Here’s a short explanation of star colors to help you better appreciate what you see when you look at the stars in the night sky. In just a couple of minutes, you’ll understand more about stars than 99% of people who’ve ever lived.
• Stars radiate light a little like glowing coals in a campfire. Just as a glowing red-hot coal is cooler than a white-hot coal, for example, so a red star is cooler than a white star, and a white star is cooler than a blue star. This was a major scientific discovery… simply by measuring the color of light coming from a star, and applying a little physics, it was possible to estimate a star’s surface temperature.
• Like most scientists, astronomers love to classify things. In the late 19th century, Harvard astronomers developed a system to classify stars not according to color, but by the strength by which hydrogen gas absorbed light at particular wavelengths. The star classes were labeled A to N in order of decreasing hydrogen absorption strength. After a time, the classes were simplified to O, B, A, F, G, K, and M. This is the Harvard spectral classification, which is still used today. So what does this have to do with star color? Keep reading…!
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• As astronomers and physicists learned more about atomic structure and the spectra of light from stars, they discovered the Harvard classification system really described the temperature of a star’s atmosphere. They discovered the type-O stars are hotter than type-B stars, and type-B stars are hotter than type-A stars, and so on. But hot stars are blue, and medium-hot stars are white, and cool stars are red.
• Here’s a summary of the dominant color and temperatures of the main classes of stars, along with examples of stars that belong to each class:
• The upshot is this: the color of a star depends on its surface temperature. But a blue star doesn’t emit only blue light, nor does a red star emit only red light. They emit visible light of all colors to some degree. It’s just that their spectrum peaks at a particular color.
• So why are there blue stars, yellow stars, red stars, but no green stars? As it turns out, there are green stars, that is, stars that radiate much of their light in the green part of the spectrum. But the total combination of the full range of colors of a “green” star appears white to our eyes. If you pass the color from a whitish star through a prism, you’ll see all the colors, including green, spread out in a continuum.
• Astronomers came to understand that bluer stars are intrinsically brighter because they are more massive than white or red stars, and more massive stars burn much faster and hotter than less massive stars. The bluish type-O stars, for example, are only 30-50 times more massive than yellow-white stars like our sun. But O stars burn a million times brighter, so they have far shorter lifetimes. O and B stars only last a few million years before they die in spectacular supernova explosions, while cooler and less massive K and M stars burn steadily for billions of years.
• Some 88% of stars in the universe seem to be the cooler type K and M. Only 1 in 3,000,000 stars are type O. Even middle-weights like our type-G Sun comprise only 8% of all known stars.
Good To Know
This relationship between star mass, luminosity, and color holds only for stars burning hydrogen in the core during the prime of their lives. For example, young and middle-aged M-type stars are small, faint and long-lived. But as stars age and start burning heavier elements in the core, bluish O and B stars, for example, evolve briefly into immensely bright M-type red stars known as red supergiants. We’ll explain this in later issues. If it sounds complicated, fear not. Even astronomy majors wrestle over this for some time before they understand how stars live and evolve.
To remember star types O-B-A-F-G-K-M in order of hotter to cooler, you can use the mnemonic “Oh Be A Fine Girl (or Guy), Kiss Me”. My personal favorite alternative: Only Bored Astronomers Find Gratification Knowing Mnemonics