\n<\/p>\n
Every night before bedtime, I take my dogs outside to do their doggy business, letting them run around in the yard on the eastern side of my house. I live under very dark skies in rural Virginia, and even though I\u2019m a card-carrying astronomer, it\u2019s always a shock to look up and see so many stars.<\/p>\n
In the winter my eyes are usually drawn to the brightest stars such as Betelgeuse and Rigel in Orion and Aldebaran in nearby Taurus. They aren\u2019t just bright; they\u2019re also colorful, standing out against the standard white appearance of most other stars in the sky. Betelgeuse is red-orange, Aldebaran is orange, and Rigel is sapphire blue.<\/p>\n
As I wrote in a previous The Universe column, \u201cThe Colors of Stars, Explained,\u201d only the brightest stars give off enough light to activate our eyes\u2019 cones, the color-sensitive cells in the retinas. But even then, the brightest stars always appear blue, red, yellow or sometimes white. In telescopic images, even fainter stars show colors. And although astronomical photographs can be a bit tricky to interpret, the star colors in them are usually pretty representative.<\/p>\n
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What you won\u2019t see, though, are green ones.<\/p>\n
The reason for this is both a fault in the stars and in ourselves: we don\u2019t see them as green because of the way stars emit light and the way our eyes see colors.<\/p>\n
Stars emit light because they\u2019re hot. They radiate that heat away as light, and in fact, their apparent color depends on their temperature. Relatively cool stars emit red light, while much hotter stars glow in blue. But it\u2019s a little more complicated than that because, in reality, they emit light across a broad range of colors in different proportions. Cool stars emit almost entirely red light, but hot stars emit blue and red light; they just emit much more blue than red. It\u2019s that mix of colors that gives a star its hue.<\/p>\n
The sun does this, too, shining in colors literally spanning the rainbow (and beyond) but not in an even distribution. A graph of solar brightness versus color shows a shape more like a lopsided bell curve, with a long tail beyond the red wavelengths. Surprisingly, the peak of that graph is in the blue-green part of the spectrum!<\/p>\n
So why doesn\u2019t the sun look teal? That\u2019s where our eyes come in. We have three kinds of cones in our eyes, each attuned to either red, green or blue light. So if an object emits or reflects red light, red cones send a strong signal to the brain while the other two kinds don\u2019t, and we perceive that object as red.<\/p>\n
Yet of course, we can see more colors than just three! That\u2019s because, say, a yellow object will trigger all three kinds of cones but by different amounts. Those signals are then mixed by the brain, and we see, in this example, yellow.<\/p>\n
The sun emits the most light in blue and green. It also emits violet, yellow, orange and red, however. When our brain is done combining and interpreting all those colors\u2019 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).<\/p>\n
This is why we don\u2019t see any green stars. As stars get hotter, we see their color shift from red to orange. And as their temperatures rise even higher, our eyes start to interpret them as bluish or white. There\u2019s no temperature where our cones combine their signals to produce green\u2014and because, in most cases, cameras are designed to mimic our eyes, stars don\u2019t look green in photographs, either.<\/p>\n
Kermit was right. It\u2019s not easy being green.<\/p>\n
I\u2019ll note that there are a couple of stars that some people say look green. Almach is a middling-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 blend into a single point of light through a telescope). Some observers have claimed that they have seen the trinary as green. I suspect this is an illusion, arising because our brain sometimes relies on contextual comparisons to interpret color. This can change the apparent color of an object quite strikingly. By comparing the bluish light from the trinary with the brighter orange light of the other star, it\u2019s possible that the trinary could look green. I\u2019ll add that in photographs, it always looks blue, contributing support to the idea that the green is illusory.<\/p>\n
Another star, Zubeneschamali in the Libra constellation, also apparently looks green to some observers. It\u2019s definitely a blue star and has no stellar companion, so it\u2019s unclear why some people see it this way. It\u2019s always looked blue to me through a telescope.<\/p>\n
And all this is not to say that there are no green celestial objects! Many nebulas\u2014gas clouds\u2014show a distinct greenish hue. In those rarefied circumstances, atoms of oxygen emit very strongly in the green part of the spectrum, overwhelming the other, fainter colors, making the nebula appear green, even in photographs.<\/p>\n
Comets can also sometimes appear emerald green! In this case, the culprit is diatomic carbon, a molecule made of two carbon atoms. Sunlight energizes the molecule, causing it to vibrate, and it then releases that energy in the form of green photons. The molecule isn\u2019t very stable, though, and gets broken down by sunlight after a day or two. This is why the main head of a comet can look green, but the tail, made of material blown off the head, tends to be a different color.<\/p>\n
Planets can get in on the action, too. Much of Earth is green for obvious reasons. Yet Uranus can appear to be very slightly green (more blue than green, but there\u2019s still a hint of the latter) because of methane in its atmosphere. The methane absorbs red light and reflects blue and green, lending the planet a pale sea-green hue.<\/p>\n
If you do happen to find yourself outside on a clear, crisp night, please do take a closer look at the stars above your head if you\u2019re able. I always do, and that wave of wonder I feel every time comes not just from their beauty; it\u2019s heightened by our understanding of how both physics and physiology combine to create that beauty.<\/p>\n","protected":false},"excerpt":{"rendered":"
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