As far as I know, Kermit the Frog hasn’t been peering through a telescope, and he isn’t singing about the heavens. But it sure fits: why is green, such a prominent color on this planet, so rare in the night sky? And why are there no green stars?

Light we call the color green is an electromagnetic wave with crests around 520 nanometers apart, a mere 20 millionths of an inch. Visible light has wavelengths from about 400 to 700 nm, which we see as colors from indigo through to red, respectively, in the familiar sequence of the spectrum. While light can be characterized precisely by wavelength, frequency or energy, the color we perceive, especially when wavelengths are mixed is a complicated result of microscopic cells and pigment molecules in our eyes. Although physics and physiology can be exacting, color is a subjective experience and very difficult to explain in precise terms.

Some stars appear green: Zuben Elschamali (“northern claw”), Beta in the constellation of Libra, The Scales, has been reported to be green. Alpha Herculis is a beautiful double star, an orange-red 3rd magnitude star paired with a green 6th magnitude secondary. Antares is a double with similar in colors, but much more difficult to split because of a smaller separation and the 6-magnitude difference in brightness. Shades of green, pastels and evocative colors, appear in the descriptions of many double stars. The fact that descriptions differ among observers certainly suggests that some subjectivity and subtleness are involved.

Stars emit green light, after all, that’s why leaves appear green and why rainbows have green in their middle. The amount of each color emitted by a star depends on its temperature according to the well-known Plank or blackbody relationship, and at any given temperature, there is a wavelength where emission peaks, mathematically described by Wien’s displacement law. Hot stars emit a lot more blue light, cooler stars more red, and their visual appearance reflects this. Mid-temperature stars, for example like the Sun, emit throughout the visible spectrum, but even at a temperature peaking in the green, that color does not dominate over the others. It is a contrast effect: stars emit green light, but not enough to appear green, only white-ish.

Color is critical to human existence and the capture and display of color is a major thrust of vision science. When all that engineering is brought to bear on star colors as determined by black-body formulae, there are demonstrably blue and red and even more pink than orange colored stars, but no green. A bit disappointing, isn’t it?

The best "green" I’ve ever seen in the sky was the striking aqua of Comet Hyakutake. Fluorescence at narrow blue and green wavelengths, combined with the response of our eye’s cones cells produces a genuine green or blue-green perception. See this for yourself in the brightest regions of M42, the Orion Nebula, or even M57, the Ring Nebula, which always appears to me as a blue-green donut. Perhaps the rarest green occurrence in the sky is that of green flash at sunset over the Pacific—but that’s another story entirely…

What if Kermit is “physically-correct”? Any way you look at it, green is a rarity in the palette of the night sky. And while the detection of color is certainly a challenge at nighttime, the sky is much more beautiful and heck of a lot more interesting for those subtle shades we must strain to see.

Moondark is written by Doug Miller, published at the Moondark web site, and printed in the Delmarva Star Gazers' Star Gazer News. This document was last revised on 24 September  2006. Text and images copyright © 2006 by Douglas C. Miller, All Rights Reserved. This material may not be reproduced in any form without prior permission.