Why Are Stars Different Colors?星星為什麼有不同的顏色？

🔊Look at Orion on a winter evening. Betelgeuse, on his shoulder, glows orange-red, while Rigel, at his foot, shines blue-white. Look carefully at other bright stars, and you will find that stars are not all white. They come in a whole range of colors: orange-red, orange, yellowish white, white, and blue-white. What makes the difference?

🔊The answer is temperature. Anything hot enough gives off light, and its color depends on how hot it is. A piece of iron in a blacksmith's fire first glows dull red, then orange, then yellow, and finally white. A star's surface works the same way. Orange-red stars like Betelgeuse are the coolest, at about 3,300 degrees Celsius. Our Sun is about 5,500 degrees, and blue-white stars like Rigel are 12,000 degrees or more. This is the opposite of a bathroom faucet, where red means hot and blue means cold!

🔊A star does not give off just one color. It gives off a whole rainbow of light mixed together, plus light our eyes cannot see. A hotter star gives off more blue light, and a cooler star gives off more red. The Sun's light is strongest in the green part of the rainbow, but it mixes in so much of every other color that the Sun is really white. It only looks yellow or orange when it is low in the sky, because the air scatters away some of its blue light.

🔊About a hundred years ago, astronomers began sorting stars by their light. At Harvard, Annie Jump Cannon sorted hundreds of thousands of stars into classes named O, B, A, F, G, K, and M, from hottest to coolest. Then Ejnar Hertzsprung and Henry Norris Russell compared each star's color with how bright it really is. Most stars fell along one band, now called the main sequence. A few were giants, far brighter than their color suggests because they are enormous, and a few were tiny, faint white dwarfs.

🔊Chinese astronomers paid attention to star colors, too. More than 2,000 years ago, the historian Sima Qian used stars as color samples: white like Sirius, red like Antares, and yellow like Betelgeuse. Yellow? Today Betelgeuse is clearly orange-red. In 2022, a team of astronomers suggested that the ancient records are right and that Betelgeuse has slowly changed color as it aged into a red supergiant. Antares, the red heart of the Scorpion, was known in China as the Great Fire.

🔊You can see star colors for yourself, but they are soft, because our eyes see color poorly in the dark. The trick is to compare: look from Betelgeuse to Rigel, or from Arcturus to Vega. A pair of binoculars, slightly out of focus, makes the colors easier to see. Use the list below to find tonight's bright stars over Changhua, sorted from the coolest to the hottest.