Light, color & astronomy

Last spring, when I took astronomy 101 we talked a lot about spectra and light. While we discussed the definitions and applications of wavelength, frequency and speed of light waves, what I was most interested in was the different types of spectra of visible light that different objects and/or conditions could produce.

First, it is important to note that sometimes light behaves like a wave while others it behaves like a particle. When light behaves like a particle, each particle is called a photon. Photons carry energy as they travel through space. The amount of energy or photon is directly proportional to the wavelength.

So, what we see by the naked eye under normal conditions is the visible light continuous spectrum of all colors. This spectrum looks like the gradation of colors we saw in class. Different types of spectra include an emission line spectrum, which looks like a dark background with spikes at certain wavelengths indicating certain colors and an absorption line spectrum, which is similar to a continuous spectrum but has some black lines where colors are missing.

Both absorption and emission line spectra appear when gas changes what types of light each atom in a given object or condition can absorb and/or reflect. Further, perceived color in astronomy almost always tells something about the temperature of an object. For example, a hotter object will have a shorter peak wavelength and will thus be brighter at all wavelengths which is why hot stars look blue or white because their peak wavelength is actually in the UV area of the wavelength spectrum while cooler stars appear yellow or red indicating a peak in the infrared part of the spectrum.