Why are there no green stars?

The entire universe in blog form
July 29 2008 8:00 AM

Why are there no green stars?

Go outside on a dark, moonless night. Look up. Is it December or January? Check out Betelgeuse, 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.

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  

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There are no green stars!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.

Take a blowtorch (figuratively!) and heat up an iron bar. After a moment it will glow red, then orange, then bluish-white. Then it'll melt. Better use a pot holder.

Why does it glow? Any matter above the temperature of absolute zero (about -273 Celsius) will emit light. The amount of light it gives off, and more importantly the wavelength of that light, depends on the temperature. The warmer the object, the shorter the wavelength.

Cold objects emit radio waves. Extremely hot objects emit ultraviolet light, or X-rays. At a very narrow of temperatures, hot objects will emit visible light (wavelengths from roughly 300 nanometers to about 700 nm).

Mind you -- and this is critical in a minute -- the objects don't emit a single wavelength of light. Instead, they emit photons in a range of wavelengths. If you were to use some sort of detector that is sensitive to the wavelengths of light emitted by an object, and then plotted the number of them versus wavelength, you get a lopsided plot called a blackbody curve (the reason behind that name isn't important here, but you can look it up if you care -- just set your SafeSearch Filtering to "on". Trust me here). It's a bit like a bell curve, but it cuts off sharply at shorter wavelengths, and tails off at longer ones.

Here's an example of several curves, corresponding to various temperatures of objects (taken from online lecture notes at UW:

Blackbody curves

The x-axis is wavelength (color, if you like) color, and the spectrum of visible colors is superposed for reference. You can see the characteristic shape of the blackbody curve. As the object gets hotter, the peak shifts to the left, to shorter wavelengths.

An object that is at 4500 Kelvins (about 4200 Celsius or 7600 F) peaks in the orange part of the spectrum. Warm it up to 6000 Kelvin (about the temperature of the Sun, 5700 C or 10,000 F) and it peaks in the blue-green. Heat it up more, and the peaks moves into the blue, or even toward shorter wavelengths. In fact, the hottest stars put out most of their light in the ultraviolet, at shorter wavelengths than we can see with our eyes.

Now wait a sec (again)... if the Sun peaks in the blue-green, why doesn't it look blue-green?

Ah, this is the key question! It's because it might peak in the blue-green, but it still emits light at other colors.

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.

Our eyes have light-sensitive cells in them called rods and cones. Rods are basically the brightness detectors, and are blind to color. Cones see color, and there are three kinds: ones sensitive to red, others to blue, and the third to green. When light hits them, each gets triggered by a different amount; red light (say, from a strawberry) really gets the red cones juiced, but the blue and green cones are rather blasé about it.

Most objects don't emit (or reflect) one color, so the cones are triggered by varying amounts. An orange, for example, gets the red cones going about twice as much as the green ones, but leaves the blue ones alone. When the brain receives the signal from the three cones, it says "This must be an object that is orange." If the green cones are seeing just as much light as the red, with the blue ones not seeing anything, we interpret that as yellow. And so on. So the only way to see a star as being green is for it to be only emitting green light. But as you can see from the graph above, that's pretty much impossible. Any star emitting mostly green will be putting out lots of red and blue as well, making the star look white. Changing the star's temperature will make it look orange, or yellow, or red, or blue, but you just can't get green. Our eyes simply won't see it that way.

That's why there are no green stars. The colors emitted by stars together with how our eyes see those colors pretty much guarantees it.

But that doesn't bug me. If you've ever put your eye to a telescope and seen gleaming Vega or ruddy Antares or the deeply orange Arcturus, you won't mind much either. Stars don't come in all colors, but they come in enough colors, and they're fantastically beautiful because of it.

Note: this is not the end of the story. There are green objects in space, and some stars do appear green... but that's for another post, coming soon. Promise.