Determining the temperature of a star from its color and its spectrum

Stars have different colors. For instance, the star Betelgeuse in Orion's shoulder is redder than most stars. (Have a look.)

Color and temperature

The reason for the different colors is that the light is (approximately) blackbody radiation, which is not sensitive to the details of the star's composition but is sensitive to the temperature of the star.

Thus astronomers can determine the surface temperature of a star by measuring its color. (To do this quantitatively, they commonly use three filters that transmit light in three different wavelength ranges. Then they take the ratio of the intensity of the light that gets through, say, the shortest wavelength filter to the intensity of the light that gets throught the medium wavelength filter.)

Spectral classification

Another way to measure the surface temperature is to examine in detail the spectral lines in the starlight. These details show something about composition, but they also show something about temperature. Consider the imaginary element Oregonium, which is found in the photosphere of all imaginary stars. We look for absorption lines from Oregonium. Here, for instance, is an event that cannot happen for hydrogen if all the atoms of hydrogen are in their gound state:

The classification by means of spectra was the result of a big project at Harvard. The Henry Draper Catalogue published around 1920, listed 224,300 stars that had been classified by astronomer Annie Jump Cannon.

The OBAFGKM classification

Astronomers commonly refer to stars by their spectral type in a scheme that arose from this project:

ASTR 122 course home page

Updated 22 Octobber 2007

Davison E. Soper, Institute of Theoretical Science, University of Oregon, Eugene OR 97403 USA