Stellar Wind
A stellar wind is a continuous outflow of plasma — primarily protons, electrons, and alpha particles — ejected from the upper atmosphere of a star at velocities ranging from hundreds to thousands of kilometres per second. In the Sun, the solar wind originates from coronal holes where the magnetic field lines are open, accelerating the corona to supersonic speeds via thermal pressure and wave-driven mechanisms. Stellar winds sculpt circumstellar nebulae, strip planetary atmospheres, modulate cosmic ray flux, and carry away angular momentum that gradually spins down rotating stars over their lifetimes.
Key Formula
dM/dt = 4π r² ρ(r) v(r)
LaTeX: \dot{M} = 4\pi r^2 \rho(r)\, v(r)
| Symbol | Meaning | Unit |
|---|---|---|
| ṁ (dM/dt) | Mass-loss rate (mass ejected per unit time) | M☉/yr or kg/s |
| r | Radial distance from star centre | metres (m) |
| ρ(r) | Wind density at radius r | kg/m³ |
| v(r) | Wind velocity at radius r | m/s |
Worked Example
Problem
The solar wind has a number density of ~5 protons/cm³ and a velocity of ~450 km/s at Earth's orbit (1 AU = 1.5 × 10¹¹ m). Estimate the Sun's mass-loss rate.
Solution
Step 1 — Convert number density: n = 5 × 10⁶ m⁻³. Step 2 — Mass density: ρ = n × m_p = 5 × 10⁶ × 1.67 × 10⁻²⁷ = 8.35 × 10⁻²¹ kg/m³. Step 3 — Wind velocity: v = 450 × 10³ = 4.5 × 10⁵ m/s. Step 4 — Shell area: A = 4π r² = 4π (1.5 × 10¹¹)² ≈ 2.83 × 10²³ m². Step 5 — Mass-loss rate: ṁ = ρ v A = 8.35 × 10⁻²¹ × 4.5 × 10⁵ × 2.83 × 10²³ ≈ 1.06 × 10⁹ kg/s. Step 6 — In solar masses per year: 1.06 × 10⁹ / (2 × 10³⁰) × (3.15 × 10⁷) ≈ 1.7 × 10⁻¹⁴ M☉/yr.
Answer
Solar mass-loss rate ≈ 1.0 × 10⁹ kg/s ≈ 1.7 × 10⁻¹⁴ M☉/yr
Stellar Wind Properties Across Spectral Types
| Star Type | Terminal Velocity (km/s) | Mass-Loss Rate (M☉/yr) | Wind Driver | Example |
|---|---|---|---|---|
| O supergiant | 2,000–3,000 | 10⁻⁶ – 10⁻⁵ | Radiation pressure on UV lines | Zeta Puppis |
| B supergiant | 500–2,000 | 10⁻⁸ – 10⁻⁶ | Radiation + thermal | Rigel |
| Wolf-Rayet | 1,000–5,000 | 10⁻⁵ – 10⁻⁴ | Extreme radiation pressure | WR 104 |
| AGB (red giant) | 10–30 | 10⁻⁷ – 10⁻⁴ | Pulsation + radiation on dust | Mira |
| Solar-type (G) | 350–800 | ~10⁻¹⁴ | Coronal heating + waves | Sun |
Interactive Tools
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Related Terms
Stellar Nucleosynthesis
Stellar nucleosynthesis is the process by which nuclear fusion reactions inside stars create heavier atomic nuclei from lighter ones, releasing energy that sustains the star against gravitational collapse. Main-sequence stars primarily fuse hydrogen into helium via the proton–proton chain or CNO cycle, while more massive stars in later evolutionary stages fuse helium, carbon, oxygen, and silicon up to iron (Fe-56), the most tightly bound nucleus. Elements heavier than iron are synthesised through neutron-capture processes (s-process in AGB stars; r-process in neutron star mergers and supernovae), making stars the principal factories of the chemical elements in the universe.
Variable Star
A variable star is any star whose observed brightness (apparent magnitude) changes over time, whether due to intrinsic physical changes in the star itself or due to geometric effects such as eclipses or rotation. Intrinsic variables include pulsating stars (Cepheids, RR Lyrae, Mira), eruptive variables (novae, flare stars), and cataclysmic variables (dwarf novae, Type Ia supernovae). Of particular cosmological importance are Cepheid variable stars, whose pulsation period is directly related to their intrinsic luminosity (the period–luminosity relation), making them crucial standard candles for measuring distances to nearby galaxies.
Binary Star System
A binary star system consists of two stars gravitationally bound to each other, orbiting their common centre of mass (barycentre) under mutual gravitational attraction. Binary systems are remarkably common, accounting for roughly half of all star systems in the Milky Way, and are the primary means of directly measuring stellar masses through application of Kepler's third law. Depending on orbital geometry, binaries may be classified as visual, spectroscopic, eclipsing, or astrometric, each revealing complementary information about the stellar components.
From Old English "steorra" (star) and Old English/Proto-Germanic "wind" (moving air). The term "solar wind" was coined by astrophysicist Eugene Parker in 1958 in his paper predicting continuous plasma outflow from the corona.