Spectral Classification of Stars JPH 02/01/08 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

Spectral Classification of Stars JPH 02/01/08 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 02/01/08

Spectral Classification of Stars JPH 02/01/08 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 –Radius 9-15 –Luminosity 90,000 – 800,000 Lifetime (M.S.): 10-1 Myr

Spectral Classification of Stars JPH 02/01/08 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 –Luminosity 95-52,000 Lifetime (M.S.): Myr

Spectral Classification of Stars JPH 02/01/08 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 –Radius –Luminosity 8-55 Lifetime (M.S.): 3 Gyr – 440 Myr

Spectral Classification of Stars JPH 02/01/08 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 –Radius –Luminosity Lifetime (M.S.): 7-3 Gyr

Spectral Classification of Stars JPH 02/01/08 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 –Radius –Luminosity Lifetime (M.S.): 15-8 Gyr

Spectral Classification of Stars JPH 02/01/08 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 –Radius –Luminosity Lifetime (M.S.): 17 Gyr

Spectral Classification of Stars JPH 02/01/08 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 –Radius –Luminosity Lifetime (M.S.): 56 Gyr

Spectral Classification of Stars JPH 02/01/08 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.