Why Are There No Green Stars?

Every night before bed, I take my dogs outside to do their dog business, letting them run around the yard on the east side of my house. I live under very dark skies in rural Virginia, and even though I’m a tenured astronomer, it’s still a shock to look up and see so many stars.

In winter, my eyes are usually drawn to the brighter stars such as Betelgeuse and Rigel in Orion and Aldebaran in neighboring Taurus. They’re not just brilliant; they are also colorful, standing out from the standard white appearance of most other stars in the sky. Betelgeuse is orange-red, Aldebaran is orange, and Rigel is sapphire blue.

As I wrote in a previous The Universe In the column “Star Colors, Explained,” only the brightest stars emit enough light to activate our eye cones, the color-sensitive cells in the retina. But even then, the brightest stars still appear blue, red, yellow, or sometimes white. In telescopic images, even the faintest stars show color. And while astronomical photographs can be a little difficult to interpret, the colors of the stars they contain are usually quite representative.

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What you won’t see, however, is Green those.

The reason for this is both a defect in the stars and a defect in ourselves: we don’t see them green because of the way the stars emit light and the way our eyes perceive colors.

Stars emit light because they are hot. They radiate this heat as light and, in fact, their apparent color depends on their temperature. Relatively cool stars emit red light, while much hotter stars glow blue. But it’s a little more complicated than that because, in reality, they emit light in a wide range of colors in different proportions. Cool stars emit almost entirely red light, but hot stars emit blue. And red light; they simply emit much more blue than red. It is this mixture of colors that gives its hue to a star.

The sun does this too, shining in colors literally spanning the rainbow (and beyond), but not uniformly. A plot of solar brightness versus color shows a shape more like an unbalanced bell curve, with a long tail beyond red wavelengths. Surprisingly, the top of this graph is in the blue-green part of the spectrum!

So why isn’t the sun teal? This is where our eyes come in. We have three types of cones in our eyes, each adapted to red, green, or blue light. So, if an object emits or reflects red light, the red cones send a strong signal to the brain while the other two types do not, and we perceive that object as red.

And yet, of course, we can see more colors than three! Indeed, for example, a yellow object will trigger the three types of cones, but at different levels. These signals are then mixed by the brain and we see, in this example, yellow.

The sun emits most light in blue and green. However, it also emits purple, yellow, orange and red. When our brain has finished combining and interpreting all the color signals from our eyes, we perceive the sun as white (not yellow, as many people think, although it is classified as a yellow dwarf star).

This is why we don’t see any green stars. As stars get hotter, we see their color change from red to orange. And as their temperature rises further, our eyes begin to interpret them as bluish or white. There is no temperature at which our cones combine their signals to produce green and since, in most cases, cameras are designed to mimic our eyes, stars do not appear green in photographs either.

Kermit was right. It’s not easy being green.

I will note that there are a few stars that some say appear green. Almach is a moderately bright star system in the constellation Andromeda. It consists of an orange giant and a trinary system of three blue stars (which are so close together that they are unresolved from our perspective; they merge into a single point of light through a telescope). Some observers have claimed to have seen the trinary as green. I suspect this is an illusion, due to the fact that our brains sometimes rely on contextual comparisons to interpret color. This can change the apparent color of an object quite strikingly. Comparing the bluish light of the trinary with the brighter orange light of the other star, it is possible that the trinary appears green. I will add that in photographs, it always appears blue, which supports the idea that green is illusory.

Another star, Zubeneschamali, in the constellation Libra, also appears green to some observers. It’s definitely a blue star and has no stellar companion, so it’s unclear why some people see it that way. It always looked blue to me through a telescope.

And all this does not mean that there are no green celestial objects! Many nebulae (gas clouds) have a distinct greenish hue. In these rare circumstances, the oxygen atoms emit very strongly in the green part of the spectrum, overwhelming the other weaker colors, making the nebula appear green, even in photographs.

Comets can also sometimes appear emerald green! In this case, the culprit is diatomic carbon, a molecule made up of two carbon atoms. Sunlight energizes the molecule, causing it to vibrate, then releases this energy in the form of green photons. The molecule is not very stable, however, and is broken down by sunlight after a day or two. This is why a comet’s main head may appear green, but the tail, made of material torn from the head, tends to be a different color.

Planets can also get in on the action. Much of the Earth is green for obvious reasons. However, Uranus can appear very slightly green (more blue than green, but there is still a hint of the latter) because of the methane present in its atmosphere. Methane absorbs red light and reflects blue and green, giving the planet a pale sea-green hue.

If you find yourself outside on a clear, cool night, take a closer look at the stars overhead if you can. I always done, and this wave of wonder that I feel each time does not only come from their beauty; this is enhanced by our understanding of how physics and physiology combine to create this beauty.