Spectral Classification of Stars

JPH 01/29/10

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 – A9Early to late A-type

Spectral Classification of Stars

JPH 01/29/10

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)

Spectral Classification of Stars

JPH 01/29/10

Spectral Classification of Stars

JPH 01/29/10

O stars

• Atmospheric temperature: > 33,000 K• Hydrogen Balmer lines: weak• Other spectral features: Ionized Helium (He+) features

sometimes 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

Spectral Classification of Stars

JPH 01/29/10

B stars

• 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

Spectral Classification of Stars

JPH 01/29/10

A stars

• 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

Spectral Classification of Stars

JPH 01/29/10

F stars

• 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

Spectral Classification of Stars

JPH 01/29/10

G stars

• 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

Spectral Classification of Stars

JPH 01/29/10

K stars

• 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

Spectral Classification of Stars

JPH 01/29/10

M stars

• 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

Spectral Classification of Stars

JPH 01/29/10

Other stars

• 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.