A color you’ll never find in a star is responsible for the universal color of star-forming regions.
If you look through a telescope’s eyepiece, distant galaxies always appears white.
But with advanced cameras that pick up individual photons, some regions show a different color: pink.
In our own galaxy, it’s the overwhelming color of star-forming regions like the Orion Nebula.
In some galaxies, the pink color can dominate a telescope’s entire field-of-view.
This isn’t some optical illusion or a false-color image; these regions and galaxies truly appear pink.
At first glance, it’s surprising, since there are no pink stars, and the majority of young starlight is preferentially blue.
But once you realize that it isn’t just stars, but gas, that can make light, the mystery solves itself.
New star-forming regions produce lots of ultraviolet light, which ionizes atoms by kicking electrons off of their nuclei.
These electrons then find other nuclei, creating neutral atoms again, eventually cascading down through its energy levels.
Hydrogen is the most common element in the Universe, and the strongest visible light-emitting transition is at 656.3 nanometers.
The combination of this red emission line — known as the Balmer alpha (or Hα) line — with white starlight adds up to pink.
Red and white make pink, explaining the color of star-forming regions.
Mostly Mute Monday tells the astronomical story of an image, object, or phenomenon in visuals, pictures, and no more than 200 words. Talk less, smile more.Ethan Siegel is the author of Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.