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Annie Jump Cannon was one of the most famous assistant of
Edward Charles Pickering in Harvard. She was an expert in stellar
classification. For the Henry Drapper catalog, she classified more than
300000 stars, an example of productivity!
1.13.5 Spectral class
If the spectral type is a characteristic of the surface temperature, a
more precise classification should take into account their luminosity. In
Yerkes, William Morgan, Philip Keenan, and Edith Kellman (MMK)
established a classification I to V which takes into account the shape of
some absoprtion lines, characteristics of gravity forces. Class I stars are
super giants while class V (like the Sun) are sub giants stars.
Astrophysists position stars on a graph based on their temperature
and luminosity. This is the Hertzsprung-Russell diagram.
1.13.6 Chemical composition and metallicity
Two stars of same class and same type have a different history, a
different genealogy. We note Z the metallicity or the ratio of atom
different than hydrogen and helium. One can position each star on a 3
dimension HR-Z diagram: temperature, luminosity, and metallicity.
We classify sars in two populations:
1. Population I: stars rich in metal (ie: non hydrogen or helium
atoms), often second or third generation stars. Part of their matter
has been created during the explosion of a supernova. Our Sun
and matter that constitutes us is a good exemple.
2. Population II: stars poor in metal, very old. We can find
them typically in globular clusters.
1.13.7 Doppler Effect
Doppler effect, explained by Christian Doppler in 1842, is a
wavelength shift between emitted light and received light when object
and observer are moving from each other. If they approach themselves,
shift is toward blue; if they are moving away, it is toward the red (such
as the "redshift" of galaxies).
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