Wolf-Rayet Stars
Manuel Wolff ; Johannes Zabl ; Jürgen Leschhorn Leonard Storz	Home Print Version	Group 142

Facts

The Wolf-Rayet class consists of very massive stars in a particular state of their lives. As a consequence of their sh ort lifetime as WR-star with only a few million years, they are very rare (~200 in our galaxy). Like most of the heavy stars, WR stars will end in a supernova, or even in a hypernova. So being a WR star is almost the last stay in a massive star’s life. (see development)
One characteristic is that there is almost no hydrogen left and consequently the WR stars have started their 2nd or 3rd nuclear fusion, burning helium or other higher elements. So they are creating elements like carbon, nitrogen, oxygen, or even up to iron. One can prove such elements by strong emission lines in their spectra.
Due to their high temperature they are strong UV-radiators.
They are surrounded by the material of their heavy stellar winds and sometimes by enormous nebulas! But why? Due to their high temperature and the huge radiation-pressure, they a are blowing out their envelope with speeds about 2000 km/s. By these winds they loose a huge amount of mass, ~one earth mass per year. This is the reason, why about 40% of the mass of the whole star is now located in the surrounding nebulas.
But the high temperature causes also other things, such as the big ionisation, which can be seen for example in carbon having lost three of its four outer electrons.
So they can nearly be classified as stars of the spectral class O, but in difference to other typical O-class stars they have less mass.

Factbook: Wolf-Rayet Stars
mass	5 M ¤ to 60 M ¤
temperature	25.000 K to 100.000 K
stellar winds and mass loss (data could be deduced from the spectra)	Average maximum velocity: 800 to 3.000 km/s Mass loss M ¤ /yr
variability	The higher the luminosity the higher the variability

 

 

Image1 -HD 50896 – WN5 – 400 to 700 nm

Image2 –HD 50896 – WN5 – 120 to 500 nm

Image2 – HD 15763 – WC5 – 400 to 700 nm

Source: Encyclopaedia of Astronomy and Astrophysics

The main source of informations about the Wolf-Rayet stars are their spectra. By the exact examination of their profiles many physical and chemical information can be deduced.
As a consequence of the heavy stellar winds, only a small percentage of the continuum arrives us as you can see both in the visible [image1] and the UV [image2].
The spectrum is determined by emission lines of the elements existing in the winds. In the WN stars especially Nitrogen [images 1 and 2], whereas in the spectra of WC [image 3] stars Carbon predominates. In both stars you can find heavy lines of helium. By the knowledge of the Doppler-effect one can derive the velocities of the blowing winds from the line-profile.

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