Stellar Physics
Stellar Wind
Continuous outflow of charged particles from stars — solar wind is just one example
A stellar wind is a continuous outflow of plasma (mostly electrons + protons) from a star. Sun's solar wind: 400-800 km/s; mass loss ~10⁻¹⁴ M_sun/yr. Massive stars (O, Wolf-Rayet) have much stronger winds — up to 10⁻⁵ M_sun/yr. Mass loss controls stellar evolution: massive stars without strong winds would supernova differently. Drives: hot stars — radiation pressure on metal lines; cool stars — magnetic field acceleration; AGB stars — radiation pressure on dust + magnetic processes.
- Sun mass loss~10⁻¹⁴ M_sun/yr
- O star mass loss~10⁻⁵-10⁻⁷ M_sun/yr
- Wolf-Rayet mass loss~10⁻⁵ M_sun/yr (extreme)
- Solar wind speed400-800 km/s
- O star wind speed~3000 km/s (much faster)
- Drive (hot)Radiation pressure on metal lines
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Why stellar winds matter
- Stellar evolution. Mass loss alters paths.
- Galactic feedback. Stellar winds heat ISM.
- Wolf-Rayet stars. Late-stage massive star physics.
- Solar system. Solar wind shapes heliosphere.
- Habitability. Winds strip planet atmospheres.
- Element distribution. Winds disperse heavy elements.
- Star formation. Winds clear molecular clouds.
Common misconceptions
- Stellar winds are uniform. Variable; clumpy.
- All stars have similar winds. Mass loss varies by 10⁹.
- Sun's wind is biggest. Sun's is mild compared to massive stars.
- Winds don't affect planets. Strip atmospheres; affect climate.
- Winds are constant. Time-variable in many ways.
- Mass loss is gentle. O-star winds are extremely energetic.
Frequently asked questions
What drives stellar winds?
Different mechanisms. (1) Hot stars (O, B): radiation pressure on UV line absorption. Many spectral lines absorb starlight; momentum transferred to ions; ions accelerated outward. (2) Sun-like (G, K, M): magnetic acceleration via coronal heating. Solar wind from coronal holes. (3) Red giants (AGB): radiation pressure on dust grains; magnetic-acoustic waves in atmosphere. Different physics for different stellar types.
What's mass loss of Sun?
~10⁻¹⁴ M_sun per year. Sun has lost ~10⁻⁴ of its mass over its lifetime. Negligible for total mass. Important: solar wind shapes solar system, drives heliosphere, affects planetary atmospheres. Earth would be different without solar wind.
What about massive stars?
O stars: 10⁵-10⁷× more mass loss than Sun. Wolf-Rayet stars: even more. Reason: more luminosity → more radiation pressure → stronger wind. Massive star can lose half its mass through wind before going supernova. Alters supernova progenitor mass — affects whether NS or BH forms.
How do winds shape evolution?
Mass loss removes energy and angular momentum. Reduces stellar mass. Affects core composition, lifetime, and end state. Without strong winds: more massive cores → more black holes vs neutron stars. With winds: more mass dispersed; less remaining for compact remnant. Key parameter in stellar models.
What's a Wolf-Rayet star?
Massive, hot, evolved star with extreme mass loss. Outer layers stripped — exposing core. Different types based on composition. WN (nitrogen-rich): hydrogen lost. WC (carbon-rich): helium lost too. Final pre-supernova state for many massive stars. Lifetime brief (~10⁵ yr).
Do winds create observable structures?
Yes. (1) Stellar wind bubbles — hot plasma cavities. (2) Bow shocks — when star moves through ISM. (3) Planetary nebulae — superwind from AGB star. (4) Solar wind shapes heliosphere. (5) Bubbles cleared in molecular clouds by winds — affects star formation. Feedback important for galaxy evolution.
Are winds steady or variable?
Often variable. Solar wind: varies with solar cycle, daily fluctuations. Massive star winds: time-variable due to clumping (overdensities), pulsations. CMEs (coronal mass ejections) — major eruption events. Magnetic activity drives much of variability.