For the cosmos, it’s all too easy being green.
Stars come in a wide variety of colors, but never green.
Stars can be red, orange, yellow, white, or blue: a spectacular but incomplete color palette.
Stars shine simply because they’re matter, heated up to a specific temperature.
They emit a broad spectrum of light, with light’s spectral peak determining what we see.
But where “green” peaks, we observe all the colors; hence they appear white.
Similarly, the hottest stars only appear bluish, as even intense violet light is joined by many other colors.
But all throughout the cosmos, we frequently observe green light.
The commonly green aurorae on Earth are an accessible example.
Comet comas frequently appear green, too.
Some dying stars — planetary nebulae — appear green as well.
There are even “green pea” galaxies out there.
These distant, brilliant, enormous gas clouds clearly shine an eerie green.
But they don’t arise the way typical star colors do.
Instead, superheated gas loses electrons, becoming ionized.
When those electrons recombine with those ions, they emit light at specific wavelengths.
To shine green, oxygen must become doubly ionized: requiring temperatures of 50,000 K or above.
With massive starbursts, nearby quasars, and cataclysmic events, “green” isn’t merely possible, but ubiquitous and mandatory.
Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more.
Starts With A Bang is written by Ethan Siegel, Ph.D., author of Beyond The Galaxy, and Treknology: The Science of Star Trek from Tricorders to Warp Drive.