Why do stars shine different colors in the night sky?
(Image credit: zhengshun tang via Getty Images)
One of the pleasures of stargazing is noticing and enjoying the various colors that stars display in dark skies.Star colors are always interesting to observe, since they add so much to the character of the constellations. These hues offer direct visual evidence of how stellar temperatures vary. A good many of the luminaries of the summer season — such as brilliant Vega, which stands about halfway up in the east-northeast sky as darkness falls — are bluish white. Still, we can easily find other, contrasting colors there as well. Look at the reddish Antares and the yellowish-white Altair. And at the top of the line of this summer's retinue, brilliant orange Arcturus holds forth in solitary splendor high toward the south.Even as you observe these stellar colors, do you notice that they're recognizable only for the brightest stars? This is due to the physiology of the eye, more specifically, the fact that the color sensors on the retina — the cones — are insensitive to faint light. Under dim illumination, the retinal rods take over. But their greater light sensitivity is offset by their color blindness. This is why faint stars tend to appear white to our eyes. However, if we look at them through a binocular or a telescope, their amplified brightness stimulates the cones, which detect their color.Colors by contrastOne of the best ways to see star colors is by contrast. Let's return to Arcturus for a moment. The classic procedure for locating this star is to follow the arc of the Big Dipper's handle southeastward. Back in the 1950s, a very popular lecturer at New York's Hayden Planetarium was Henry M. Neely (1879-1963), who had a favorite ditty for locating Arcturus and another bright star of late spring/early summer: "Follow the arc to Arcturus and speed to Spica."Spica shines with a distinct bluish tint. Move your eye rapidly back and forth between Arcturus and Spica to see the great difference in their respective orange and blue hues.
