There Are No Green Stars
When astrophysicists peer into the universe, they see a nearly perfect rainbow spectrum of stars, from cooler red ones on one side to hotter blue ones on the other. In the center of the spectrum, they should see green stars, but they don’t due to the nature of stars and the limitations of the human eye.
Let There Be Light
A star begins its life as a cloud of hydrogen gas. Due to gravity, the cloud condenses. If the cloud is big enough, it will have enough gravity to create the temperature and pressure needed in its core to fuse hydrogen together (along with the help of quantum tunneling). The original hydrogen atoms have slightly more mass than the helium they fuse into, and this missing mass is converted into energy, as shown by Einstein’s E=mc2. Small amounts of mass (m) are converted into incredible amounts of energy (E) when multiplied by the speed of light squared (c2), about 9 x 1016 m2/s2. This is what generates the majority of light from stars, although bigger stars can fuse helium into heavier elements and these heavier elements into even heavier ones, all of which releases energy, until it tries to fuse iron, causing a supernova event.
But what is this light? Light is simply perpendicular waves in the electrical and magnetic fields. When one goes up and down, the other goes left and right. Together they are called an electromagnetic wave, or EM wave. EM waves include the X-rays fired at you by your doctor, the microwaves heating your left overs, the radio waves bringing you your favorite tunes, the light detected by your eye, etc. It’s all the same stuff, the only differences being its wavelength, which is the distance between each crest or trough, and its frequency, which is the rate at which it oscillates.
This is the EM spectrum:
The EM Spectrum and Black-body Radiation
The wavelength and frequency of light coming out of a star can be modeled by a black-body. Black-bodies are theoretical entities that emit electromagnetic waves, the amount of which is determined by their temperature alone, without any other complicating factors. While black-bodies are theoretical, the data they produce matches real world data fairly closely.
Below is a black-body plot, which demonstrates the relationship between wavelength/frequency and temperature. As can be seen, the higher the temperature, the shorter the wavelength and therefore the higher the frequency. For example, a 3000K object generates light mainly in the infrared part of the EM spectrum. An object like this would only be faintly visible to us, as the graph barely overlaps with the visible spectrum. The majority of light from a 4000K object is just outside of what we can see, but it produces enough within the visible spectrum to make it likely visible. A 5000K object peaks in the orange part of the visible spectrum, while a 6000K object peaks within the yellow. To us, these objects would appear orange and yellow, as the majority of light is orange and yellow.
If you follow the pattern, the peak goes up and to the left as temperature increases. This means that the hotter an object gets, the more its peak moves from right to left along the EM spectrum in the chart above. Extremely energetic objects like the accretion disk of a black hole or a neutron star can peak in the X-rays or even gamma rays.
The Universe Staring Back at Itself
“We are stardust brought to life, then empowered by the universe to figure itself out—and we have only just begun.” –Neil DeGrasse Tyson
When it comes to stars, the light we see matches closely with an ideal black-body. Blue stars appear blue because they are hotter and emit most of their light in the blue part of the visible spectrum. And red stars are red because they are cooler and emit a majority of red light.
This is demonstrated in the Hertzsprung–Russell diagram, or HR diagram. While this is not as simple to read as the black-body plot above, it still demonstrates the connection between surface temperature and color.
There’s a glaring omission though: there are no green stars. Even our Sun is in the lighter part of the yellow spectrum, pushing into white, where green should be. What’s going on here? The answer has to do with how our eyes processes light from the stars.
The human eye has evolved to detect three colors and their many combinations. More specifically, we only have three types of cones in our eyes, each of which detect EM waves associated with red, green, or blue. Colors in between are perceived due to more than one cone being activated at the same time, as explained by the chart below. Here, we can see each cone is dedicated to either small wavelengths, medium wavelengths, or long wavelengths, yet they overlap substantially, especially the medium wavelengths/green cone and the long wavelengths/red cones.
So, let’s stitch all of the above together. Imagine a star with its black-body curve peaking in the middle of the green band in the chart above. Yes, the majority of its light is green, and the green cone would detect this. However, if you look at the shape of a black-body curve, you’ll see that it would spread out and down across the chart as well. That is, a black-body curve that peaks in the green, would also encompass large amounts of both red and green light, thus triggering all three cones, which we perceive as white light.
In other words, a star that peaks in the blue or the red will appear as such because its black-body curve doesn’t overlap nearly as much with the rest of the visible spectrum. Blue stars don’t trigger the red cone, and red stars don’t trigger the blue cone. Green stars, though, triggers all three.
So, when you think about it, it’s strange that we don’t actually see the Sun in its true green because our eyes didn’t evolve the ability to do so.
