Looking Up: Searching for green starlight
Most stars appear white to the eye, although a closer look will reveal hints of blue, yellow, orange and red. What about green?
Alas, astronomers say there are no green stars, but I still find it hard to believe.
Most stars appear white to the eye, but that is primarily because our retinas are not sensitive to color in low light conditions. In a backyard telescope, beautiful swathes of stellar clouds, such as the Great Nebula in Orion (M42, visible in the south in mid-March evenings just below the famous three “belt stars” of Orion), appear white. Long exposure color photographs bring out vivid colors, which can do a disservice to the stargazer if one expects the same thing at the telescope eyepiece.
Brighter colored stars do stand out in shades of blue, yellow, orange and red. Many others still appear white. With the unaided eye, you can appreciate the orange-red star Betelgeuse on the upper left corner of Orion, or the bright yellow star Capella high overhead. Brilliant Sirius gleams to the lower left of Orion and is distinctly bluish-white. Aim binoculars at the outer, top tip of the Big Dipper “bowl,” which is named Dubhe and is vivid orange.
Stars vary in color with their surface temperature and are classified by “spectral types” listing stars by letters. Cooler stars with spectral types K or M radiate most of their energy in the red end of the spectrum. Hotter stars, such as spectral types O and B, emit mostly at blue and ultraviolet wavelengths. Our sun is of spectral type G and is yellowish-white.
Spectroscopes reveal the stellar spectrum, and in the rainbow of light, there is always a component of green! Even if the star’s light is predominantly another shade, there is still green light! Unfortunately, our eyes do not perceive a star as green because those with the right temperature to mainly emit green light, emit starlight in a manner that is blended by our eye as white.
Spectrums of stars also reveal dark lines where elements making up the star - such as hydrogen or helium - absorb narrow frequencies of the light, which show up as thin dark lines in the color. Like a fingerprint, astronomers are able to learn the chemical makeup of stars and other celestial bodies this way.
Occasionally we can even see green sunlight. When the conditions are right at sunset or sunrise, a green spot of light is visible above the upper rim of the disk of the sun. Called the “green flash,” it normally is seen for no more than a second or two. On rare occasions, it has been seen like a green ray shooting up from the point of sunset or sunrise.
The green flash is caused by refraction, with the component colors of the sunlight split. Green flashes are more likely to be seen in stable, clean air, when more of the light of the low sun reaches the observer’s eyes without being scattered in the atmosphere. They usually are seen on a flat horizon such as over the ocean.
Now, where are the purple stars?
What about the planets? The Earth has lots of green. The planet Uranus has a bluish-green hue as a result of methane in its atmosphere. Venus, so bright in the evening sky (look southwest), is really white.
Last quarter moon occurred Jan. 17-18. After this, watch for the early morning crescent, waning towards new moon on Jan. 24. The moon, by the way, is also not green despite the cheese it’s supposed to be made of.
Keep looking up!
Peter Becker is managing editor at The News Eagle in Hawley, Pennsylvania. Notes are welcome at news@neagle.com. Please mention in what newspaper or website you read this column.