Dr. Aaron Titus | Department of Physics, High Point University
PHY1050      Astronomy of Stars, Galaxies, and the Cosmos
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spectral class

The spectral class of a star tells us the temperature of the star.

The absorption spectrum of a star tells us the composition of a star as well as the temperature of a star because the intensity and presence of certain spectral lines are a function of temperature.

When absorption spectra of stars were first collected by astronomers, they were grouped together by noticing similarities such as the intensity and presence of certain spectral lines. At the time, it was not understood what variable (or variables) that determined these differences in the spectra of stars.

Later, it was learned that temperature was the variable that affected the differences in stellar spectra. After this was understood, it gave us a new technique for determing the temperature of a star.

To determine the temperature of a star: (1) collect the spectrum of the star, (2) compare it to the spectra of stars of known temperature, (3) determine the spectral class of the star, and finally (4) deduce the temperature of the star.

The spectral classes are defined as: (from Astronomy, by Zeilik)

Class Color Surface Temperature (K) Features Example
O Bluish-white 30,000 Fewer absorption lines. Lines mostly consist of highly ionized atoms and helium. Weak hydrogen lines.
B Bluish-white 11,000-30,000 Lines of neutral helium. Hydrogen lines are stronger than in O-type stars. Rigel
A Bluish-white 7,500-11,000 Strong hydrogen lines. Lines of singly ionized magnesium, iron, calcium, and others. Weak lines of some neutral metals. Sirius
F Blish-white to white 6,000-7,500 Hydrogen lines are weaker than for A-type stars. Lines of singly ionized metals and neutral metals. Procyon
G White to yellowish-white 5,000-6,000 Strong lines of ionized calcium. Lines of ioned and neutral metals,. Weak hydrogen lines. Sun
K Yellowish-orange 3,500-5,000 Dominated by lines of neutral metals. Aldebaran
M Reddish less than 3,500 Strong lines of neutral metals and molecules. Betelgeuse

An example spectrum for each class is shown below:

Note that each class is also broken into subclasses from 0 (hottest) to 9 (coolest) with each subclass identified by certain spectral lines. The figure above actually shows spectra for O4, B5, A0, F0, G0, K0, and M2 stars. Sun is a G2 star with a surface temperature of 5800 K.

Thus, to get the temperature of a star, astronomers use a spectrometer to record the absorption spectrum of the star. Then, a computer (or a person) compares the spectrum to a catalog of spectra in order to determine the spectral classification of the star. Because we know the temperatures of the various classes of stars, this gives the temperature of the star.






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