Every night before I go to sleep, I take my dogs outside and let them run around the yard on the east side of my house. I live in rural Virginia under very dark skies, and even though I’m a card-carrying astronomer, it’s always a shock to look up and see so many stars.
In winter my eyes usually go to the brightest stars, such as Betelgeuse and Rigel in Orion and Aldebaran in nearby Taurus. They’re not just bright; They are also colorful, and stand in contrast to the standard white appearance of most other stars in the sky. Betelgeuse is red-orange, Aldebaran is orange, and Rigel is sapphire blue.
As I wrote in the previous world Column, “Star Colors, Explained”, Only the brightest stars release enough light to activate the color-sensitive cells in the retina, the cones of our eyes. But still, the brightest stars always appear blue, red, yellow or sometimes white. Light stars also show color in photographs taken through a telescope. And although astronomical photographs can be a little difficult to interpret, the colors of the stars in them are usually quite representative.
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What you won’t see, however, are Green The ones.
The reason is a flaw in both the stars and us: Because of the way stars emit light and the way our eyes see colors, we don’t see them as green.
Stars emit light because they are hot. They radiate that heat as light, and in fact, their apparent color depends on their temperature. Relatively cool stars emit red light, while hotter stars glow blue. But it’s a little more complicated than that because, in fact, 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 emit far more blue than red. It is the mixture of colors that gives a star its color.
The sun does the same, literally shining in colors spanning the length of the rainbow (and beyond), but not in uniform distribution. The graph of solar luminosity versus color shows a shape resembling an unbalanced bell curveWith a long tail beyond red wavelengths. Surprisingly, the peak of that graph is in the blue-green part of the spectrum!
So why doesn’t the sun look teal? This is where our eyes come in. Our eyes have three types of cones, each corresponding 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 see that object as red.
Yet, we can see more than just three colors! This is because, say, a yellow object will trigger all three types of cones, but in different amounts. Those signals are then mixed by the brain, and we see, in this example, yellow.
the sun emits most of Blue and green light. However, it also emits violet, yellow, orange and red colors. When our brain combines and interprets all those color signals from our eyes, we see the Sun as white (not yellow, as many people think, although it is classified as a yellow dwarf star).
This is the reason why we do not see any green stars. As stars become hotter, we see their color change from red to orange. And as their temperature rises even higher, our eyes begin to perceive them as blue or white. There is no temperature where our cones combine their signals to produce green – and because, in most cases, cameras are designed to mimic our eyes, stars don’t even look green in photographs.
Kermit was right. Being green is not easy.
I will note that there are some stars that some people say look green. Almach is a medium-bright star system in the constellation Andromeda. It consists of a triple system of an orange giant and three blue stars (which are so close together that they are unresolved from our viewpoint; they blend into a single point of light through a telescope). Some observers have claimed to have seen Trinetra as green. I suspect this is an illusion that arises because our brain sometimes becomes dependent on relevant comparison Interpreting color. It can change the apparent color of an object quite dramatically. By comparing the blue light of Trinetra with the bright orange light of another star, it is possible that Trinetra may appear green. I would also add that it always looks blue in photographs, which supports the idea that the green color is deceptive.
another star, zubeneschamali Even in the Libra constellation, some observers clearly see green. It is definitely a blue star and has no stellar companion, so it is unclear why some people see it that way. Through binoculars it always appears blue to me.
And all this does not mean that there are no green celestial bodies! Many nebulae—gas clouds—show a distinctive green color. In those rare circumstances, oxygen atoms emit very intensely in the green part of the spectrum, overpowering other lighter colors, causing the nebula to appear green even in photographs.
Comets can sometimes even appear emerald green! In this case, the culprit is diatomic carbon, which is a molecule made of two carbon atoms. Sunlight energizes the molecule, causing it to vibrate, and then it releases that energy as green photons. However, the molecule is not very stable, and is broken down by sunlight after a day or two. This is why the main head of the comet may appear green, but the tail, which is made up of material blown from the head, is a different color.
Planets may also get in on the action. Most of the Earth is green for obvious reasons. As yet Uranus may appear very light green (More blue than green, but still has a hint of green) due to the methane in its atmosphere. Methane absorbs red light and reflects blue and green, giving the planet a pale sea green color.
If you find yourself outside on a clear, beautiful night, please take a close look at the stars above your head if you are able. I Always They do, and that wave of wonder I feel every time doesn’t just come from their beauty; This is enhanced by our understanding of how physics and physiology work together to create that beauty.
