What is a Wolf Rayet star
The WR stars are particularly massive stars and can be found on the mass scale in the range between 20 and 100 solar masses and thus belong to the class of hypergiants. The surface temperature of normal Wolf-Rayet stars is between 30,000 and 60,000 Kelvin.
WR stars repel large amounts of matter into their surroundings. These stellar winds are accelerated to up to 4000 km / s by the star's radiation, which superimposes strong, very broad emission lines on the otherwise continuous spectrum. A WR star can lose up to 1000 solar masses per year, that is, up to one solar mass in ten thousand years. The rate of mass loss can increase to ten times this episode by episode.
Wolf-Rayet stars are divided into two main categories (named after the predominant element of the emission lines; the order also applies to the development over time):
The WN type shows mainly emission lines of nitrogen.
The Toilet type shows carbon and oxygen emission lines. WO stars are an extension of the toilet type and are very rare; with them the oxygen lines dominate.
These elements come from the nuclear fusion of the Wolf-Rayet star, which becomes visible when it blows off its hydrogen-rich atmosphere.
Hypergiant With a mass of up to 100 times the mass of the Sun, they are at least as massive as supergiants. They are not necessarily larger (in terms of volume) than supergiants, but they are usually more massive. This extends up to the Eddington limit, a theoretical maximum limit of the star's mass (around 120 solar masses), above which a star generates so much radiation that it repels its outer layers. Some hypergiants appear to have more than 100 times the mass of the Sun, although they may initially have been 200 to 250 times the mass of the Sun. This fact challenges the current theory of star formation and evolution.
With a thousand to million times the luminosity of the sun, hypergiants are the brightest stars. However, their temperatures are very different. They fluctuate from 3,500 Kelvin up to 35,000 K. Almost all hypergiants show variations in terms of their luminosity. The reasons lie in instabilities in their interior at moderate temperatures and high pressures.
Compared to the lifetimes of other stars, the hypergiants are extremely short-lived with a duration of 1 to 3 million years. Then they explode as a supernova or even a hypernova. It is believed that a hypergiant would leave a black hole after such a formidable ending. However, it could also be completely destroyed by a pair instability supernova. This mechanism was only recently discovered.
Due to the unusually high initial mass of the Wolf-Rayet stars, the development is characterized by extreme conditions:
- It is very fast, in 5,000 to 100,000 years the star will go through all phases until the catastrophic end.
- The end phase is determined by the high mass and is characterized by extreme explosions, which underlie the supernova types Ib and Ic, up to and including hypernova.
The typical evolution of a Wolf-Rayet star depends on the initial mass of the original star. It should be noted that mass loss already takes place during the development to the Wolf-Rayet star, so that the masses of the WR stars can be significantly lower than the initial masses.
Stars with an initial mass of more than 75 Solar masses develop from a star with the spectral class O via a hydrogen-rich WN to an LBV star (bright blue variables). The development continues from a low-hydrogen WN star to a toilet star and ends with a type Ic supernova.
With 40 to 75 Solar masses the development corresponds to that of the 75 solar mass stars, but here the phase of the hydrogen-rich WN star is skipped.
With an initial mass of under40 Solar masses develop from the O star into an LBV star or a red supergiant. After it has developed into a hydrogen-poor WN star, it ends as a type Ib supernova.
Luminous blue changer (LBV for short; engl. luminous blue variable), also called S-Doradus star after the star S Doradus, denotes stars of the stellar class of 50–150 solar masses and variable luminosity.
These stars, which belong to the so-called hypergiants (luminosity class 0), have the greatest mass that a star can have (Humphreys-Davidson limit) and shine for a short time with a luminosity that can be millions of times that of our sun . Due to the high surface temperature of approx. 30,000 to 50,000 Kelvin, they appear blue and belong to the O spectral class. LBVs pulsate irregularly and are often surrounded by a gas cloud, as they repel huge amounts of their shell and can thus lose up to 10 solar masses. After their phase as LBVs, which lasted only about 40,000 years, they can develop into Wolf-Rayet stars and end in a supernova or (so far hypothetical) hypernova explosion. The LBV is one of the rarest classes of stars. Only a few LBV are known, six of them in the Milky Way and a few more in its neighboring galaxies of the local group. The reason for this is the short duration of this phase of 5,000 to 100,000 years.
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