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Wolf-Rayet Stars


ROUGH NOTES ONLY - NOT A FINISHED PAGE: Wolf-Rayet stars represent an evolutionary phase in the lives of massive stars during which they undergo heavy mass loss. They are characterised by an extraordinary spectrum which is dominated by emission lines of highly ionised elements.

Keywords: Wolf-Rayet, W-R, WR stars, massive star evolution


Wolf-Rayet stars are evolved, massive, extremely hot (up to ~50,000 K) and very luminous (105 to 106 L¤). They are extremely rare, reflecting their short lifespan.

Their surface composition is extremely exotic, being dominated by helium rather than hydrogen, and typically showing broad wind emission lines of elements like carbon (WC type), nitrogen (WN type), or oxygen: the products of core nucleosynthesis. The presenceor absence of hydrogen, respectively, is used to distinguish the so-called ‘late’ type WN stars (WNL) from the ‘early’ (WNE) types.

Intense stellar winds drive mass loss rates of several 10-5 up to 10-4 M¤ per year; the latter are at least three or four times that expected for other hot, O-type or B-type stars.

“The dense stellar winds completely veil the underlying atmosphere so that anatmospheric analysis can only be done with dynamical, spherically extended model atmospheres, such as those developed by Hillier (1991), Hamann (1994), and Schmutz (1991). Significant progress has been achieved in that respect so that W-R stars can be placed onthe HRD with some confidence (see e.g., Hamann, Koesterke, & Wessolowski 1993). Almost all of the Galactic WNL stars have an observable amount of hydrogen at their surface; some have more than 50% hydrogen (Hamann et al. 1991; Crowther et al. 1995). This property of WNL stars is opposed to other W-R subtypes. The ionising spectrum of WNL stars is very similar to what is observed in luminous O stars (Esteban et al. 1993). Despite their similar luminosities and effective temperatures, W-R and O stars differ drastically in their masses. The most luminous O stars have current masses around 80 M¤ (see, e.g., Kudritzki et al. 1991), whereas WNL stars in binary systems have an average mass of 20 M¤ (Cherepashchuk 1991). Standard evolutionary models assume that heavy mass loss reduces the mass of O stars, so that W-R stars are the low mass descendants of previously massive O stars (Maeder 1990). The close relationship between WNL and extremely luminous O supergiants is also suggested by the similar spectral morphology of the least extreme WNL stars and the most extreme O stars (Walborn 1974; Walborn et al. 1985, 1992)” (Nota et al. 1996, p. 384).

Related Topics

Further Reading

  • Stars and Their Spectra – James Kaler (introductory/intermediate)
  • Modern Astrophysics, Introduction to – Carroll & Ostlie (intermediate)
  • Stellar Astrophysics, Introduction to (3 vols) – Böhm-Vitense (advanced)
  • The Stars: Their Structure and Evolution (2nd ed.) – R.J. Tayler (advanced)
  • Evolution of massive stars. A confrontation between theory and observation (Vanbeveren et al. 1994) (professional)
  • Wolf-Rayet Stars and Interrelations with other Massive Stars in Galaxies (van der Hucht et al. 1991) (professional)



      The first spectral studies of Wolf-Rayet objects were undertaken by the French astronomers, C. Wolf and G. Rayet, after whom the class is named. Instead of dark absorption lines, the stellar winds give rise to strong emission bands of highly ionised elements (Conti & Massey 1989). Lines are broadened in the hottest subtypes, which is believed to be correlated with v.

      The velocity of a stellar wind, initially denoted n0 where it leaves the ‘surface’ of the star, increases with distance. At some point, far from the star, it reaches a maximum known as the terminal velocity and denoted v. For hot winds (Abbott 1982) it is thought to be about three times the escape velocity. (Also see Stellar Winds.)

      Their surface composition is highly anomalous, being dominated by helium rather than hydrogen. The latter is apparently absent in many cases.

      “WR stars can broadly be divided into nitrogen-rich WN stars and carbon (and oxygen) rich WC (and WO) stars. The principal difference between the two subtypes is believed to be that the N-enrichment in WN stars is merely a by-product of H-burning, whilst the C in WC/WO stars is a sign of the fact that He-burning products have reached the stellar surface. As a result, WC stars are thought to be more evolved than WN stars” (Puls et al. 2008, p. 57).

      Three sequences and a transitional phase are recognised by Langer 1990:

      • WNL: the so-called ‘late’ type WN Wolf-Rayet; substantial hydrogen present, nitrogen predominates over a normal carbon component; most massive; large radius [→ sidebar]; coolest and brightest of the W-Rs; the youngest (least evolved) sequence (see Chu et al. 1983); thought to comprise a He burning core and H burning shell
      • WNE: ‘early’ WN Wolf-Rayet; hydrogen absent (only the He burning core remains), nitrogen predominates over carbon; smaller radius; hot and bright
      • WN+WC: this spectral type is sometimes associated with WN+WC binary systems, but has been identified in single stars where it is recognised as a rare (i.e. short lived) transition type
      • WC: hydrogen absent, surface carbon presence increasing from 10-4 to around 40%, oxygen prominent, nitrogen absent; least massive; the hottest though least luminous type; oldest (most evolved) of the W-R sequences; some authors recognise the oxygen-rich examples as a separate WO sequence

      The spectral lines show high levels of ionisation including the presence of ionised helium, He+, but also C+3 or N+3, which suggests temperatures equal to or exceeding those of O-type stars.

      Radii are very difficult to determine at the best of times, and especially so where strong mass loss makes the concept of a stellar ‘surface’ problematic. However, a few estimates have been made from eclipsing binaries, such as 11 R¤ for the late type CQ Cephei (HD 214419, WN7+O9) and ~3 R¤ for the earlier V444 Cygni (HD 193576, WN5+O6). This correlation of large radius with late type versus smaller radius with early type is assumed to persist across all W-R sequences.

      “As for ordinary stars, they are binned into subclasses, in which higher values mean later (i.e. cooler) types. Calibration of W-R stars in Galactic open clusters and in the Large Magellanic Cloud, yields a very tight correlation between Mv and subclass [see van der Hucht et al. 1988]. ... With mean bolometric correction

      WR Magnitudes(22665 bytes)

      Fig. 1: Approximate Mv and Mbol correlations for WC and WN stars(after Moffat et al. 1989, p. 231).

      Mbol – Mv » -4.5 ± 0.2 (1)

      for most W-R subclasses (Smith & Maeder 1989), this implies that the total luminosity also increases systematically from earlier to later WN or WC subtypes (e.g. Mbol » -11 for WN7to -8 for WN4 or Mbol » -9 for WC8 to -8 forWC5)” (Moffat et al. 1989, p. 230).

      Thus the hottest W-Rs are the least brilliant; onthe H-R diagram, the W-R zone slopes downward to the left and narrows like a funnel as shown in fig 2. (After Moffat et al. 1989, p. 233.)

      Note that the positions plotted in the figure refer to the cores of the stars [→ sidebar].

      All stellar observations are necessarily derived from radiation emitted at a variety of physical depths within the atmospheres of the stars, and thus represent a kind of average. We normally interpret them using the simplifying assumption that they are all emitted at a certain optical depth (since the physical depth cannot be known a priori), denoted t . Values of t = 2/3 and t = 1 are commonly used. For stars with exceptionally strong stellar winds, such as Wolf-Rayet stars, however, the region corresponding to an optical depth of t » 1 might well occur within the wind, far from the “surface” of the star.

      Mass and Luminosity

      Variability/Mass Loss

      “A second development was the result of the realization that WR winds are clumped (e.g. Moffat et al. 1988) leading to a decrease in the empiricalWR mass-loss rate by a factor 2-4 (e.g., Hamann & Koesterke 1998). Because on the one hand mass-loss rates have been down-revised, whilst on the other hand luminosities have been revised in an upwards direction, the burden for radiative acceleration to drive WR mass loss to values exceeding the single-scattering limit has become less severe. Nevertheless, compared to O-stars,WRs display significantly larger wind densities and mass-loss rates (factors >~10 for the latter, see Fig. 17). In contrast, the terminal velocities of WR stars are comparable to those from O-stars, of the order of 1000 kms-1(late WNs and WCs) to 2500 kms-1and higher (early WNs and WCs), see Table 2 in Crowther (2007)” (Puls et al. 2008, p. 57-58).




      “We re-iterate that a classification as a WR star reflects purely spectroscopic terminology – signaling the presence of strong and broad emission lines in the spectra which are produced by strong winds. Such spectra can originate in evolved stars that have lost a significant amount of their initial mass, or alternatively from an object that has formed with a large initial stellar mass and luminosity. This latter group of WR stars may thus include objects still in their core H-burning phase of evolution” (Puls et al. 2008, p. 57).

      Occurrence and Examples

      Some of the most massive stars known are Wolf Rayet stars.

      The WN5h star, VFTS 682, which is located about 30 pc from the young massive cluster R136 in the Large Magellanic Cloud. VFTS 682 is estimated at ~130 M¤ (131 ± 25 M¤) and luminosity log L/L¤ = 6.48 ± 0.2 (Rubio-Diez et al. 2016).

      Within the R136 cluster itself, lies R136a1, estimated to be ~170 M¤ and luminosity log L/L¤ = 6.64 (Rubio-Diez et al. 2016).


      Abbott, D.C. 1982: The theory of radiation driven stellar winds and the Wolf-Rayet phenomenon. In de Loore, C.W.H.; Willis, A.J. (ed.) 1982: Wolf-Rayet stars: Observations, physics, evolution. International Astronomical Union Symposia 99 : 185-196.

      Chu, Y.; Treffers, R.R.; Kwitter, K.B. 1983: Galactic ring nebulae associated with Wolf-Rayet Stars. VIII. Summary and atlas. The Astrophysical Journal Supplement Series 53: 937-944.

      Conti, P.S.; Massey, P. 1989: Spectroscopic studies of Wolf-Rayet stars. IV. Optical spectrophotometry of the emission lines in Galactic and Large Magellanic Cloud stars. The Astrophysical Journal 337: 251-271.

      Crowther, P. 2007: Physical properties of Wolf-Rayet stars. Annual Review of Astronomy and Astrophysics 45: 177-219.

      Crowther, P.A.; Smith, L.J.; Hillier, D.J.; Schmutz, W. 1995: Fundamental parameters of Wolf-Rayet stars. III. The evolutionary status of WNL stars. Astronomy and Astrophysics 293: 427-445.

      Esteban, C.; Smith, L.J.; Vilchez, J.M.; Clegg, R.E.S. 1993: Spatially resolved spectroscopy of Wolf-Rayet ring nebulae. Part 4: The fundamental parameters of the central stars. Astronomy and Astrophysics 272: 299.

      Hamann, W.-R. 1994: Spectral analyses of Wolf-Rayet stars: Theory, results, conclusions. In Vanbeveren, D.; van Rensbergen, W.; de Loore, C. (ed.) 1994: Evolution of massive stars. A confrontation between theory and observation. Springer : 237-251.

      Hamann, W.R.; Dünnebeil, G.; Koesterke, L.; Schmutz, W.; Wessolowski, U. 1991: Spectral analyses of Wolf-Rayet stars - hydrogen abundances in WN subtypes. Astronomy and Astrophysics 249: 443-454.

      Hamann, W.R.; Koesterke, L. 1998: Spectrum formation in clumped stellar winds: consequences for the analyses of Wolf-Rayet spectra. Astronomy & Astrophysics 335: 1003-1008.

      Hamann, W.R.; Koesterke, L.; Wessolowski, U. 1993: Spectra analysis of the Galactic Wolf-Rayet stars - a comprehensive study of the WN class. Astronomy and Astrophysics 274: 397.

      Hillier, D.J. 1991: Diagnostics of Wolf-Rayet atmospheres. In Crivellari, L.; Hubeny, I.; Hummer, D. (ed.) 1991: Stellar atmospheres: Beyond classical models. NATO Advanced Research Workshop. Kluwer : 317-329.

      Kudritzki, R.P.; Puls, J.; Gabler, R.; Schmitt, J.H.M.M. 1991: Hot Stars - what can BE Learned fromExtreme Ultraviolet Spectroscopy. In Malina, R.F.; Bowyer, S. (ed) 1991: Extreme Ultraviolet Astronomy. Pergamon : 130-154.

      Langer, N. 1990: Models of Wolf-Rayet Stars. In Garmany, C.D. 1990: Properties of hot luminous stars. Proceedings of the First Boulder-Munich Workshop, Boulder, CO, Aug. 6-11, 1988. Astronomical Society of the Pacific Conference Series 7 : 328-339.

      Maeder, A. 1990: Tables for massive star evolution at various metallicities. Astronomy and Astrophysics Supplement Series 84: 139-177.

      Moffat, A.F.J.; Drissen, L.; Lamontagne, R.; Robert, C. 1988: Spectroscopic evidence for rapid blob ejection in Wolf-Rayet stars. Astrophysical Journal 334: 1038-1043.

      Nota, A.; Pasquali, A.; Drissen, L.; Leitherer, C.; Robert, C.; Moffat, Anthony F. J.; Schmutz, W. 1996: O stars in transition. I. Optical spectroscopy of Ofpe/WN9 and related stars. Astrophysical Journal Supplement 102: 383-410.

      Puls, J.; Vink, J.S.; Najarro, F. 2008: Mass loss from hot massive stars. Astronomy and Astrophysics Review 16: 209-325.

      Rubio-Díez, M.M.; Najarro, F.; García, M.; Sundqvist, J.O. 2016: Re-examing the Upper Mass Limit of Very Massive Stars: VFTS 682, an isolated ~130 M¤ twin of R136’s WN5h core stars. In Eldridge, J.J.; Bray, J.C.; McClelland, L.A.S.; Xiao, L. (ed.) 2016: The Lives and Death-Throes of Massive Stars. Proceedings IAU Symposium 329 : 131-135.

      Schmutz, W. 1991: Observations versus atmospheric models of WR Stars (review). In van der Hucht, K.A.; Hidayat, B. (ed.) 1991: Wolf-Rayet Stars and interrelations with other massive stars in galaxies. Proceedings of the International Astronomical Union 143 : 39.

      Smith, L.F.; Maeder, A. 1989: The bolometric corrections and the Ṁ : L relation for Wolf-Rayet stars. Astronomy and Astrophysics 211: 71-80.

      van der Hucht, K.A.; Hidayat, B. (ed) 1991: Wolf-Rayet Stars and Interrelations with other Massive Stars in Galaxies. Proceedings of the International Astronomical Union 143: 1-680.

      van der Hucht, K.A.; Hidayat, B.; Admiranto, A.G.; Supelli, K.R.; Doom, C. 1988: The galactic distribution and subtype evolution of Wolf-Rayet stars. III. Astronomy and Astrophysics 199: 217-234.

      Vanbeveren, D.; van Rensbergen, W.; de Loore, C. (ed.) 1994: Evolution of massive stars. A confrontation between theory and observation. Springer: 1-490.

      Walborn, N.R. 1974: Some morphological properties of WN spectra. Astrophysical Journal 89: 269-271.

      Walborn, N.R.; Ebbets, D.C.; Parker, J.W.; Nichols-Bohlin, J.; White, R.L. 1992: Ultraviolet and optical spectral morphology of Melnick 42 and Radcliffe 136a in 30 Doradus. Astrophysical Journal 393: L13-L16.

      Walborn, N.R.; Nichols-Bohlin, J.; Panck, R.J. 1985: International Ultraviolet Explorer Atlas of O-Type Spectra from 1200 to 1900 Å. NASA RP-1155.

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