Classification of Stellar Spectra

• Essentially all stars appear as point sources. Only differences are brightness and spectra.

• Many differences in spectra due to temperature– Pseudo blackbody (with chromosphere outside

photosphere)– Relative strength of absorption lines (due to

different stages of ionization and energy states)

Hydrogen Absorption Lines in Visible• Classification originally based on strength of Balmer

absorption lines (2 -> n). A type stars have strongest lines– (OBAFGKM) Harvard Classification obafgkmrns.html

http://astronomy.swin.edu.au/cosmos/h/harvard+spectral+classification

Hydrogen Balmer Series

• Balmer series due to transitions from n=2 to n>2

• Strength of absorption lines depends on fraction of atoms that are neutral and in the n=2 state

• Maxwell-Boltzmann Statistics:

Saha Equation• Relative number of atoms in ith ionization state

also determined using Maxwell-Boltzmann statistics– Derivation complicated due to the continua of

possible states for free electrons– Result is the Saha equation:

• Partition function (effective number of states):

• Ionization energy:

Hydrogen mostly ionized above 12,000 K

Thus the fraction of hydrogen atoms in the n=2 energy state is given by fractionof neutral atoms times the fraction of neutral atoms in n=2.

Calcium Lines

• Abundance of calcium is 1/500,000 that of hydrogen

• Lower ionization energy (6.1 eV) makes calcium essentially all singly ionized (Ca II)

• Roughly 265/266 Ca II atoms in ground state– 1s22s22p63s23p64s1

– 4s to 3d produce calcium H & K lines– Even though abundance is low, the high fraction of

calcium in ground state produce large absorption lines

Abundances: H – 91%He – 8.9%O - .05%C – 0.03N – 0.01Ne - .01Si - .003Mg - .003Fe – .003

Solar Spectrum

D: SodiumE & G: IronF: Hydrogen (H beta)C: Hydrogen (H alpha)H & K: Calcium II

Hertzsprung-Russell Diagram

Hipparcos catalog 2010: 16,631 stars with relative distance precision better than 10%

Luminosity vs Spectral Classification

Hertzspring-Russell Diagram

HR Diagram for Nearest Stars

Stellar Radii

Along constant radius line

• Sirius A

• Sirius B

• Betelgeuse

• d = 640 ly• L = 105 Lsun

• R = 1,200 Rsun=4.5 AU

• M = 19 MSun

Correlation between mass, luminosity, and position in MS branch: