Colors of the Winter Sky

Carnegie, Jan/Feb 2004 by Radzilowicz, John G

Most people think of stars as scintillating points of white light. Few think that stars have any color. However, a careful look reveals that, in fact, there is a wide range of colors to be found among the stars.

The cold, clear, nights of winter provide a perfect time to seek out some of the more colorful stars to be found in our skies. That's because some of the brightest stars in the sky are winter stars, and bright stars show their colors best of all.

This time of year, you can find dazzling and colorful stars rising high above the southern horizon. The bluish tint of the stars Sirius and Rigel is easy to spot. You'll also find the orange-red glow of Betelgeuse and Aldebaran fairly easy to distinguish. And with relatively little effort, you'll spot the yellow hue of Capella and Procyon as well.

All of these stars are found in major constellations that dominate the winter nights. Linking the stars forms another type of star picture called an asterism. This asterism is called the Winter Hexagon (or sometimes the Winter Circle). The proximity of these stars makes it easy to compare them with each other, and the contrast makes the colors that much easier to discern.

The color of a particular star tells us something important about it. That's because the surface temperatures of the stars determine the colors we see. Red and red-orange stars are coolest. They have surface temperatures around 3000 degrees. Yellow stars are about twice as hot as the red. And blue and blue-white stars are the hottest, topping out around 12,000 degrees or so.

On the next clear night, test your own perception of star colors by trying to spot some of the colorful members of the Winter Hexagon.

By John G. Radzilowicz, director of the Henry Buhl, Jr. Planetarium and Observatory