If Brown Isn’t a Color, What Color are Brown Dwarfs?

by Fraser Cain on January 6, 2009

Artist's impression of a brown dwarf. Image credit: NASA/JPL

Artist's impression of a brown dwarf. Image credit: NASA/JPL

We’ve talked about brown dwarfs here on Universe Today for years and years. These are the “failed stars”; objects with too little mass to fully ignite nuclear fusion in their cores. Instead of blazing with red, yellow or the white light of our own stars, they’re heated by the gravitational collapse of material. They’re called brown dwarfs, but you might be surprised to know that they aren’t actually brown. In fact, it’s impossible to have brown light. So what color are they?

The term “brown dwarf” was originally coined by Jill Tarter in 1975 to describe these objects, and there were other suggestions for names, like planetar and substar. But the name “brown dwarf” stuck. And here’s the problem, as described by Jill Tarter, “it was obvious that we needed a color to describe these dwarfs that was between red and black. I proposed brown and Joe (Silk) objected that brown was not a color.”

Brown isn’t a color?!

Not for astronomers. When they consider the color of a star, astronomers are talking about the wavelength of the light being emitted. Stars emit light at various wavelengths, and whatever photons are mostly being emitted are what we see. Yellow stars emit primarily yellow photons, red stars emit mostly red photons, etc. But you can’t have a star emit brown photons because the “color” brown is a de-saturated yellow. Brown dwarfs can’t be brown because it’s impossible to emit brown light. So what color are they?

Dr. Kenneth Brecher is a professor at Boston University and the primary investigator for Project LITE. This is a research project that uses a variety of experiments to understand how people see color. I highly recommend you check out the Project LITE website and take a look at the Flash experiments they have available. You’ve probably seen some of these optical illusions in the past, where spinning wheels of black-and-white can actually create different colors in our brains. Brecher demonstrated one of these color wheels for me – it’s a CD that can spin like a top. At rest, you see black-and-white, and then spin up the disk and you can see red, green and blue. Very cool stuff (totally unconnected from the color of brown dwarfs).

The color of a brown dwarf

The color of a brown dwarf

Brecher did a presentation at the American Astronomical Society Meeting about the actual color of brown dwarfs. He even had a flashlight that shines a light the color of brown dwarfs. Unfortunately, I didn’t catch a photo of it, but check out Nature’s blog, they got one. It’s sort of a dull orange color. But here’s the cool part. There’s no way to actually see the color of a brown dwarf unless you’re having the photons strike your eyeballs.

All you color theory folks might want to know the hexidecimal code: EB4B25. And here are the RGB values: R-235, G-75, B-37

So what color would an isolated brown dwarf look like? Dr. Brecher had a slide in his presentation that shows the color – we’ve extracted it and made it bigger. I think it looks kind of reddish orange, but then color is in the eye of the beholder.


Fraser Cain is the publisher of Universe Today. He's also the co-host of Astronomy Cast with Dr. Pamela Gay.

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