Winds

The BPT law

BPT Part 3.12, in Winds: Why Small Galaxies Hold Fewer Metals: "Where many explode together their hot bubbles merge and break out of the disk, and a wind of gas, carrying the fresh metals with it, leaves the galaxy at hundreds of kilometers a second … The wind takes the gas and leaves the stars. How much it carries out is set by energy: a share of the supernovae's energy lifts gas out of the galaxy's well." Its Wind Mass Loading is η_w = 2·ε_w·E_SN·n_SN/v_esc², "the gas a galaxy's supernovae blow out, per mass of stars formed", with ε_w about a tenth, E_SN = 10⁵¹ erg and n_SN about one per hundred Suns; "η_w ≈ 4 at an orbit of 60 km/s … 1.5 at 100 km/s and 0.4 at 200". Its Halo Escape Speed is v_esc = v_c·√(2(1 + ln(r_t/r))), about 2.6·v_c for gas in the disk.

Its game reading: "Each supernova in a galaxy drives a wind by Wind Mass Loading, 2ε_w E_SN / v_esc² of gas for each supernova, its v_esc 2.6 times the galaxy's measured circular speed: the gas, with the metals it holds, fresh ones too, is taken from the supernova's cloud and leaves from it at the escape speed, outward from the galaxy's center, too hot to form stars for the galaxy's cooling time; the stars stay." The ruling quoted at stars.js:119 (2026-10-28): "build that".

The established physics

A tenth of a supernova's 10⁵¹ erg left as motion (Thornton et al. 1998); energy-driven winds, η_w ∝ v_c⁻² (Dekel & Silk 1986; Murray, Quataert & Thompson 2005).

The game's math

WIND_PER_SN = 2 · 0.1 · 10⁵¹ erg / (M☉ · c²) = 1.12 × 10⁻⁴      (stars.js:124)
want        = n · WIND_PER_SN / β_esc² · S.sun                    cells of gas blown out by n supernovae
q           = min(1, want / gas in the cloud)

Every gas body of the cloud gives q of its gas and metals. The removed gas from each cloud becomes one wind body (stars.js:242): at its mass-weighted place, moving at the galaxy's velocity plus vesc outward from the galaxy's center, all gas, wind: true, hot until flipNow + hot. β_esc, vesc and hot come from the galaxy (b.gal, set by updateGalaxies(), Chapter 4, Part 4.14, and Chapter 6). Bodies outside any galaxy blow no wind.

Worked case: β_esc = 2.6 × 200 km/s / c ≈ 1.73 × 10⁻³: one supernova blows 1.12 × 10⁻⁴ / 3.0 × 10⁻⁶ ≈ 37 Suns of gas, for an η_w of about 0.4 per 100 Suns of stars (one supernova per hundred Suns), matching BPT Part 3.12's 0.4 at 200 km/s.

In the game

A wind is gas, so it is drawn as gas: a new body of faint haze in the data's color (scene3d.js:781–782) that leaves the supernova's cloud outward from its galaxy's center at the escape speed, while the stars it came from stay put. Being hot, it forms no stars until its heat has passed; then, wherever it has come to, in the gas between the galaxies or back in a galaxy, it is ordinary gas again. Since the gas blown out per supernova goes as 1/β_esc², a galaxy with a low escape speed loses far more of its gas this way than a large one. No HUD line or banner names a wind; the game keeps the wind's total mass and metals on the star tallies (S.windMass, S.windZ, stars.js:242) but does not show them.

The scale model

The wind's mass is in the level's Suns; its speed is the galaxy's escape speed in the game's cells a flip; its heating time a share of the life.

Fixed numbers