Star Color
 Stars give off light at every wavelength, but peak in intensity at a certain
wavelengths
 We can determine the color of a star by measuring the peak wavelength
o Pass the light through a spectrograph (measures light intensity at each
wavelength)
o Pass the light through a color filter (only let light pass through at certain
wavelengths) and measure the wavelength intensity
 Most common color filters are U (ultraviolet), B (blue), and V
(photovisual)
 Wavelength vs. Intensity (Figure 24-2)
o Radiation follows a fairly smooth curve
o Peak in radiation intensity at certain wavelengths
o Each line represents a different star (with different Temperatures)
 Notice the hotter stars
peak in the blue (they
appear to be the color
blue in the sky) and
cooler stars peak in the
red
 Blue hot is hotter than
red hot
Star Color and Temperature
 There is a set of physical laws that govern this relationship
o Planck's law dictates the peak intensity of the radiation
 Considers stars close to an idealized case of a blackbody something that absorbs all the radiation that falls on it (perfect absorber and
perfect emitter)
o Wein's displacement law relates temperature to wavelength
 λmax T = constant
 In other words:
 If T increases, λmax decreases
 If T decreases, λmax increases
 Example: If Star A has a Temperature of 10,000 K and a
wavelength twice that of Star B, what is Star B's
Temperature?
o Stefan-Boltzmann law relates energy emitted with temperature
 Energy = σ T4
 Note that if temperature increases even slightly, energy output
increases greatly
 Example: If a star's temperature doubles, how much more energy
will it be giving off?