Re: Hues of colors in space

Hi Russell,

I think that you're asking how the colors that we see in interstellar gas, and in stars relates to the chemical makeup of the gas and stars. There are actually a couple of factors that come into play; the chemical makeup, and the temperature.

If you pass the light from a star through a prism, you will see that it emits light at all wavelengths, but more at some wavelengths than at others. The wavelength where most of the light from a star is emitted is determined primarily by the star's temperature. Hot stars emit most of their light at bluer wavelengths, while cool stars emit most of their light at redder wavelengths. The effect is similar to heating an iron rod with a blowtorch. The rod begins to glow a dull red as it is heated. As it gets hotter, it glows orange, then yellow, and finally white.

A good pair of stars that illustrate this are Rigel and Betelgeuse. They are the brightest stars in the constellation of Orion ("The Hunter"), which is visible at night at this time of year in the Southern sky. Betelgeuse is a cool red giant, while Rigel is a much hotter blue giant. Their color difference is apparent to the unaided eye, and is very obvious in even a small telescope.

Hot gas, such as an interstellar gas cloud, emits a very different spectrum of light. Such a source of light may look white (a mercury vapor streetlamp, for example), but a prism will show that the source is emitting only at certain wavelengths. This is because of the structure of the atoms in the gas. An atom consists of one or more negatively-charged electrons orbiting around a nucleus that contains neutral neutrons and positively- charged protons. The electrons can't have just any orbits around the nucleus, however, but can only be in certain specific ones, which physicists usually call "energy levels" because each orbit is characterized by an energy above the bottom.

In terms of energy, you can think of the energy levels as like a stairway. The bottom of the stairway represents the smallest energy level, what we call the "ground state" of the electrons. To move electrons up the stairs takes energy, just like pushing a bowling ball up the stairs. In a hot gas, the energy is often supplied by collisions between the atom and another atom, or between the atom and a free electron. Some of the energy of the collision can go to raise an electron to a higher energy state.

Like a bowling ball on a stairway, however, the electrons don't like to stay in a higher energy state, and will rapidly "fall down" to lower energy levels. As they fall, they release energy, which appears in the form of photons ("packets" of light).

The key thing is that the spacing of the energy levels is different for each atom, which means that the difference in energy as the electrons fall from one level to the next depends upon the chemical element. This shows up in the wavelength of the photons that are emitted--bigger energy differences lead to the emission of bluer photons, while smaller differences lead to redder photons being emitted. The electrons in a single atom actually fall through many different energy levels, with many different spacings, so that photons of many different wavelengths may be produced.

The pattern of photons gives what we call the emission spectrum of a particular chemical element, and which is different from that of all other elements. By passing the light from an interstellar gas cloud through a prism, we are therefore able to look at the patterns of emission lines that we see, and determine the chemical makeup of the gas cloud.

We can do the same thing in stars, only in reverse. As I said above, a star emits light at all wavelengths. Above the hot surface of the star, however, is cooler gas. The cool gas absorbs light at the wavelengths that correspond to the energy differences between the energy levels. The actual spectrum that we see from star is therefore light at all wavelengths, except that light from certain wavelengths has been removed, what we call an absorption spectrum. From the position of the absorption lines, we can tell the chemical makeup of the star.

You can find some examples of the emission spectra for some of the chemical elements at http://www.achilles.net/~jtalbot/data/elements/index.html.