Copyright © Karl Dahlke, 2023
When a substance gets hot enough, it glows. This was the idea of the light bulb - run electricity through a tungsten filament so that it gets hot and puts out a bright white light.
Rewind 1,000 years and consider a camp fire. The flames are superheated gases, and they emit infrared radiation, also known as heat, along with red, orange, and yellow light. Place an iron rod in the flames and leave it there for 20 minutes. It won't get as hot as the flames, but it might become red hot. Pull it out and carry it away from the fire, and view it on a dark night. It glows red. Eventually it cools, and the red glow fades.
Remember that red light has the lowest energy of all the colors of the rainbow. As the iron rod heats up, the first color it will produce is red. Faint red, just visible on a dark night, occurs at 460 °C, or 860 °F. Here is a table of shades of red, and the temperatures needed to produce them.
| Color | degrees C | degrees F |
|---|---|---|
| Faint Red | 500 | 930 |
| Blood Red | 580 | 1075 |
| Dark Cherry | 635 | 1175 |
| Medium Cherry | 690 | 1275 |
| Cherry | 745 | 1375 |
| Bright Cherry | 790 | 1450 |
Take a cherry red hot iron rod into a pitch dark room and run its light through a prism, as described in the previous chapter. You will see a band of red, fading away into orange. The light of any glowing hot object is always a smear of colors. It is never a single line, like one note on the piano. After all, we can't expect every atom in the iron rod to be exactly the same temperature. There will be variations, and different energy levels of electrons and shells, producing a range of light.
Allow the iron rod to cool, until it is faint red, then look at its light through the spectrum again. There is just one thin line of red, but it is still emitting a range of radiation. The rest is infrared, the light that carries heat, the light that we can't see.
As an object gets hotter, its smear of energy grows wider. To illustrate, press down on the leftmost notes of a piano with three fingers, and pretend like you are hearing the notes between these notes as well. Move your hand to the right, corresponding to a higher temperature. As you approach middle c, you have to press your entire hand down on the keyboard, spanning almost an octave. Further to the right, you need a hand and three fingers, and at the right of the piano, you will use both hands to span a dozen notes. The spread of radiation is wider as the object gets hotter. The frequency increases, and the spread increases. This applies to iron rods, and to stars.
Let's apply this idea to stars. There are big stars, little stars, stars that burn fast and furious and die in a few million years, and stars that burn slow and steady for ten trillion years. The temperature of the star determines its color. Let's begin with stars that are just 3,000 °C. They have enough energy to radiate red light, and maybe a little orange. Such a star might be small, a red dwarf, or large enough to reach the orbit of Jupiter, a red giant, but in either case, the surface is red hot. You can find these red stars in the sky. They are in the minority but they're there. Betelgeuse is most notable, as the tenth brightest star in the night sky. It is located in the constellation Orion,
Turn up the heat, and the star generates almost half of the rainbow. To our eyes, it shines orange, or pale yellow.
When a star is the temperature of our sun, 6,000 °C, 10,000 °F, it covers the entire rainbow, and infrared below, and ultraviolet above. It is like pressing your entire hand on the piano, from C4 to C5. The star is white, because all the colors together make white. Sometimes our sun appears yellow during the day, or red at sunset, but these are atmospheric effects. People on the Space Staation know that our sun is pure white.
So why can't a star be green? If the star is hot enough to produce green, it also produces the colors around green. You can't just hit one note on the piano with one finger, you have to use your whole hand. Green brings in yellow below, and blue above. A star that is the perfect temperature to produce green, produces all the colors, and shines white.
Other colors are also impossible. A star can't be purple. Purple is red + blue, but if a star was the right temperature for red and blue, it would have to generate all the colors in between. It would be white.
Make the star hotter still, and it produces more blue and less red. We are climbing through and beyond the traditional rainbow. It shines blu-white. A very large and very hot star emits mostly ultraviolet radiation. Blue is at the bottom of its energy smear, thus it looks blue to us. Here are the possible colors, from coolest to hottest.
The vast majority of visible stars in the night sky are white, but all colors are there if you look for them. Blue is particularly rare, since those stars are short-lived. Such a star would have to be born yesterday, to be seen by us today. You can find blue stars in the Pleiades. You'll need at least a hand telescope to zoom in on them and see the color.
We have never found, in the night sky, an isolated color, or a single frequency radio wave. The natural processes that generate energy are noisy, and sloppy. They produce a wide range of signals. If we ever found a green star, or a single color, or a 165.5 megahertz radio wave in the sky, that would indicate extraterestrial intelligence and technology.