Correct a giant's light by its type, not by the cooler dwarf its colour reads as

3392f06 read every star's bolometric correction off the dwarf sequence at its colour, so a giant
got the correction of the cooler dwarf of that colour, which is larger. Antares, M1 Ib at B−V 1.87,
took an M5 dwarf's −3.26 and came out 172 023 L☉ and 1 516 R☉, a 7.06 AU sphere, against the 680 R☉
Ohnaka et al. (2013) measure; 119 Tau 2 838 against 587, Menkar 204 against 89.

isGiant (spectral.ts) reads luminosity class I to III off a type's primary component, or HYG's g
and c prefixes, and a giant keeps its type's correction. Drawn radius against the published one
(no planets, so derived), before and after: Antares 2.23 -> 1.01, 119 Tau 4.84 -> 1.43, Menkar 2.29
-> 1.80, Scheat 1.92 -> 1.42, Aldebaran 1.95 -> 1.10, Mirach 1.96 -> 1.27, 41 Com 2.14 -> 1.56;
Betelgeuse 0.76 -> 0.89. It is not better everywhere: Gacrux goes from 1.23 to 1.40 and Arcturus
from 1.02 to 0.88. 10 808 stars have a giant's type, 10 794 of them a colour. Against the archive's
own luminosity for the 130 giant hosts, the median error moves from 0.038 to 0.052 dex and the 90th
percentile from 0.246 to 0.204; 17 are off by over 1.5 times, against 19.

Controls, each failing its named test (1 of 781): the giant given the dwarf's correction, a giant
companion read as the primary, a IV read as a I, and the prefixes ignored.

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

src/app/shared/astro/spectral.spec.ts

@@ -1,6 +1,6 @@
 import { describe, expect, it } from 'vitest';
 
-import { dwarfSequenceAtColor, parseSpectralClass, SPECTRAL_CLASSES, spectralTypeFromColor, spectralTypeToColorIndex, temperatureToColorIndex } from './spectral';
+import { dwarfSequenceAtColor, isGiant, parseSpectralClass, SPECTRAL_CLASSES, spectralTypeFromColor, spectralTypeToColorIndex, temperatureToColorIndex } from './spectral';
 
 describe('parseSpectralClass', () => {
   it('reads a clean class and subclass', () => {
@@ -46,6 +46,20 @@ describe('parseSpectralClass', () => {
   });
 });
 
+describe('isGiant', () => {
+  it('reads luminosity classes I to III off the primary, and the giant and supergiant prefixes', () => {
+    for (const type of ['M1Ib + B2.5V', 'K5III', 'M2II-IIIvar', 'C7Iab', 'K0IIIb', 'gK0', 'cM2']) {
+      expect(isGiant(type), type).toBe(true);
+    }
+  });
+
+  it('leaves dwarfs, subgiants, a dwarf with a giant companion and the unclassified alone', () => {
+    for (const type of ['G2V', 'B2IV', 'F0IVn', 'M5Ve', 'K1V + M3III', 'g-k', 'Unknown', 'DA', '']) {
+      expect(isGiant(type), type).toBe(false);
+    }
+  });
+});
+
 describe('spectralTypeToColorIndex', () => {
   it('places the Sun near its real B-V of 0.65', () => {
     expect(spectralTypeToColorIndex('G2V')).toBeCloseTo(0.626, 2);