THE "HENRY DRAPER" CLASSIFICATION-TYPES O-M
Type	Main Characteristics	Subtypes	HD Criteria	Typical Stars
O	Hottest stars, continuum strong in UV	Oa	O II λ 4650 dominates	BD+35°4013
		Ob	He II λ 4686 dominates (in emission)	BD+35°4001
		Oc	He II λ 4686 dominates (in emission), lines narrower	BD+36°3987
		Od	Absorption lines dominate; only He II, O II in emission	ζ Pup, λ Cep
		Oe	Si IV λ 4089 at maximum	29 CMa
		Oe5	O II  λ 4649, He II  λ4686 strong	τ CMa
B	Neutral helium dominates	B0	C III  λ 4650 at maximum	ε Ori
		B1	He I  λ 4472 > O II λ 4649	β CMa, β Cen
		B2	He I lines at maximum strength	δ Ori, α Lup
		B3	He I lines are disappearing	π4 Ori, α Pav
		B5	Si λ 4128 > He λ 4121	19 Tau, φ Vel
		B8	λ 4472 = Mg λ 4481	β Per, δ Gru
		B9	He I λ 4026 just visible	λ Aql, λ Cen
A	Hydrogen lines decreasing from  maximum strength at A0	A0	Balmer lines at maximum strength	α CMa
		A2	Ca II K = 0.4 Hδ	δ CMa, ι Cen
		A3	K = 0.8 Hδ	α PsA, τ3 Eri
		A5	K > Hδ	β Tri, α Pic
F	Metallic lines becoming noticeable	F0	K = H + Hδ	δ Gem, α Car
		F2	G band becoming noticeable	π Sgr
		F5	G band becoming continuous	α CMi, ρ Pup
		F8	Balmer lines slightly stronger than in sun	β Vir, α For
G	Solar-type spectra	G0	Ca λ 4227 = Hδ	α Aur, β Hya
		G5	Fe λ 4325 > Hγ on low resolution images	κ Gem, α Ret
K	Metallic lines dominate	K0	H and K at maximum strength	α Bov, α Phe
		K2	Continuum becoming weak in blue	β Cnc, υ Lib
		K5	G band no longer continuous	α Tau
M	TiO bands dominate	Ma	TiO bands noticeable	α Ori, α Hya
		Mb	Bands conspicuous	ρ Per, γ Cru
		Mc	Spectrum fluted by the strong bands	W Cyg, RX Aqr
		Md	Mira variables, Hγ , Hδ in emission	χ Cyg, ο Cet