Star Color

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Whenever you have a good look at the stars on a clear night, you'll get to notice the range of colors that the stars have to offer. What causes the difference in colors? Different temperatures. Suppose you put a poker into a hot furnace - hot enough to make it start glowing. The first color you could discern would be red. As the poker gets hotter, this glowing hue would progress from red to orange, to yellow, then white, and finally as blue hot - assuming the poker somehow stayed solid. That color/temperature relationship works the same way with the star's light output, the coolest stars appearing red and the hottest as blue. The color ranging is represented scientifically by the spectral class which categorizes the light output with a letter (M, K, G, F, A, B, O), based on from readings from spectroscopic instruments mounted on telescopes. For example, our Sun is listed as a G type star, having a yellow color and surface temperature of about 6,000 Kelvins. There are subdivisions between the letters, but that's not terribly important for this conversation. This spectral classification spans an extraordinary scope of temperatures - as seen below:

Color Temperature (Kelvins)
O 28,000 - 50,000
B 10,000 - 28,000
A 7,500 - 10,000
F 6,500 - 7,500
G 5,000 - 6,000
K 3,500 - 5,000
M 2,500 - 3,500

Here are the colors and temperatures of some of the brighter stars in the sky:

Sun - G

Capella - G

Rigel - B

Betelguese - M

Procyon - F

A good way to discern the dramatic differences that star hues can present is to view some double stars at a star party. If you ask most amateur astronomers, their idea of the prettiest double stars feature the widest color contrasts - say a hot bluish star matched with a cooler orange star. Having such a pair close together in the eyepiece illustrates the variations between stars very attractively indeed. But when an observer gets up to the eyepiece to view such a combination, having clearly set spectral classifications, what will they see? Well, that depends. This is where the "human equation" comes into play. For better and worse, how our senses perceive the world differs from one person to another. Many folks will see a yellow-pink star when others see it as red because of optical quirks from one eyeball to another. A common form of inconsistent color sensitivity is the Purkinje Effect, where the eye sensitivity shifts as brightness changes. In addition, our eyes change as we get older. Showing in a number of ways, like losing dark adaptation, aging also manifests itself in altering the basic tint that the eye views. Many older people have their eyes viewing through a progressively "warmer", yellowing tones. And of course, we articulate what we're viewing differently, too.

The result of these individual differences between observers is that they seldom concur on one particular color combination when viewing double stars. They're within the same general area of the spectrum, mind you. But there is seldom an exact agreement. But just don't take my word for it. A description for the double star Cor Caroli out of Burnham's Celestial Handbook demonstrates this very clearly. In the words of the experienced astronomers who supplied the observations for this particular object, the coloring of the pair ranged from "flushed white & pale lilac", "pale yellow and fawn", "white and pale olive blue", and "white and pale copper". Confusing? Perhaps, yet it emphasizes how amateur astronomy is a human endeavor, rather the domain of machines.

One other interesting fact about stars - there are no green ones!