Wolf-Rayet star
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Wolf-Rayet stars (often referred to as WR stars) are evolved, massive stars (over 20 solar masses), and are losing their mass rapidly by means of a very strong stellar wind, with speeds up to 2000 km/s. While our own sun loses 10-14 of its own mass on a yearly basis, a Wolf-Rayet star loses 10-5 solar masses a year. These stars are also very hot: their temperature are in the range of 25,000 K to 50,000 K.
[edit] Description
Wolf-Rayet stars are a normal stage in the evolution of massive stars, in which strong, broad emission lines of helium and nitrogen ("WN" sequence) or helium, carbon, and oxygen ("WC" sequence) are visible. Due to their strong emission lines they can be identified in nearby galaxies.
About 150 Wolf-Rayets are known in our own Milky Way Galaxy, about 100 are known in the Large Magellanic Cloud, while only 12 have been identified in the Small Magellanic Cloud. Wolf-Rayet stars were discovered spectroscopically in 1867 by the French astronomers Charles Wolf and Georges Rayet using visual spectrometery at Paris Observatory.
Some (roughly 10% of the galactic) central stars of planetary nebulae are - despite their lower masses - also of the WR-type, i.e. they show emission line spectra with broad lines from helium, carbon and oxygen.
It is possible for a Wolf-Rayet star to go into a "collapsar" stage in its death throes - this is when the core of the star collapses to form a black hole, sucking in the surrounding material. This is thought to be the precursor of a long gamma-ray burst.
The best known (and most visible) example of a Wolf-Rayet star is Gamma Velorum (γ Vel), which is a bright star visible to those located south of 40 degrees northern latitude. One of the members of the star system (Gamma Velorum is actually four stars) is a Wolf-Rayet star. Due to the exotic nature of its spectrum (bright emission lines in lieu of dark absorption lines) it is dubbed "the spectral gem of the southern sky".
[edit] See also
[edit] References and further reading
- [1] Some Wolf-Rayet stars in binaries are close enough that we can image a rotating "pinwheel nebula" showing the dust generated by colliding winds in the binary system, from Aperture Masking Interferometry observations.
- [2]Wolf-Rayet Stars: Spectral Classifications
- [3]ApJ 525:L97-L100 Nov. 10, 1999. Monnier, Tuthill & Danchi: Pinwheel Nebula Around WR98a (PDF)
- [4]ApJ Jan. 3,2005. Dougherty, et. al.: High Resolution Radio Observations of the Colliding Wind Binary WR140 (PDF)
- [5]A catalog of northern Wolf-Rayet Stars and the Central Stars of Planetary Nebulae (Harvard)
