Temperature Blackbody Radiation • Heated gas radiates electromagnetic energy, called blackbody radiation. intensity low energy high energy • Higher temperatures create higher energies. – Cooler stars red – Hotter stars blue frequency Measuring Temperature • Measure the star’s spectrum. • Measure the wavelength with the peak intensity. • Measure the intensity of the star with a red filter. • Measure again with a green filter, then a blue filter. • Convert the wavelength to a temperature. • Match to the right temperature profile. peak wavelength 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. 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 – Our Brother Andy Found Green Killer Martians Type O B A F G K M Temperature 35,000 K 20,000 K 10,000 K 7,000 K 6,000 K 4,000 K 3,000 K Spectral Classes Some bright stars Sun Sirius Alpha Centauri Vega Capella Rigel Betelgeuse Aldebaran class G2 A1 G2 A0 G8 B8 M1 K5 • Each type is split into 10 classes from 0 (hot) to 9 (cool). • Temperature and luminosity are not the same thing.