Transcript: When a hot object like a star is surrounded by a cooler layer of gas or has an intervening cool material we see an absorption spectrum. Always they’re at the smooth thermal radiation reflecting the temperature of the object itself, but in the case of an outer or intervening cool layer of gas the radiation is absorbed by the cooler gas exciting those atoms that then reradiate in all directions causing notches in the spectrum or lowering of the intensity at the exact position corresponding to the spectral transitions of the elements in that gas. Thus the smooth thermal spectrum has superimposed on it an absorption spectrum where the wavelengths of the absorption features tell us about the chemical composition of the gas. Fraunhofer in the mid-nineteenth century observed an array of dark absorption lines in the spectrum of the sun, and this was our first clue that the sun was made of hydrogen and helium.