H. E. Smith | Spring 2007 |
Physics 5 - Lecture Summary #9 Stellar Spectra |
We may consider three principal types of spectra which appear when the light from an object is broken up into its component wavelengths or "dispersed":
An absorption spectrum is produced when a continuum passes through "cooler" gas. Photons of the appropriate energies are absorbed by the atoms in the gas. Although the photons may be re-emitted, they are effectively removed from the beam of light, resulting in a dark or absorption feature. The atmospheres of stars act as a cooler blanket around the hotter interior of a star so that typical stellar spectra are absorption spectra.
The systematic classification of stars in terms of absorption features and the understanding that such spectral classification is essentially a sequence in atmospheric temperature was due to Annie J. Cannon at Harvard.
Stellar Spectral Classification | |||||||
---|---|---|---|---|---|---|---|
Spectral Type |
Atmospheric Temperature (K) |
Hydrogen (Balmer) Features |
Other Features | M/M | R/R | L/L | Main Sequence Lifetime |
O | >33,000 K | weak | Ionized Helium (He+) features sometimes in emission Strong UV continuum | 20-60 | 9-15 | 90,000-800,000 | 10-1 Myr |
B | 10,500-30,000 K | medium | Neutral He absorption | 3-18 | 3.0-8.4 | 95-52,000 | 400-11 Myr |
A | 7,500-10,000 K | strong | H features maximum at A0 Some features of heavy elements, eg Ca+ |
2.0-3.0 | 1.7-2.7 | 8-55 | 3 Gyr - 440 Myr |
F | 6,000-7,200 K | medium | 1.1-1.6 | 1.2-1.6 | 2.0-6.5 | 7-3 Gy | |
G | 5,500-6,000 K | weak | Ca+ H&K, Na "D" Sun is G2V |
0.9-1.05 | 0.85-1.1 | 0.66-1.5 | 15-8 Gy |
K | 4,000-5,250 K | v. weak | Ca+, Fe, Strong molecules, eg CH, CN |
0.6-0.8 | 0.65-0.80 | 0.10-0.42 | 17 Gy |
M | 2,600-3,850 K | v. weak | Molecules, eg TiO Very red continuum |
0.1-0.5 | 0.17-0.63 | 0.001-0.08 | 56 Gy |
L | 1300-2,500K | Metal hydride molecular bands (CrH & FeH); neutral metal lines | ~ 0.08 | ||||
T | < 1300 | Methane bands | < 0.08 |
Click on the spectral type above to see spectra of the appropriate type or click here to see Spectral Types O - G and here for Spectral Types G - M
Click here for a selection of Spectral Sequence mnemonics
The temperature of the stellar photosphere determines the rate and severity of collisions between molecules, atoms and ions which in turn determines:
Among the other things that we may determine from the absorption spectrum are: density, chemical composition, magnetic field strength, and radial velocity. These are all secondary effects compared to temperature.
The Hertzsprung-Russell Diagram
The Sun
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Conducted by Gene Smith, CASS/UCSD.
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Prof. H. E. (Gene) Smith
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Last updated: 28 Jan 2000