383 695 of the 455 608 stars read "Spectral type Unknown", every Gaia star among them, although
380 884 of them carry a colour. The card now gives those the type of the dwarf whose colour is
nearest, in the colour's own system, marked as an estimate: TRAPPIST-1, BP-RP 4.90, reads
"Spectral type ~M8, from colour", which is how it was classified (M8 V). A star with neither a
type nor a colour gets no subtitle rather than the literal "Unknown".
The colours are Pecaut & Mamajek's mean dwarf sequence (2013, ApJS 208, 9, table 5), as Mamajek
maintains it online (version 2022.04.16, which carries Gaia BP-RP), from B0 to M8.5. B-V cannot
tell O types apart (the whole sequence spans 0.03 of it), and BP-RP turns back past M8.5 and is
tabulated only from B9. A colour outside the table gets no estimate: 186 BP-RP and 41 B-V colours,
among them the blue Gaia DR3 5612323414549657984 at BP-RP -0.15.
380 657 stars get an estimate: F 97 840, G 144 327, K 105 984, M 29 142, A 3 272, B 92. Checked
against catalogued types:
- 19 433 HYG dwarfs with a B-V: 74.3 % within two subclasses, 95.5 % within five.
- 146 Gaia exoplanet hosts the archive types as dwarfs, from BP-RP: 88.4 % within two
subclasses, 98.6 % within five.
The estimate assumes a dwarf, so a giant reads later than it is (Pollux, K0 III at B-V 0.99,
reads K3). It also ignores reddening. The comment says both.
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
SenrokaiandClaude Opus 5.5
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