Stellar Winds and Mass Loss

## Stellar Winds and Mass Loss

All stars lose mass through stellar winds — streams of charged particles expelled from their surfaces. The rate and mechanism vary enormously, from the Sun's gentle breeze to the violent outflows of Wolf-Rayet stars that reshape their surroundings.

### The Solar Wind

The Sun loses mass at a modest rate through its solar wind:

| Property | Value |
|----------|-------|
| Mass loss rate | 2 x 10^-14 M_sun/year (~1.5 million tons/second) |
| Velocity (slow wind) | 300-400 km/s |
| Velocity (fast wind) | 600-800 km/s |
| Temperature | ~100,000 K (corona) |
| Particle density at Earth | ~5 particles/cm3 |
| Reach | ~120 AU (heliopause, confirmed by Voyager 1 in 2012) |

The solar wind creates the heliosphere — a bubble of solar influence extending well beyond the planets. Voyager 1 crossed the heliopause in August 2012 at 121 AU, entering interstellar space.

### Massive Star Winds

O and B stars drive powerful winds through **radiation pressure** on spectral lines — photons from the star's interior transfer momentum to metal ions in the atmosphere, which drag the surrounding gas outward.

| Star Type | Mass Loss Rate (M_sun/yr) | Wind Speed (km/s) |
|-----------|---------------------------|--------------------|
| Sun (G2 V) | 2 x 10^-14 | 400-800 |
| B supergiant | 10^-7 to 10^-6 | 500-2,000 |
| O supergiant | 10^-6 to 10^-5 | 2,000-3,000 |
| Wolf-Rayet | 10^-5 to 10^-4 | 1,000-5,000 |
| Red supergiant | 10^-6 to 10^-4 | 10-30 |
| AGB star | 10^-7 to 10^-4 | 10-15 |

A Wolf-Rayet star can lose an entire solar mass in 100,000 years. This mass loss strips the star's outer hydrogen and helium layers, exposing the carbon, nitrogen, and oxygen cores — explaining why Wolf-Rayet stars show spectra dominated by these elements.

### AGB Winds and Dust Formation

On the asymptotic giant branch (AGB), low- and intermediate-mass stars experience pulsation-driven mass loss. As the star pulsates, material is levitated to cooler regions where it condenses into dust grains. Radiation pressure on these grains drives them outward, dragging gas along.

This process creates circumstellar shells rich in carbon or oxygen dust, depending on the star's C/O ratio:
- **Carbon stars** (C/O > 1): Produce soot-like carbon dust, silicon carbide
- **Oxygen-rich stars** (C/O < 1): Produce silicate dust, alumina

### Impact on the Interstellar Medium

Stellar winds:
- Enrich the interstellar medium with heavy elements (carbon, nitrogen, oxygen)
- Create cavities and bubbles in the ISM (e.g., the Local Bubble, ~1,000 ly across)
- Trigger star formation by compressing neighboring gas clouds
- Shape planetary nebulae and supernova remnants

The **Local Bubble** — the region of low-density, hot gas surrounding the Sun — was likely carved by multiple supernovae and stellar winds from the Scorpius-Centaurus OB association over the past 10-20 million years.