1. Temperature

2. Blackbody Radiation
Heated gas radiates electromagnetic energy, called blackbody radiation.
Higher temperatures create higher energies.
Cooler stars red
Hotter stars blue
low energy
high energy
frequency
intensity

3. Measuring Temperature
Measure the star’s spectrum.
Measure the wavelength with the peak intensity.
Convert the wavelength to a temperature.
Measure the intensity of the star with a red filter.
Measure again with a green filter, then a blue filter.
Match to the right temperature profile.
peak
wavelength

4. Absorption Lines
Ionized gases at a star’s surface absorb specific frequencies of light.
Dark lines in a star’s spectrum
Since gases ionize at different temperatures, the lines tell the temperature of the star.

5. Spectral Types
The types of spectra were originally classified only by hydrogen absorption, labeled A, B, C, …, P.
Eventually arranged by temperature.
O, B, A, F, G, K, M
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Type Temperature
O 35,000 K
B 20,000 K
A 10,000 K
F 7,000 K
G 6,000 K
K 4,000 K
M 3,000 K

6. Spectral Classes
Some bright stars class
Sun G2
Sirius A1
Alpha Centauri G2
Vega A0
Capella G8
Rigel B8
Betelgeuse M1
Aldebaran K5
Each type is split into 10 classes from 0 (hot) to 9 (cool).
Temperature and luminosity are not the same thing.