Spectral type

The spectral type is an indication of the surface temperature of a star since the temperature determines which emission lines will be present and how strong they will be. From hottest to coolest stars are grouped into categories O, B, A, F, G, K, and M. Each letter is then subdivided into 10 numbers, from hotter to cooler 0, 1, 2, 3, 4, etc.

Class	Effective temperature (K)	Conventional color description	Actual apparent color	Mass	Radius	Luminosity (bolometric)	Hydrogen lines	Fraction of all main-sequence stars
O	≥ 30,000 K	blue	blue	≥ 16 M☉	≥ 6.6 R☉	≥ 30,000 L☉	Weak	~0.00003%
B	10,000–30,000 K	blue white	deep blue white	2.1–16 M☉	1.8–6.6 R☉	25–30,000 L☉	Medium	0.13%
A	7,500–10,000 K	white	blue white	1.4–2.1 M☉	1.4–1.8 R☉	5–25 L☉	Strong	0.6%
F	6,000–7,500 K	yellow white	white	1.04–1.4 M☉	1.15–1.4 R☉	1.5–5 L☉	Medium	3%
G	5,200–6,000 K	yellow	yellowish white	0.8–1.04 M☉	0.96–1.15 R☉	0.6–1.5 L☉	Weak	7.6%
K	3,700–5,200 K	orange	pale yellow orange	0.45–0.8 M☉	0.7–0.96 R☉	0.08–0.6 L☉	Very weak	12.1%
M	2,400–3,700 K	red	light orange red	0.08–0.45 M☉	≤ 0.7 R☉	≤ 0.08 L☉	Very weak	76.45%
L	1,300–2,400 K	red brown	scarlet	0.005–0.08 M☉	0.08–0.15 R☉	0.000,05–0.001 L☉	Extremely weak
T	500–1,300 K	brown	magenta	0.001–0.07 M☉	0.08–0.14 R☉	0.000,001–0.000,05 L☉	Extremely weak
Y	≤ 500 K	dark brown	black	0.0005–0.02 M☉	0.08–0.14 R☉	0.000,000,1–0.000,001 L☉	Extremely weak