Why do stars appear different colors in the night sky?

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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 contrast

One 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.

A small telescope or even a pair of steadily held binoculars will readily split Albireo into two tiny points of light of beautiful contrasting colors: the brighter one a rich yellowish orange, the other a deep azure blue, both placed very close together. A stunning view will come with a telescope magnifying between 18x and 30x.

Stars hot and cool

Earlier, we referred to Antares as being reddish, which is how it's always described. But actually, that isn't correct. What we regard as "red" stars (which are catalogued as spectral class M) are really yellow-orange and approximately the same color as an old-fashioned incandescent light bulb. Both it and M stars have about the same 3,000 Kelvin color temperature.

Our eyes evolved to take advantage of the radiation emitted by the sun, which is an average star as far as temperature and color are concerned. Very hot and cool stars, on the other hand, are strongest in the ultraviolet and infrared ranges, respectively.

Starlight is similar to what physicists call black-body radiation — the electromagnetic waves given off by a body that emits and absorbs radiation 100% efficiently. (Anything that truly absorbed all the light falling on it would be black, hence the name). We know that the hotter a star, the more energy it emits at every wavelength — for very hot stars, the peak emissions are directed toward the shorter (bluer) wavelengths.

Simply put, the location of the peak emissions determines the star's color. We can break this all down into two rather simple laws that are immediate consequences of blackbody radiation.

For an object at a particular temperature, the total energy radiated at all wavelengths is given by the Stefan-Boltzmann law, after its two discoverers, Josef Stefan (1835-1893) and Ludwig Boltzmann (1844-1906). The Stefan-Boltzmann law states that the rate at which an object radiates heat is proportional to the fourth power of the absolute temperature, expressed in degrees Kelvin. Thus, if the temperature doubles, the energy output increases 16 times. If it triples, the output increases 81 times.

The second law is called Wien's law, named for Wilhelm Wien (1864-1928). It states that the wavelength of a star's peak output is inversely proportional to the temperature. If the temperature doubles, the peak wavelength is halved. Wien's law has a very colorful consequence, which can be demonstrated by the coils on an electric stove. As their temperature increases, we first feel infrared radiation, then see it glowing dull red, followed by bright red and still brighter orange. If it could continue heating without melting, the coils would turn yellow, white, then blue-white while becoming extremely brilliant — just like stars.

So, the color is determined by Wien's law, while the total radiation (both visible and invisible) is determined by the Stefan-Boltzmann law.

If you're looking for a telescope to get a closer look at the night sky our best telescopes for deep space guide may help. We also have a guide to astrophotography for beginners, which covers everything from equipment to shooting modes and more.

Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, Sky and Telescope, The Old Farmer's Almanac and other publications.