If you gaze into the early evening sky now, you might notice a few spots of color. The bright star Betelgeuse is orange. Arcturus is yellow orange. And Spica has a hint of blue. The colors tell us that the stars shine brightest at different wavelengths. Orange stars are cool, so they shine at longer wavelengths. Blue stars are hot, so they shine at shorter wavelengths. Yet that range represents only a tiny sliver of the electromagnetic spectrum. And astronomers use the entire spectrum to study the universe. Electromagnetic waves are produced by everything from clouds of dust to exploding stars. The longest are radio waves. Each wave — the point from one peak to the next — spans yards to miles. So astronomers use giant radio dishes — some of them hundreds of feet across — to catch the waves. Among other things, the waves tell us about some of the coldest objects in the universe, such as big clouds of gas and dust. The shortest waves are gamma rays. A wave can be smaller than the nucleus
