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 every year, a Wolf-Rayet star loses 10−5% solar masses a year. These stars are also very hot: their surface temperatures are in the range of 25,000 K to 50,000 K.
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Observation history
In 1867, astronomers using the 40 cm Foucault telescope at the Paris Observatory, discovered three stars in the constellation Cygnus that displayed broad emission bands on an otherwise continuous spectrum.[1] The astronomer's names were Charles Wolf and Georges Rayet, and thus this category of stars became named Wolf-Rayet (WR) stars.[2] Most stars display absorption bands in the spectrum, as a result of overlaying elements absorbing light energy at specific frequencies. The number of stars with emission lines is quite low, so these were clearly unusual objects. The nature of the emission bands in the spectra of a Wolf-Rayet star remained a mystery for several decades. Edward C. Pickering theorized that the lines were caused by an unusual state of hydrogen, and it was found that this "Pickering series" of lines followed a pattern similar to the Balmer series, when half-integral quantum numbers were substituted. It was later shown that the lines resulted from the presence of helium; a gas that was discovered in 1868.[3] By 1929, the width of the emission bands was being attributed to the Doppler effect, and hence that the gas surrounding these stars must be moving with velocities of 300–2400 km/s along the line of sight. The conclusion was that a Wolf-Rayet star is continually ejecting gas into space, producing an expanding envelope of nebulous gas. The force ejecting the gas at the high velocities observed is radiation pressure.[4] In addition to helium, emission lines of carbon, oxygen and nitrogen were identified in the spectra of Wolf-Rayet stars.[5] In 1938, the International Astronomical Union classified the spectra of Wolf-Rayet stars into types WN and WC, depending on whether the spectrum was dominated by lines of nitrogen or carbon-oxygen respectively.[6]
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 230 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. Some (roughly 10%) of the 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, pulling 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 at least six 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 Southern Skies".[7]
See also
References
- ^ Huggins, William (1890-1). "On Wolf and Rayet's Bright-Line Stars in Cygnus". Proceedings of the Royal Society of London 49: 33-46. Retrieved on 2007-09-06.
- ^ Murdin, P. (2001). Wolf, Charles J E (1827-1918). Bristol: Institute of Physics Publishing. DOI:10.1888/0333750888/4101.
- ^ Fowler, A. (1912). "Hydrogen, Spectrum of, Observations of the principal and other series of lines in the". Monthly Notices of the Royal Astronomical Society 73: 62-105. Retrieved on 2007-02-04.
- ^ Beals, C. S. (1929). "On the nature of Wolf-Rayet emission". Monthly Notices of the Royal Astronomical Society 90: 202-212. Retrieved on 2007-09-10.
- ^ Beals, C. S. (1933). "Classification and temperatures of Wolf-Rayet stars". The Observatory 56: 196-197. Retrieved on 2007-09-10.
- ^ Swings, P. (1942). "The Spectra of Wolf-Rayet Stars and Related Objects". Astrophysical Journal 95: 112-133. Retrieved on 2007-09-10.
- ^ Hoffleit. The Bright Star Catalogue, 5th Revised Ed.. Retrieved on August 08, 2007.
External links
- [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)
