceeded the Eddington limit for at least 10 years without destroying the star. With the low temperature of the eruption of Eta Carinae is not typical for the class of supernova impostors. The cause of the great eruption instability in the core zone of the massive star is suspected, the power multiplied. The outer layers of the star expanded time and have been worn by a strong stellar wind. Some fell on the companion, and the released gravitational energy was the main source of the increase in brightness. The mass transfer has probably extended the orbital period of the binary star out of 5 on the measured today 5.5 years. During the outbreak, there was every 5 years to an increase in brightness when the two stars were especially close to their elliptical orbit.

Homunkulusnebel

η Carinae is surrounded by a sprawling bipolar nebula, also known as Homunkulusnebel because of his appearance on photographic plates. The fog is in the form of two opposing cones whose tips in η Carinae have their origin, and measures at an apparent size of 18 " from end to end just over 0.5 light-years. Through their propagation speed of up to 700 km / s, which was estimated with the help of various recordings from 1945 to 1995 from their proper motion, the cloud can be on the outbreak in the 1840s traced, which presumably responsible for the then drop in brightness as they obscured the star and swallowed most of his light already. on recordings that are made ​​in the space of a year, the first visible changes in their size can be identified.

The cones are aligned in the direction of the axis of rotation of the star. In the direction of the two cones, so the rotational poles, pushes the star still enormous amounts of matter from. From Earth, η Carinae is exactly along seen through one of the cone walls. This is the light by a factor of 100, about 5 magnitudes weaker than in the light of the nebula. Other LBV also have such a bipolar nebula, but by the much higher contrast they appear less magnificently on images.

Equatorial plate

Perpendicular to the propagation direction of the cone-shaped cloud, in the equatorial plane as said, there is a relatively flat plate which is also made of fortgeschleudertem material. The velocity estimates for they yield a higher speed than that of the bipolar cloud and show that they must have been ejected much later than this, in the 1890s. Since η Carinae was closely watched by his big outbreak in the 1840s, though short term increase was found in the records of its brightness curve in this period.

In bipolar clouds around other far less serious star (see planetary nebula ), it was assumed a dense equatorial disk, which can escape the ejecta only at the two poles of the star unhindered. Since η Carinae itself now also exits in the plane of the equatorial disc material at high speed, one is not sure what mechanisms actually work here.

Cloud material and energy release

The material of cloud and the disk consists of gas with a high content of nitrogen and dust. It is heated by the star, so that in the gas cloud, many chemical compounds may arise ( Cosmochemistry ). As a result, the Homunkulusnebel also radiates in the infrared region and is one of the brightest infrared objects in the Milky Way at all. Since the infrared radiation is in contrast to visible light is able to penetrate the dust, it is possible to observe in this wavelength range, the most part concealed, faces away from us Cloud half. This allowed the mass of the two clouds on each about one and the equatorial disk are estimated to be about half a solar mass. The existence of dust in the ejecta of the star is attributed to the fact that it is cooled down with increasing distance and so allowed the formation of dust particles.

From mass and velocity of propagation of bipolar clouds whose kinetic energy was calculated, which provides information about the extent of eruptions. Thus, it corresponds to the amount of energy that liberates our sun in 200 million years, which is on the order of 2.1042 J. For the equatorial disc results in about half the value, since although they have a higher propagation velocity, but contains less mass.

Older outbreaks

A little removed from Homunkulusnebel is older ejecta, which may have been thrown in a similar outbreak in the 15th century. Images of X-ray satellites Chandra from 1999 also have a horseshoe-shaped ring with a diameter of about 2 light years realize is closed by the on another major outbreak more than a thousand years ago. In the X-ray range, also shows that the gas has in the immediate vicinity of the central star a temperature of about 60 million Kelvin and the outside of the ring where the gas collides with the interstellar matter and is braked, about 3 million Kelvin.

The cause of such bursts is not yet understood. A likely assumption is that they are caused by accumulated radiation pressure of the huge brightness, i.e., that the pressure of the outwardly directed radiation at some point outweigh the inward gravity, so that the hydrostatic equilibrium collapses briefly and the starburst -like large quantities of matter of its outer shells repels.

They show in any case that the star is highly unstable and has reached the end of its life cycle. It is thought that he at least once in a thousand years goes through a major outbreak and that it will probably explode within the next 100,000 years as a supernova. This makes it a very interesting object of study because let him watch the final stages of stellar evolution and their transitions.

Radiation fluctuations

The observations of recent years have shown that the brightness of the star continues to rise. The cause is not known. Probably the bolometric brightness Eta Carinae does not change, but a change in the density of the absorbent material in the immediate area leads to an increase of the optical brightness. Superimposed on the increase are more periodic fluctuations:

• Within 5.5 years, the emitted X-ray radiation increases gradually. Towards the end it grows dramatically and then drops abruptly by a factor of 100 on a three-month minimum, until a new cycle begins.
• There is also a 85.1 -day variation, occur in the short-term radiation spurts. This could be caused by a pulsation of the star, i.e., by periodic expansion and contraction of the star cases.

Theories

In the spectrum of η Carinae periodic variations were found, which indicated that it is η Carinae is a binary star system in which the two components orbit each approximately 5.54 years. With this same period, the minima occur in the X-ray radiation from the central area, which could be explained by the masking as a double star component by the other. The X-rays could be produced by the collision of the stellar winds of the two components, as might play a role covering operations. It has so far not yet a coherent model of this system are set up, which all observed phenomena at the same time explained, but recently the proportion of the companion to the total light could be detected in the ultraviolet wavelength range, so that there are hardly any on the binary star hypothesis of doubts.

There are several theories for the description of the mechanism that caused the emergence of the bipolar clouds of Homunkulusnebels: A states that the magnetic field of the star have bundled the stones being thrown plasma in two preferred directions. Another leads the clouds back to the influence of gravity of the companion star, while a third, the rotation of the star blames the extremely high luminosity in the Eddington limit in the interaction. The latter is favored by the most recent data; but there is still no unanimity of doctrine.

In addition, the astronomer Eric Sven Johansson presented on the basis of spectrographic studies of η Carinae in 1996, the theory that immediately around the star arises ultraviolet laser light. Such laser phenomena were not yet observed in nature until then, radiant energy in the microwave range weaker cosmic maser does.

One of the newer, less common hypothesis is that η Carinae is possibly also a triple star system consisting of two "normal" stars with less than 60 solar masses and a neutron star with severe accretion disk that orbits the secondary component closely.

The uniqueness of η Carinae

The sight, the η Carinae provides is unique. This is due to the relative proximity to Earth, compared with other LBVs, and to the fact that the light from the central star is strongly attenuated against the light of the nebula. Thus, the mist is apparent not only in pictures but also spectral lines of the nebula appear by a factor of hundred more than without this slowdown. Therefore η Carinae was considered a unique object also itself a long time. However, there are increasing signs that η Carinae, we see him from a different angle, would differ only slightly from other LBVs in the upper mass range. For example, all studied in sufficient detail LBVs in bipolar nebulae like the homunculus.

Archäoastronomische studies

Of further interest is for astronomers to find historical evidence for even earlier outbursts in order to make statements about their incidence. Finding one, however, so far been virtually no results, only a myth of the Sumerians from the 4th to the 3rd millennium BC, whose god Enki, which should be published in the form of a variable star, with η Carinae might be related. Proved this is not, however, especially as no information can be found, supposed to have been the place in the sky Enki.