'''Wolf-Rayet stars''' are massive stars that are in the stage of their stellar evolution where they undergo heavy mass loss. These stars are most recognized for their unusual spectrum, which is dominated by the emission lines of highly ionized [[element|elements]]s.
==History==
The first Wolf-Rayet stars were identified in 1867 at the [[Paris]] Observatory when astronomers '''Charles Wolf''' and '''Georges Rayet''' observed three stars in the constellation of [[Cygnus]] that displayed unusually broad emission bands in their [[spectrum]]. The cause of these emission bands remained unknown for several decades. At first, the emission bands were thought to be some unusual state of hydrogen, but were later shown to be helium, an element identified for the first time in 1868.<ref>http://adsabs.harvard.edu/abs/1912MNRAS..73...62F</ref> Later, emission lines of [[carbon]], [[oxygen]], and [[nitrogen]] were identified in the spectra.<ref>http://adsabs.harvard.edu/abs/1933Obs....56..196B</ref>
Later observations showed the usually wide emission bands were due to the [[Doppler effect]] caused by the gas surrounding the [[star]] moving at extremely high speeds of around 300-2400 300–2400 km/s.<ref>http://adsabs.harvard.edu/abs/1929MNRAS..90..202B</ref> The [[International Astronomical Union]] in 1938 classified Wolf-Rayet stars officially according to their spectra. These classifications are WN, for spectrum dominated by lines of nitrogen, and WC for spectrum dominated by carbon and oxygen.<ref>http://adsabs.harvard.edu/abs/1942ApJ....95..112S</ref> Later in 1990, the WN classification was subdivided into two sub-categories called WNL and WNE.<ref name="nz">http://www.peripatus.gen.nz/astronomy/wolraysta.html</ref>
==Portrait==
Wolf-rayet stars are massive stars nearing the end of their lives (possibly evolved from massive class O stars). Typically they have a mass of over 20 solar masses, with surface temperature running between 25,000K to 50,000K, making them among the largest and hottest stars in the universe. Their atmospheres are unusually thick in comparison to other stars. One of the most notable features of a Wolf-Rayet star is the incredible mass loss due to intense stellar winds of up to 2000 km/s. All stars lose mass over time, however the rate of mass loss for a Wolf-Rayet star is very high, between 10<sup>-4−4</sup> and 10<sup>-5−5</sup> solar masses a year. In comparison, the [[Sun]] losses only around 10<sup>-14−14</sup> solar masses a year.<ref name="nz">http://www.peripatus.gen.nz/astronomy/wolraysta.html</ref>
The ferocious winds are caused when the various elements, such as carbon, oxygen and nitrogen, are created in the star's core reach the surface over time. As these elements accumulate on the surface, they absorb much of intense light of the star, causing a powerfully strong stellar wind to start blowing from the surface of the star. This wind becomes so thick, it obscures the star itself.<ref>http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/980603a.html</ref>
The mass loss caused by the furious stellar winds sloughing off the outer layers of the star's atmosphere considerably shortens the remaining life of the star. Eventually the Wolf-Rayet star will run out of fusible material and will end its life as a [[Supernova#Type_1b_and_IcType 1b and Ic|type Ib supernova]].<ref name="nz">http://www.peripatus.gen.nz/astronomy/wolraysta.html</ref>
There are only around 230 Wolf-Rayet stars known in our own [[Milky Way|galaxy]], with a further 100 in the [[Magellanic_CloudsMagellanic Clouds#Large_Magellanic_CloudLarge Magellanic Cloud|Large Magellanic Cloud]] and a dozen in the [[Magellanic_CloudsMagellanic Clouds#Small_Magellanic_CloudSmall Magellanic Cloud|Small Magellanic Cloud]]. Of all of these, the best known is '''Gamma 2 Velorum''', a multiple star system of six stars of which at least one is a Wolf-Rayet star. Gamma 2 Velorum is visible to the unaided eye in the skies below 40 degrees south latitude and is nicknamed "Spectral Gem of Southern Skies".
====Wolf-Rayet Sequence Classifications====