What are Wolf-Rayet stars?

Wolf-Rayet (WR) stars are the evolved descents of the most massive, extremely hot (temperatures up to 200,000 K) and very luminous (10⁵  to 10⁶ solar luminosities, L_⊙) O stars, with masses 25 – 30 solar masses (M_⊙) for solar metallicity. WR stars possess very strong stellar winds, which reach velocities up to 3,000 km/s. These winds are observed in the broad emission line profiles (sometimes, even P-Cygni profiles) of WR spectra in the optical and UV ranges. These strong winds are also attributed to atmospheres in expansion. Actually, these winds are so strong that they are peeling the star and converting it in a nude nucleus without envelope. Indeed, WR stars have ejected their unprocessed outer Hydrogen-rich layers. WR stars typically lose 10⁻⁵ M_⊙ a year; in comparison the Sun only loses  10⁻¹⁴  M_⊙  per year.

Hα image of the Population I Wolf-Rayet star WR 124 (WN8) showing a young circunstelar envelope that is ejected at velocities highest than 300 km/s. The chaotic and filamentary structure created forms the M 1-67 nebula. The star is located at about 4.6 kpc from the Sun. At the left, image obtained by the author using the IAC-80 telescope, combining filters Hα (red) Hα continuum (green) and [O III] (blue). The right Hα image was obtained by the Hubble Space Telescope WFPC2 (Grosdidier et al. 1998). Note that the large arcs of nebulosity extend around the central star yet with not overall global shell structure. Furthermore, numerous bright knots of emission occur in the inner part of the nebula, often surrounded by what appear to be their own local wind diffuse bubbles. The dashed square in the IAC-80 image indicates the size of the HST image.

This is Figure 2.1 in my PhD Thesis.

WR stars were discovered by French astronomers Charles Wolf and Georges Rayet in 1867. They found that three bright galactic stars located at Cygnus region have, rather than absorptions lines, broad strong emission bands superposed to the typical continuum of hot stars. In 1930 C.S Beals correctly identified these features as emission lines produced by high ionized elements as helium, carbon, nitrogen and oxygen.  The intriguing spectral appearance of WR stars is due both their strong stellar winds and highly evolved surface chemical abundance. In 1938, WR stars were subdivided into WN (nitrogen-rich) and WC (carbon-rich) depending on whether the spectrum was dominated by lines of nitrogen or carbon-oxygen , respectively. Not until the 1980s did it became clear that WR stars represent an evolutionary phase in the lives of massive stars during which they undergo heavy mass loss. 

The mass-loss occurs via a continuous stellar wind which accelerated from low velocities near the surface of the star to velocities that exceed the surface escape speed. Their spectra, originated over a range of radii with the optical continuum forming close the stellar core and the emission lines in the more external areas (even beyond 10 stellar radii), indicate that the WR stars are embedded in luminous and turbulent shells of ejecta owing outwards at speeds comparable to the expansion velocities of novae although, in the case of WR stars, the expanding shell is being constantly fed with material from the main body of the star.

WR stars are extremely rare, reflecting their short lifespan. Indeed, they live for only some few hundred of thousands years, and hence only few WR stars are known: about 500 in our Milky Way and 100 in the Large Magellanic Cloud. Indeed, because of their peculiarities (brightness and broad emission lines), WR stars can be detected in distant galaxies. A galaxy showing features of WR stars in its spectrum is known as a Wolf-Rayet galaxy.

I compiled the main characteristics of WR stars in Chapter 2 of my PhD Thesis. A recent review about the properties of WR stars was presented by Crowther (2007).