Here we consider Stellar Spectra. We will, roughly speaking, discuss how spectra are formed, and then look at the ways in which stellar spectra are used to give the temperatures of stars and the chemical compositions of stellar atmospheres. As mentioned earlier, we will be concerned with the continuous spectra and line spectra of the Celestial objects. To get a feel for what these terms mean, we will first consider the Solar spectrum and then look at some lamps.
By poor resolution, we mean that details in the spectrum which have very nearly the same wavelengths (energies) cannot be separated (seen as discrete features). Because spectral lines, in general, cover quite small ranges in wavelength, most prisms are not good enough to see the individual features; they get smeared-out by the prism.
If we used a different kind of tool to break up the Solar radiation into its constituent colors (one that was better at resolving spectral lines), we would see:
Note that the above plot simply shows how much total energy is carried by photons of particular wavelengths in the spectrum. The peak of the plot shows at which wavelength the star would appear the brightest. Note--the continuous part of the spectrum that I drew resembles the spectrum of a blackbody radiator. This is also true for most stars. We find that Sun can be represented fairly well by a blackbody of temperature ~ 5,800 Kelvins. (The Wien law, W(max) = 30,000,000 Angstroms / T(K), tells you that the peak emission falls
W(max) = 30,000,000 Angstroms / 5,800 Kelvins = 5,200 Angstroms.
Note that 1 Angstrom = 0.00000001 centimeters.)
We now move on to the primary topics for today's lecture: