The Star Types

The BPT law

BPT Part 2.9 (The Stars, by Their Birth Mass): "Once a clump has collapsed, its mass decides everything that follows: how hot its surface is and so its colour, how bright it is, how long it lives, how it dies and what it leaves behind." Its table runs from the hottest class to the coolest: below 0.013 M☉ planetary-mass objects; brown dwarfs (L, T, Y) 0.013–0.075, deep red to infrared, burning deuterium briefly and cooling for ever; red dwarfs (M) 0.075–0.5, "none has died yet"; orange dwarfs (K) 0.5–0.8; yellow dwarfs (G), the Sun, 0.8–1.2; F stars 1.2–2, white; A and late B stars 2–8, blue-white; massive B and O stars 8–25, blue; very massive O stars and Wolf–Rayet stars 25–130, blue to ultraviolet; and Population III giants of 130–250 and above 250 Suns, both marked "metal-poor". BPT Part 3.14 (The Game's Reading) says how the game is to show them: "blue-white giants bright and few in young clusters, red giants and supergiants large and orange, red dwarfs small and dim, brown dwarfs fainter still, white dwarfs as small sharp points, neutron stars blinking as pulsars".

The established physics

Spectral classes O B A F G K M (Cannon & Pickering 1918; Morgan, Keenan & Kellman 1943); end states by core mass (Woosley, Heger & Weaver 2002; Heger et al. 2003), all cited in BPT Part 2.9.

The game's math

STAR_TYPES stars.js:28 lists nine classes with min, max (Suns) and a blackbody tint rgb. typeOf(m) stars.js:40 returns the first class with m < max. REMNANT_RGB stars.js:39 tints white dwarfs and neutron stars. The B class runs 8–25 and O 25–130 (the book's "A and late B" 2–8 is the code's A class).

Brown dwarfs are drawn and tallied as type BD but are not counted as stars (they never fuse hydrogen, 2026-10-29): S.bd (stars.js:199) and the HUD's "shining" count subtracts them (stars.js:259).

In the game

Each type has its own look, set by starLook(s) scene3d.js:661 and shared by the bodies' stars and the galaxies' own. Color is a tint of the class's blackbody color on white, "as the eye sees them" (ruling quoted at scene3d.js:663–664: "the red stars should appear more as points of white light and the blue need to maintain the light as well"):

• O, B and Population III stars: the largest ordinary points, sized by the logarithm of their luminosity and a third larger again (scene3d.js:669).
• Red dwarfs: small and a little dimmed (scene3d.js:671); brown dwarfs: faint, small, magenta-red (scene3d.js:670).
• Other classes: sized by the logarithm of their luminosity (scene3d.js:672).
• Giants: a clear orange, supergiants (8 Suns or more) larger and redder (scene3d.js:668; Part 5.6).
• White dwarfs: small pale blue-white points; neutron stars: tinier, pale blue (scene3d.js:666–667). One neutron star in five blinks as a pulsar, each with its own period of 0.25 to 1.45 seconds and a short bright flash as its beam sweeps past (scene3d.js:800, 1042–1045).

The torus's skin also carries the summed light: paintStarlight() scene3d.js:890 adds each living star's n·L in its type's color to the cell it stands in, every third step, so the skin carries the light-weighted color of the stars on it.

The Stars line names the classes in order: "N shining · O n B n A n F n G n K n M n · brown dwarfs n · giants n · white dwarfs n · neutron stars n · supernovae n · stellar wells n" (stars.js:259). A tap in a galaxy reads its stars as "N stars, X% young and blue", young and blue meaning living stars of two Suns or more not yet giants (app.js:2974, 3028).

The scale model

Exact: no scale model. Types are by birth mass in the level's Suns.

Where the game and the theory differ

• Above 130 Suns every star is class III whatever its metals. BPT Part 2.9 reserves Population III for metal-poor gas; a metal-rich star of 130–150 Suns, which the Kroupa sampler can draw (Part 5.5), is typed III too.

Fixed numbers