# Green Stars?

Most recent answer: 10/22/2007

Q:
Why are there no green stars? We have red, yellow, and blue. Why does black body radiation skip green?
- Wayne
El Cerrito, CA, US
A:
Your eye and brain decide what color light is by comparing amounts of light energy in different frequency ranges. Green corresponds to light concentrated in the middle of the visible frequency range. The way black-body radiation works is that it includes waves at all frequencies up to a temperature-dependent limit, with the content of the higher frequencies gradually falling off. There’s an exact formula for how the distribution of frequencies depends on temperature. There’s no temperature for which thermal radiation includes the middle part of the visible spectrum but not also the low (reddish) end. So those temperatures don’t give green but the color you get when you combine green and red light, which comes out yellowish.

Mike W.

There is a very small percentage of "blue" stars.   These are typically very young, very massive, and very hot, ten times the temperature of the sun.  (Although they give off light throughout the visible range, that thermal radiation formula say that the blue part somewhat outweighs the rest.) They usually end up as supernovae. For more information on stellar color/temperature see the wikipedia article at

LeeH

(published on 10/22/2007)

## Follow-Up #1: thermal colors

Q:
So if a blue star cools a bit and the main color output shifts towards green - at the same time there’s suddenly a split output with a second peak lower in the red area, so we interpret the two as yellow? Then if we had to rely on BB radiation and our eyes only, we would know the other colors in proper order, but never suspect that green light is there in the spectrum between yellow and blue? Thanks again
- wayne
El Cerrito, CA, US
A:
There is no split in the output into components. There’s a smooth distribution of frequencies, building up to a single peak and then falling down, but the frequency of the peak depends on temperature. There is one sort of ’split’ though, because our eyes have three separate types of color receptors. We can see what the ratios are between the firing rates of those receptors, but we can’t directly see anything further about the distribution of frequencies. So something might look green because it consisted of a single frequency of green light, triggering those receptors to different degrees,  or because it was a mixture of several different frequencies that also happened together to trigger the three receptors to the same degrees.
So it just happens that there’s no temperature for which the black-body spectrum concentrates enough of the power in the middle frequency range, away from the red and blue extremes, to be seen as green. Setting the temperature so the peak is in the green still has enough outside the green range to be perceived as yellow.
So if we really had our eyes only, we’d not directly realize that there was a green component. Of course with our eyes and a prism, we can see the green component along with all the others.

Mike W.

(published on 10/22/2007)

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