Stellar Evolution
Wolf-Rayet Star
Massive, hot stars with extreme mass loss — pre-supernova phase
A Wolf-Rayet (WR) star is a massive, evolved star (>20 M_sun progenitor) that has lost its outer hydrogen envelope through extreme stellar winds, exposing the helium- or carbon-rich layers. Surface temperatures: 25,000-200,000 K. Spectra show broad emission lines (not absorption) — sign of strong stellar wind. Three subtypes: WN (nitrogen-rich), WC (carbon-rich), WO (oxygen-rich). End-state: collapse → supernova or black hole. Many gamma-ray burst progenitors are Wolf-Rayet stars. Brief phase (~10⁵ yr) before final collapse.
- Surface temperature25,000-200,000 K (very hot)
- Mass loss rate~10⁻⁵ M_sun/yr (extreme)
- Wind speed~3000 km/s
- Lifetime~10⁵ years (brief phase)
- SubtypesWN, WC, WO (composition)
- DiscovererWolf and Rayet, 1867 (Paris Observatory)
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Why WR matter
- Massive star evolution. Late stage of high-mass stars.
- Pre-SN physics. Direct precursors to core-collapse SN.
- Gamma-ray bursts. Some WRs are progenitors.
- Stellar winds. Most extreme winds known.
- Element distribution. Carbon, nitrogen via stripping.
- Galactic chemistry. Major source of metals.
- Stellar physics. Test mass-loss models.
Common misconceptions
- WR are stable phase. Brief and unstable.
- WR are dim. Brightest stars in galaxy.
- WR don't supernova. Almost all do.
- WR are different from regular stars. Late phase of massive star evolution.
- WR can't have hydrogen. Some early WN have hydrogen.
- WR don't matter for Earth. Affect galactic chemistry.
Frequently asked questions
How do Wolf-Rayet stars form?
Originally massive O-stars (>20 M_sun). Strong stellar winds during main sequence and post-MS phases gradually strip outer hydrogen. Eventually hydrogen envelope is mostly removed. Helium core exposed (WN). Further stripping reveals carbon (WC) or oxygen (WO). Process gradual: 100,000+ years. Outcome depends on initial mass and rotation.
What are the subtypes?
(1) WN — nitrogen-rich. Hydrogen burned in shell; nitrogen from CNO cycle. Hottest. (2) WC — carbon-rich. Helium core exposed; carbon from triple-alpha. (3) WO — oxygen-rich. Late stage; nearing core collapse. Different stages of envelope stripping.
Why are spectra emission lines (not absorption)?
Strong stellar wind produces emission. Wind is hot, dense, and emits radiation. Emission overwhelms absorption from photosphere. Broad lines (1000-3000 km/s) reflect wind expansion. Atypical spectrum — unique to Wolf-Rayet stars and similar massive star phases.
How do they end?
Core collapse supernova (Type Ib or Ic — no hydrogen, no helium). Many become black holes due to mass. Some explode as long-duration gamma-ray bursts. Specific outcome: depends on initial mass, mass loss, rotation. Massive ones go directly to black hole; less massive form neutron star + supernova.
How are they observed?
Optical spectroscopy reveals broad emission lines. Photometric variability. UV very bright. Infrared dust emission (especially WC stars). Many in binaries — companion observable. Catalogs: Galactic WR catalog has ~700 confirmed; expected ~10³ in galaxy.
Where are they found?
Star-forming regions, OB associations. Mostly in galactic disk. Some isolated. Notable: WR 11 (gamma Velorum) — closest WR star, 1100 ly. WR 124 — runaway WR, surrounded by nebula. WR 6 (EZ CMa) — eclipsing binary with WC. Limited number of well-known examples.
Could they hit Earth?
Distance protects. Even nearby WR (1000+ ly): unlikely to disrupt Earth. Concern: gamma-ray burst from collapsing WR, beamed toward Earth. Estimated risk: very low. Past mass extinctions don't appear to be from WR or GRBs. Earth survives despite massive star deaths nearby.