When you look at the grill on an electric stove as it heats up, you see it turn from black to bright red, and you know by the color that it’s hot enough to blister your fingers.
Astronomers determine stars’ temperatures in much the same way, by studying their colors. Of course, stars are a lot hotter than a stove: the coolest are red.
Progressively hotter ones are orange, yellow, green, blue and, finally, violet. Blue and violet stars are rare; red stars are common because, for one thing, they burn out more slowly.
The next time you are out on a clear night (a dry winter sky far from city lights is best) take a look at Orion, a large constellation suggesting the shape of a hunter brandishing a club and a lion skin. The bright red star at Orion’s shoulder is Betelgeuse (pronounced “beetle juice”); the foot star of Orion is Rigel, a clear blue star.
These colors are visible with the naked eye.
Astronomers determine the color of light emitted by a star with equipment called a spectrophotometer. The light waves are then characterized by wavelength. Astronomers use a mathematical formula that involves the speed of light and some other universal constants to convert wavelengths to temperature: the longer the wavelength, the lower the energy, the cooler the star.
With that formula, they have calculated that Betelgeuse is about five thousand degrees Fahrenheit, and Rigel is about four times as hot.
Knowing how hot a star is, astronomers can begin to understand its mass, its distance from Earth and how much it has aged.