(c) Anthony K. Grafton 2003

The Color of Stars

When you gaze up at the stars, what do you see? If you look very carefully, you’ll be able to tell that not all stars appear to be the same color. Those differences in color can tell scientists important things about stars.

Put simply, stars that appear blue in color are hotter, and stars that appear red are cooler. Of course, “hotter” and “cooler” are relative terms since most any star will have a temperature of thousands of degrees. But why are the red stars the cooler ones? Don’t we usually think of “red hot” when something is at a high temperature?

To understand how the colors of stars are related to their temperature, let’s first think about light. Light can be thought of as a wave. Waves usually have evenly-spaced peaks and valleys. The distance from one peak to the next is called the wavelength. The wavelength of light determines most of its properties.

When we see differences in color, what we’re really seeing are differences in the wavelengths of light emitted or reflected by objects. Human vision covers only a small part of all the possible wavelengths of light, and we call that small part the visible spectrum. The visible spectrum contains the colors red, orange, yellow, green, blue, indigo, and violet (remember ROY G. BIV for short). Light on the red side of the spectrum has longer wavelengths than light on the blue side of the spectrum.

The wavelength of light is also related to the energy possessed by the light. The shorter the wavelength, the more energy the light contains. Hotter stars give off light with more energy, or shorter wavelengths, and so they appear bluish in color. Cooler stars, on the other hand, give off light with longer wavelengths and appear reddish in color. There are mathematical formulas that can precisely relate the color of a star with its temperature.

Temperature is not the only thing that starlight can tell us. A careful examination of the light given off by a star can reveal the composition, age, and life history of that star as well. Based on its color, what can you deduce about the temperature of our Sun? Is it on the hot side, the cool side, or somewhere in the middle?