Wolf–Rayet star

Wolf -Rayet stars, also abbreviated WR stars in the literature, are the exposed cores formerly massive stars. They are after the French astronomers Charles Wolf (1827-1918) and Georges Rayet (1839-1906) named.


The previously measured masses of Wolf- Rayet stars are 10-256 solar masses, although originally a theoretical upper limit at about 150 solar masses was expected. The surface temperature is 30000-120000 K.

WR -star encounter large amounts of matter from their surroundings. These star winch are accelerated by the radiation of the star at up to 4000 km / s, which is superimposed on the continuous spectrum strong, very broad emission lines. A WR star can lose up to 10-4 solar masses per year. Episode, the mass loss rate can even increase to ten times of it. The stellar wind of carbon-rich Wolf -Rayet stars with a late spectral type WC condensed into particles of dust. This occurs at a longer distance, where the dust is not dissociated by intense ultraviolet radiation. It is not, this is a continuous process, but form discrete clouds around the Wolf- Rayet star. As a result, there will be variations in brightness due to the variable absorption by the carbon-rich dust.

Furthermore WR stars are formed in close binary systems. Starts a massive star to move away from the main sequence and expands from there, he can exceed the Roche limit. The outer atmosphere is no longer bound to the star and can flow. Further development of the star leads to further expansion and the outer layers are lost. What remains is a WR star with a spectral signature that shows the thermonuclear reactions of hydrogen burning and / or helium burning in the former nucleus. The best-known example of a WR star in a binary system V444 Cygni is.


Wolf -Rayet stars are divided into two main categories ( designation according to the predominant element of the emission lines, order also applies to the time evolution, see below):

  • The WN - type mainly shows emission lines of helium and multiply ionized nitrogen.
  • The WC - type mainly shows emission lines of oxygen and multiply ionized carbon. WO- stars represent an extension of the WC type and are very rare; with them the oxygen lines dominate.

These elements come from the nucleosynthesis of the Wolf -Rayet star, which are visible when it blows off its hydrogen-rich atmosphere.


The typical development of a Wolf -Rayet star depends on the initial mass of the original star. It should be noted that already takes place during the development of the Wolf- Rayet star mass loss, so that the masses of WR stars can be significantly lower than the initial mass.

Despite extensive surveys such as the Palomar Transient Factory, it is not successful with the precursors of supernovae to identify the type Ibc to recordings before the outbreak. It should be at the precursors to luminous Wolf -Rayet star with an absolute visual magnitude, which are approximately 150 times higher than the average Wolf -Rayet stars. Simulations of massive WR stars, which develop in supernovae of Type Ibc, show an almost complete loss of their helium atmosphere shortly before core collapse. Here, the surface temperature rises to values ​​in excess of 150,000 K and according to Wien's displacement law most of the radiation is emitted in the far ultraviolet region. Therefore, Wolf -Rayet star just before its core collapse quite faint stars with absolute visual magnitudes MV of -2 which is roughly a factor of 100 fainter than most WR stars. The lifetime of massive Wolf -Rayet stars should be in the order of 500,000 years of computational simulations.

According to the collapsar model rapidly rotating Wolf -Rayet stars could also be the precursor of long GRBs. First, the connection between long GRBs and supernovae has now been verified by observations of type Ibc and secondly, in the optical spectra of the long gamma-ray bursts blue-shifted absorption lines at speeds of 3000-4000 km / s has been demonstrated. The properties of these blue-shifted absorption lines fit to an interaction of the supernova with circumstellar matter, which is caused by the stellar wind of a Wolf -Rayet star.

Central stars of planetary nebula

Based on morphological similarities of the spectrum ( strong and broad emission lines ) are also referred to about 10 % of the central stars of planetary nebulae as a Wolf -Rayet star. This is to lower mass stars ( about 0.6 solar masses, initial masses less than 8 solar masses ) with a hydrogen- poor atmosphere.

To avoid confusion, has for these objects the engl. Abbreviation WR- CSPN (Wolf -Rayet - Central Star with Planetary Nebula ) or [WC ] ( square brackets ), and occasionally [ WR ] enforced.

It is believed that [ WR ] CSPN arise from post-AGB stars with a helium flash, where the majority of the hydrogen is mixed in the star down and burned. The residual atmosphere consisting essentially of helium, carbon, and oxygen. The star then evolves into a white dwarf.

The mass loss rates due to the strong stellar wind are about 10-7 to 10-5 solar masses per year, which is about 10 - to 100 -fold higher than normal, hydrogen-rich central stars. The development of the discovered [WC ] star runs from a [WC ] - PG1159 on a star to a hydrogen- poor white dwarf, which can be explained by a simple cooling sequence.

The central star of IC4663 and pB8 are [ WN ] star whose atmosphere consists of 95 % helium. The [ WN ] star could arise from the merger of two white dwarfs, as this would explain the reason for the high proportion of neon and nitrogen in the atmosphere of the central star of a planetary nebula. An alternative scenario is a diffusion -induced nova. Here, the helium burning ignites again in a post-AGB star and thereby triggered by the strong convection is mixed material from the CNO core area in the atmosphere.

Interestingly, observations of planetary nebulae found no systematic difference between such ordinary and hydrogen- poor ( WR ) central stars. This suggests that the development of the hydrogen- poor central star is random.


  • γ Velorum in the constellation of sails of the ship, visible with the naked eye, type of toilet
  • Called WR 102ka, also Peony nebula star, currently second- brightest star in the Milky Way
  • WR 124 with a planetary nebula M1 -67 ( → example image above and section below).
  • NGC300 X -1: Wolf-Rayet/Schwarzes hole binary star system
  • WR 7 with surrounding ring nebula NGC 2359 ( ducks fog or Thor's Helmet )