Read a colour past either end of the dwarf table at that end, where the star has no type instead

dwarfSequenceAtColor answers null outside Pecaut & Mamajek's table, B−V −0.301 to 2.16 and BP−RP
−0.12 to 5.1, and effectiveTemperatureK then fell back on the type, which Gaia's stars do not have
and carbon stars' parser does not read. 219 stars with a measured colour got no temperature and so
no radius, and were drawn at the Sun's radius in the Sun's colour: 110 white dwarfs within 50 pc,
38 Gaia stars redder than BP−RP 5.1 (Gaia DR3 6439125097427143808, an ultracool dwarf 4.0 pc away),
HD 46687, La Superba and the other carbon stars, and an O8 star.

Past the table, the colour is now read at the row it is past (clampToTable), but only for a star
with no readable type: beside a type an off-table colour is more often the bad measurement — HD
49748 is G5 V at B−V −0.32 — so the type still wins there, as it did. The luminosity reads the same
point, so a star past the red end also gets the M8.5 row's G−V and correction. A type's own colour
past the table, which only O types have, is read at B0. Carbon and S stars (C, N, R, S) now count
as giants, so their correction stays their type's, not an M8.5 dwarf's −5.78.

Measured on the shipped catalogue: stars without a temperature 3 053 -> 2 834, without a radius
3 077 -> 2 858; all 292 stars with an off-table colour now have a temperature, against 73. Gaia DR3
6439125097427143808 is 2 420 K and 0.110 R☉ (M8.5 V: 0.104); the 110 white dwarfs a median 0.018 R☉
at 10 700 K (0.0013 to 0.034); HD 46687 212 R☉ at 2 420 K and La Superba 133; audit #16's Gaia DR3
5612323414549657984, k1 Pup, B6 V at BP−RP −0.15, 203 L☉ and 4.1 R☉ against 143 and none (FLAME
gives 336 and 3.49). The 2 851 stars left without a radius have neither a colour nor a readable
type, or no band.

Controls, each failing its named test: no clamp for an untyped star (2 of 784 failed), the clamp
winning over a type, an O type's colour unclamped, the luminosity off the unclamped sequence, and
carbon stars not counted as giants (1 of 784 each); clampToTable ignored (3 of 784).

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/app/shared/astro/stellar.ts

@@ -1,4 +1,4 @@
-import { dwarfSequenceAtColor, isGiant, parseSpectralClass, SpectralClass, spectralTypeToColorIndex } from './spectral';
+import { DwarfSequencePoint, dwarfSequenceAtColor, isGiant, parseSpectralClass, SpectralClass, spectralTypeToColorIndex } from './spectral';
 
 /**
  * Stellar luminosity, derived from the two things the star catalogue actually measures.
@@ -133,7 +133,7 @@ export function luminositySolar(star: StellarPhotometry): number | null {
   // dwarf, whose correction is larger. Antares, M1 Ib at B−V 1.87, read as an M5 dwarf's −3.26
   // came out 1 516 R☉ against the 680 Ohnaka et al. (2013) measure, and 119 Tau 2 838 against 587;
   // its type's −1.55 gives 690.
-  const sequence = star.colorIndex != null ? dwarfSequenceAtColor(star.colorIndex, star.colorSystem) : null;
+  const sequence = sequenceAtColour(star);
   const absoluteV = absolute - (star.magnitudeBand === 'G' ? (sequence?.gMinusV ?? 0) : 0);
   const bolometric =
     absoluteV + (sequence && !isGiant(star.spectralType) ? sequence.bolometricCorrectionV : bolometricCorrection(star.spectralType));
@@ -141,6 +141,22 @@ export function luminositySolar(star: StellarPhotometry): number | null {
   return Math.min(Math.max(luminosity, MIN_LUMINOSITY_SOLAR), MAX_LUMINOSITY_SOLAR);
 }
 
+/**
+ * The dwarf sequence at a star's colour — at the end of the table a colour is past, where the star
+ * has no type to go by instead. Past the red end are the ultracool dwarfs Gaia measures redder than
+ * BP−RP 5.1, M8.5; past the blue end, its white dwarfs and hot stars bluer than −0.12, B9, and
+ * B−V's O stars. Unclamped, they had no temperature and were drawn at the Sun's: Gaia DR3
+ * 6439125097427143808, an ultracool dwarf 4.0 pc away, and 110 white dwarfs within 50 pc, all at
+ * 1 R☉. Beside a type, an off-table colour is more often a bad one than an extreme star — HD
+ * 49748, G5 V, at B−V −0.32 — and the type is read instead.
+ */
+function sequenceAtColour(star: StellarPhotometry): DwarfSequencePoint | null {
+  if (star.colorIndex == null) {
+    return null;
+  }
+  return dwarfSequenceAtColor(star.colorIndex, star.colorSystem) ?? (parseSpectralClass(star.spectralType) ? null : dwarfSequenceAtColor(star.colorIndex, star.colorSystem, true));
+}
+
 /** The Sun's effective temperature, the IAU 2015 nominal value. */
 export const SOLAR_EFFECTIVE_TEMPERATURE_K = 5772;
 
@@ -154,8 +170,8 @@ export function effectiveTemperatureK(star: StellarPhotometry): number | null {
   if (star.distancePc === 0) {
     return SOLAR_EFFECTIVE_TEMPERATURE_K;
   }
-  const measured = star.colorIndex != null ? dwarfSequenceAtColor(star.colorIndex, star.colorSystem) : null;
-  return (measured ?? dwarfSequenceAtColor(spectralTypeToColorIndex(star.spectralType)))?.temperatureK ?? null;
+  // A type's colour past the table is an O star's, which B−V no longer tells apart from B0.
+  return (sequenceAtColour(star) ?? dwarfSequenceAtColor(spectralTypeToColorIndex(star.spectralType), 'B-V', true))?.temperatureK ?? null;
 }
 
 /**