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O B A F G K M Hottest to Coldest (Temperature sequence) Classification scheme due to Annie Jump Cannon (Harvard) (non-alphabetic ordering merely an historical accident)

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O b a f g k m hottest to coldest temperature sequence

O B A F G K M

Hottest to Coldest (Temperature sequence)

Classification scheme due to Annie Jump Cannon (Harvard) (non-alphabetic ordering merely an historical accident)

Within each letter class also a numerical class 0-9 (Hot to Cold) for further refinement. Then a Roman Numeral (e.g., I, II, V) for luminosity class

Sun = G2V

A0 – A9

Early to late A-type


O b a f g k m hottest to coldest temperature sequence

Luminosity classes (LC)

Second parameter (pressure-dependent)

  • Super-supergiants 0 0-Ia Ia-0

  • Supergiants: Ia Iab Ib

  • Bright giants: IIa IIab IIb

  • Normal giants: IIIa IIIab IIIb

  • Subgiants IVa IVab IVb

  • Main Sequence V (also “dwarfs”)

a,ab,b: subclasses (in decreasing luminosity order)

Transition classes (Ib-II also used)


O b a f g k m hottest to coldest temperature sequence

Atmospheric temperature: > 33,000 K

Hydrogen Balmer lines: weak

Other spectral features: Ionized Helium (He+) featuressometimes in emission, strong UV continuum

Parameters (in solar units)

Mass 20-60

Radius 9-15

Luminosity 90,000 – 800,000

Lifetime (M.S.): 10-1 Myr

O stars


O b a f g k m hottest to coldest temperature sequence

Atmospheric temperature: 10,500-30,000 K

Hydrogen Balmer lines: medium

Other spectral features: Neutral He absorption

Parameters (in solar units):

Mass 3-18

Radius 3.0-8.4

Luminosity 95-52,000

Lifetime (M.S.): 400-11 Myr

B stars


O b a f g k m hottest to coldest temperature sequence

Atmospheric temperature: 7,500-10,000 K

Hydrogen Balmer lines: strong

Other spectral features: H features maximum at A0, some features of heavy elements, eg Ca+

Parameters (in solar units):

Mass 2.0-3.0

Radius 1.7-2.7

Luminosity 8-55

Lifetime (M.S.): 3 Gyr – 440 Myr

A stars


O b a f g k m hottest to coldest temperature sequence

Atmospheric temperature: 6,000-7,200 K

Hydrogen Balmer lines: medium

Other spectral features: Ca+ lines continue to increase as Balmer lines weaken thru class (from 0 to 9)

Parameters (in solar units):

Mass 1.1-1.6

Radius 1.2-1.6

Luminosity 2.0-6.5

Lifetime (M.S.): 7-3 Gyr

F stars


O b a f g k m hottest to coldest temperature sequence

Atmospheric temperature: 5,500-6,000 K

Hydrogen Balmer lines: weak

Other spectral features: Ca+ H&K, Na "D"Sun is G2V

Parameters (in solar units):

Mass 0.9-1.05

Radius 0.85-1.1

Luminosity 0.66-1.5

Lifetime (M.S.): 15-8 Gyr

G stars


O b a f g k m hottest to coldest temperature sequence

Atmospheric temperature: 4,000-5,250 K

Hydrogen Balmer lines: very weak

Other spectral features: Ca+, Fe,strong molecules, eg CH, CN

Parameters (in solar units):

Mass 0.6-0.8

Radius 0.65-0.8

Luminosity 0.10-0.42

Lifetime (M.S.): 17 Gyr

K stars


O b a f g k m hottest to coldest temperature sequence

Atmospheric temperature: 2,600-3,850 K

Hydrogen Balmer lines: very weak

Other spectral features: Molecules, eg TiO ,very red continuum

Parameters (in solar units):

Mass 0.08-0.5

Radius 0.17-0.63

Luminosity 0.001-0.08

Lifetime (M.S.): 56 Gyr

M stars


O b a f g k m hottest to coldest temperature sequence

W = Wolf-Rayet (WR) stars – very massive – earlier than O-type – losing mass at high rate

WN = nitrogen rich

WC = carbon rich

L = “stars” redder than M. May not be true stars (i.e., H burning) – brown dwarfs.

"R", "N", "S" stars are cool stars with particular types of molecular bands.

Other stars