 We know a few things about light. We know that light is electromagnetic radiation created by moving electrons. With help from Jupiter's moon Io, we know that light travels across empty space at 186,000 miles per second. And as we have seen, stars vary in apparent brightness, and knowing the star's distance, we can use the inverse square law to find its intrinsic luminosity. The other thing we know is that stars have different colors. And very interestingly, color tells us a lot more than you might think. But to understand color, we need to know more about light. Light, as we currently understand it, has a dual nature. We can view it as a particle, like a photon, or we can view it as a wave that can interfere with itself. Although we haven't been able to reconcile these two incompatible views with one underlying understanding, what we do know has turned out to be surprisingly sufficient for the distance ladder. For color, we view light as a wave. Here's a simple wave. It has a repeating cycle, a wavelength, and a frequency in cycles per second. Newton showed that the sun's light can be dispersed into the colors of the rainbow with a crystal. This effect comes from the wave nature of light. Different colors represent different light frequencies. The higher the frequency, or inversely, the shorter the wavelength, the more it's bent by the crystal. This produces a spectrum of light with blue and violet at the high frequency end, and red at the low frequency end. An important relationship between energy and light is that a light's energy is directly proportional to the frequency. So when physicists see color, they think energy. Red is low energy light, and blue is high energy light. Here we see the full electromagnetic spectrum with visible light in the middle. Radiation with longer wavelengths and smaller frequencies than red light is called infrared, and still longer wavelengths are called radio waves. Moving up the energy scale, radiation with shorter wavelengths than violet light is called ultraviolet. Still shorter wavelengths are called x-rays, and the maximum energy radiation is called gamma rays. Celestial objects shine in radio to infrared, visible light, and ultraviolet to x-rays.