Continuous, Emission and Absorption Spectra

By studying the spectra of objects in the universe, you can do "remote learning," that is, from very far away you can figure out the object's chemical composition and it's velocity. Both of which are very important in astronomy. Let's start by distinguishing between continuous, emission and absorption spectra.

If you understand the picture at right, then you are in a very good position!

Continuous Spectrum

We know that blackbodies emit a well understood spectrum. This spectrum is continuous. That is, a blackbody emits at least some radiation at every wavelength. At right, in Part (a) of Figure 5-14, we see a picture of a a hot blackbody and it's continuous emission spectrum. That is, if you look directly at a blackbody, you will see emission at all wavelengths. "But wait!" you say. "You told me I see everyday objects such as shirts and dogs because of reflected light, NOT emitted light."
This is true. Blue and red shirts both have the same temperature. But both do emit some visible radiation, the key is that they emit only a tiny, tiny amount. Effectively (but not exactly) none!

Key Points:

• A blackbody emits a continuous spectrum
• If you look directly at a blackbody, you will see this continuous spectrum

Absorption Line Spectrum

If there is an absorbing material (such as a cloud of cooler gas) between you and the blackbody, then you will not see a completely continuous spectrum.
Part (b) at right shows a cloud of cool gas between the blackbody and us, and says that we see an "absorption line spectrum," an almost-continuous spectrum, if you will.
Like a gold prospector, panning for gold in a pile of dirt, the cloud of cool gas is "looking" for certain types of light in the "pile of light" (the continuous spectrum) coming from the blackbody.
If the prospector finds a nugget, he gets excited and keeps it, but passes along the dirt (who collects dirt?).
Similarly, if the gas cloud finds light at a wavelength (color) that excites its atoms, the gas cloud will keep that light. But will transmit the light that is unexciting.
In this way, gas cloud plucks out certain colors from the continuous spectrum before passing the rest on to us, the observers. So what do we see?
An absorption line spectrum. An almost-continuous spectrum that has certain colors removed (absorbed).

Key Points:

• Clouds of gas absorb certain wavelengths (colors) of light
• A continuous spectrum that hits a cloud of cool gas will be partially absorbed
• The transmitted spectrum is called an absorption line spectrum (because certain lines are absorbed), and is continuous except for the colors that were absorbed by the gas.

Emission Line Spectrum

The cool gas cloud does not ONLY absorb. In fact, anything that absorbs must also emit. In the context of the picture at right, if we look at only the cloud of gas (i.e. without the blackbody in our line of sight), we will see an emission line spectrum. This is shown in Part (c) at right.
The question is, as always, why?. Well, we know that the cloud of gas absorbs certain wavelengths of light from the blackbody. Each absorbed photon excited one atom in the gas. But an excited atom will not stay excited forever.
Eventually, the excited atom will return to its unexcited (or, "ground") state. The ground state has less energy than the excited state, and because we know that energy must be conserved, the atom will emit a photon whose wavelength (color) is exactly the same as the photon it initally absorbed.
So a cloud of cool gas that absorbs certain colors of light will also emit those same colors. If we look at the cool gas without the blackbody in the line of sight, we will see an emission line spectrum, and the colors of the lines we see are exactly the same colors that were missing from the absorption line spectrum.

Key Points:

• Anything that absorbs also emits.
• A cloud of cool gas that absorbs certain colors from a blackbody will emit exactly those colors as the gas atoms de-excite
• If we look at the cloud without the blackbody in our line of sight, we will see an emission line spectrum.
• The lines of emission have the same color as the absorption lines in the absorption line spectrum
• If you added an emission line spectrum and an absorption line spectrum, you would get a continuous spectrum.