|Gene Smith's Astronomy Tutorial|
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|
|O||>33,000 K||weak||Ionized Helium (He+) features|
sometimes in emission
Strong UV continuum
|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
|K||4,000-5,250 K||v. weak||Ca+, Fe,
Strong molecules, eg CH, CN
|M||2,600-3,850 K||v. weak||Molecules, eg TiO
Very red continuum
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.
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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: 16 April 1999