Lecture #11: Thermal spectrum vs. Line radiation


I. A thermal spectrum is a smooth hump, peaking at a frequency that is proportional to the temperature. A given sized area of any object at a given temperature will look more or less the same as any other object with that size and temperature
-- this holds exactly if the objects are "blackbodies", i.e., if they absorb all light that hits them and reradiate light based purely on their own surface temperature. There are no perfect blackbodies, but all objects in the solar system, including the Sun, are reasonably good blackbodies.
-- sometimes thermal emission is called blackbody emission, though it sounds like a contradiction because a hot blackbody will be a little brighter than a hot whitebody. For us, all that matters is the temperature, and we'll treat all high-density objects as blackbodies.
II. Line emission and absorption
--At low density, gases will only absorb or emit photons at specific frequencies. The spectra of these emissions or absorptions look like "lines", so these are called spectral lines. They encode an enormous amount of information about the composition and general state of any low-density astronomical gas that is being observed.