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 elements.
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. Later, emission lines of carbon, oxygen, and nitrogen were identified in the spectra.
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 km/s. 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. Later in 1990, the WN classification was subdivided into two sub-categories called WNL and WNE.
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 10-4 and 10-5 solar masses a year. In comparison, the Sun losses only around 10-14 solar masses a year.
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.
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 type Ib supernova.
There are only around 230 Wolf-Rayet stars known in our own galaxy, with a further 100 in the Large Magellanic Cloud and a dozen in the 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
- WNL - Also called late type WN Wolf-Rayet stars. Typically the earliest sequence in the evolution of these stars. They are the most massive of Wolf-Rayet stars and have substantial hydrogen present with a hydrogen burning shell and a helium burning core. Spectra emission lines are dominated by nitrogen.
- WNE - Also called early WN Wolf-Rayet stars. The spectra emission lines are still nitrogen dominated. Hydrogen is absent from the star with the helium burning core remaining. Hotter and brighter then WNL types.
- WN+WC - Transitional classification that is short lived. Usually associated with binary systems but also found with single Wolf-rayet stars.
- WC - Hottest but least luminous of the classes of Wolf-Rayet stars. Nitrogen is absent from the spectra. Hydrogen is absent from the star and the surface is increasingly represented by carbon. This is the most evolved of the Wolf-Rayet sequences. Particularly oxygen-rich examples are classified as WO.