Why are there no green stars?

[n2doc]

Go outside on a dark, moonless night. Look up. Is it December or January? Check out Betegeuse, glowing dully red at Orion’s shoulder, and Rigel, a laser blue at his knee. A month later, yellow Capella rides high in Auriga.

Is it July? Find Vega, a sapphire in Lyra, or Antares, the orange-red heart of Scorpius.

In fact, any time of the year you can find colors in the sky. Most stars look white, but the brightest ones show color. Red, orange, yellow, blue… almost all the colors of the rainbow. But hey, wait a sec. Where are the green stars? Shouldn’t we see them?

Nope. It’s a very common question, but in fact we don’t see any green stars at all. Here’s why.

More:

http://blogs.discovermagazine.com/badastronomy/2008/07/29/why-are-there-no-green-stars/

Snip:

Look at the graph for an object as hot as the Sun. That curve peaks at blue-green, so it emits most of its photons there. But it still emits some that are bluer, and some that are redder. When we look at the Sun, we see all these colors blended together. Our eyes mix them up to produce one color: white. Yes, white. Some people say the Sun is yellow, but if it were really yellow to our eyes, then clouds would look yellow, and snow would too (all of it, not just some of it in your back yard where your dog hangs out).

OK, so the Sun doesn’t look green. But can we fiddle with the temperature to get a green star? Maybe one that’s slightly warmer or cooler than the Sun?

It turns out that no, you can’t. A warmer star will put out more blue, and a cooler one more red, but no matter what, our eyes just won’t see that as green.
The fault lies not in the stars (well, not entirely), but within ourselves.

[JohnnyRingo]

1. It's good to find another "Bad Astronomy" reader

I visit Phil Plait's site every day. It's a great mix of politix and cosmos.
Phil has a patented way of unfolding the secrets of the universe in a way laymen can comprehend, a trait formerly shared by Sagan. His informative videos can also be found at http://www.hulu.com/

Here's another really cool stopover for stargazers:
http://apod.nasa.gov/apod/astropix.html

"Astronomy Picture of the Day" features a new pic every day with an explanation written by a professional astronomer.

[renate]

2. I get the part about how our eyes don't perceive the green

But a picture of a red star or a blue star or a yellow star appears red or blue or yellow, of course. And we can look at a non-radiant green object (such as a picture of a green star) and see green. (We can also look at radiant green objects like neon signs or glowsticks just fine, but maybe the lower temperature accounts for that.) So I'm still not sure why even pictures of green stars don't exist.

[supernova]

3. Two reasons that I can think of

1) A really fun way to look at a star is to look at its light graph, the graph of the wavelengths of light the star emits. Do you remember High School or Jr High chemistry? The teacher would throw different elements on the bunsen burner and the flames would turn different colors? Copper is an element that burns blue/green. Going back to the light graph, you can see what elements the star is made of (mostly), by looking at what colors it is burning.

So, I suppose that if you wanted a really green star, you'd have to find one that was primarily made of copper, or any other green-burning element.

edit: Here's an online lesson that explains it better than I can:

http://www.learner.org/teacherslab/science/light/color/spectra/index.html

2) The Doppler effect. The Doppler effect is that any object moving away from you (and all us celestial bodies are moving away from each other), will shift to the red-end of the light spectrum. Just like sound coming towards you sounds higher and that same sound that is now past you sounds lower pitched. Light does the same thing. How far that red-shift is (now that you know certain elements produce certain colors, is how far away the star is.

[caraher]

4. The article linked in the OP explains it nicely

First, flame spectra are very different from stellar spectra, as they are dominated by a lot of light at the discrete frequencies that create those vivid colors. Those are determined by the energy levels of the element in the flame

Star color is determined primarily by temperature, not chemical composition - which in any case varies little from star to star - they're all mostly hydrogen and helium as they "burn," so if elemental composition were a dominant factor they'd all have the same color!

Second, I think the Doppler shift is small given the relative velocities of visible individual stars, which are all pretty much going to be in our galaxy. (I should check the numbers to verify this.) The correlation between red-shift and distance is something that operates more at intergalactic distance scales; from Doppler shifts alone you directly determine the component of relative velocity along the line of sight to the star, not distance. But when we look at an intergalactic scale we find that the relative velocities do depend on distance because of the (accelerating!) expansion of the universe. Fascinating stuff, but not why we perceive individual stars to have the colors we see - they are our near neighbors and don't move terribly fast relative to us.

The whole key is understanding the figure linked in the article:



Every star has a spectrum that follows a continuous curve like the three shown above, where the exact shape (and therefore distribution of light frequencies depends essentially on temperature alone. Now human visual perception kicks in. We evolved with the Sun as our "reference" light source, so any light source with the spectral distribution of the Sun will be seen as "white light." If our source is cooler, the red end of the spectrum outweighs the blue and and we perceive the star to be reddish; if hotter, it looks "bluer." Those perceptions do track the peak wavelengths of the spectra to some degree, but the peak alone does not determine the perception. It happens that green is the color of the peak for a source whose temperature matches that of the sun. Any source of light whose spectrum is dominated by temperature whose temperature is near that of the sun has a peak wavelength that, were it the only color present, would be perceived as green; but the distribution of colors overall is that of a "white light" source.

A photo of such a source will reflect white light to our eyes with approximately unchanged frequency distributions, so there's no reason images of stars should appear green when they appear white to the naked eye.

Color perception is a rather subtle field, one that I only know a little about. About 10 years ago I read an article about a group that had managed to create a laser beam that appeared brown. If you look at the wavelength you'd expect an orange beam, I think, but by tweaking the intensity of scattered light and background sources it could be made to appear brown. Eyes have nonlinear spectral responses, and there's a lot of processing between the initial photon detection and the brain perceiving an image, so it's easy to get it wrong when trying to predict what something will look like based on simple arguments.

Great question - one that had never occurred to me before!

[caraher]

5. An interesting article on color perception

Color perception is more about "software" than the light detection "hardware" in the eye... and the software is adaptable.

[Jim__]

6. I agree - interesting article.

I am curious as to what they mean when they say that "every one has the same color experience":

Each subject was asked to tune the color of a disk of light to produce a pure yellow light that was neither reddish yellow nor greenish yellow. Everyone selected nearly the same wavelength of yellow, showing an obvious consensus over what color they perceived yellow to be.

Once Williams looked into their eyes, however, he was surprised to see that the number of long- and middle-wavelength cones¿the cones that detect red, green, and yellow¿were sometimes profusely scattered throughout the retina, and sometimes barely evident. The discrepancy was more than a 40:1 ratio, yet all the volunteers were apparently seeing the same color yellow.

"Those early experiments showed that everyone we tested has the same color experience despite this really profound difference in the front-end of their visual system," says Hofer.

All the subjects identify this color experience with the word "yellow", but I don't believe we know that their experience is actually the same.

[Tesha]

7. Stars are basically "black-body" emitters.

Stars are basically black-body emitters. See the "black
body curve" chart below; the colors of stars (as perceived
by the human visual system) basically follow that line and
nowhere does that line enter a region we'd describe as
"green".



Tesha

[hunter]

8. That's a sweet chart.

Thanks.