Red supergiant

A red supergiant (English red supergiant, RSG ) is a very extended star that has reached the end of its development. It has a surface temperature of less than 4300 K and shows the spectral characteristics of an Riesens with a very small acceleration of gravity due to their large radii.

Characteristics

It is similar in appearance to a red giant, but is much larger and more massive. A supergiant, the 10 - to 30- fold mass and up to 1500 times the radius of the sun. The luminosity can increase up to 100,000 times the solar luminosity. The surface temperature is between 3450 and 4300 K and corresponds to the spectral types late K to M. The red supergiant enriched by its stellar wind with mass loss rates of 10-6 to 10-3 solar masses per year, the interstellar medium with heavy elements at. In the outflowing stellar wind are often formed stellar masers of OH, H2O and SiO. Due to their short lifetime of a few million years include the red supergiant to the young population I. In addition, formed from the outflowing stellar wind dust shells, which is why these stars show a strong infrared excess. Dust formation leads to a strong absorbance in the visible and near infrared to red supergiants. Determination of the Bolometric luminosity is therefore often afflicted with this class of stars with a large error.

Variability

All Red supergiants are among the variable stars and assigned to the slow irregular variable stars or the semi- regular variable stars. Are the cause of radial vibrations of the atmosphere of the RG in the fundamental, the first or even the second harmonic, with periods on the order of a few hundred days. In addition, variations are detected in the light curves with cycle lengths of up to 4000 days, which are referred to as secondary long periods. Its cause is not yet understood. In addition to the regular components of the brightness changes can occur irregular variability caused by giant convection cells on the surface of RSG in red supergiant. The convection cells can take up to 20 percent of the surface. In the near infrared show the red supergiant a good defined period-luminosity relationship. The high luminosity of the RSG therefore allows a determination of distance to a much larger space than any other pulsating variables such as at the Cepheids. In addition to the brightness of the spectra of red giants are variable. For HV 11423 wanders in the Small Magellanic Cloud within months between an early K and an average M- spectrum back and forth.

Development

After the end of its hydrogen burning of the radiation pressure decreases. It outweigh the gravitational forces that have emerged in a contraction of the core and felt this heat to more than 100 million Kelvin. Below this temperature the star reaches the helium burning in its core. The next firing process, the carbon burning, will start from 500 million Kelvin. More stars from 13 solar masses reach the Neonbrennen and if further stages of nucleosynthesis. The continued presence of hydrogen burning zone is slowly continuing toward stellar surface.

By the disturbance of the inner equilibrium of the gas pressure rises in the inside of the star. This leads to an expansion of the outer layers of gas, to cool the case, and emit light predominantly in the red spectral range. The expansion reaches its maximum when the star has found his inner balance. These can range up to a thousand times the radius of our sun. For comparison, in our solar system, this would correspond roughly to the orbit of Jupiter, or about the 5.2 times the Earth's orbit.

Until the star the Hertzsprung -Russell diagram ( HRD short ) achieved in the supergiant range, it loses about 25% to 33% ( empirically uncertain parameters) its mass. This is the point at which most of the giants in the HRD fall down and do not make it to the supergiants.

A very massive star moves even several times horizontally through the HRD, becomes the red supergiant and eventually wanders back. The exact course of development and the mass limits are strongly dependent on the chemical composition of the star, and the mass loss by stellar winds. These Masseauswürfen the star may lose considerable mass by episodes with a strong stellar wind.

Part of the Red supergiants evolve into supernovae of type IIP (plateau ). These core -collapse supernovae occur when the core of the star no more fuel is available for thermonuclear reactions in which energy is released and the degeneracy pressure can no longer prevent gravitational collapse. This scenario is supported by numerous supernovae of type IIP in nearby galaxies, where the previously proven red supergiant by the core-collapse supernova could no longer be observed. The interaction of Supernovaejekta with the stellar wind of the red giant's About supports this interpretation. The result of such supernovae is a neutron star or black hole. The other red supergiant hike due to strong mass losses back in the blue part of HRDs and explode as a supernova of type Ib or Ic, while they are in the phase of a Wolf -Rayet star.

The development paths for non-rotating, non-magnetic stars with solar metallicity shows the following table, where the paths of development can significantly change through mass exchange in binary systems is:

Circumstellar Covers

Due to strong winds variable star Red supergiants can lose up to 50% of their former main sequence mass. The mass loss rate between 10-3 solar masses per year varies in some outbreaks, and may drop to values ​​of 10-7 MSonne. The high mass loss should be a result of high radiation pressure in combination with atmospheric activity due to theoretical considerations. In the radio area have been proven to Red supergiants lines of carbon monoxide, water, ammonia, hydrogen sulfide, silicon oxide and the hydroxyl. With the measured outflow velocity and the radial distribution of the stars of the stellar wind the last thousand years can be reconstructed in the supergiants. The Abströmgeschwindigkeiten lie in the order of a few tens of kilometers per second and the typical temperature of the gas in the circumstellar envelope between 300 and 700 K. With the high mass loss rates include the red supergiant with the important sources of the interstellar medium with heavy elements accumulate.

Examples

Betelgeuse and Antares A are well-known red supergiants. The biggest known star WOH G64 has the approximate 2000 times the diameter of the sun.