Racial profiling has been a hot topic in our country for years now. Trying to figure out something about someone based on color is a matter for heated debate in the public forum. But in astronomy we judge based on color all the time. We have to. And here it’s pretty reliable.

There is no way to travel to a star and stick a thermometer in it to see how hot it is. We can’t land on these balls of gas and perform experiments. About the best we can do to find out how a star works is to try to replicate in our laboratories what we see out there, then infer things about that far distant star based on our findings. 

One of the characteristics of a star is its color. “Color?!” you may exclaim, “I thought all stars were just twinkling white things.” Not at all.

Stars come in all sorts of colors: red, orange, yellow, white, blue. But why? They are colored that way because of their temperatures. You have seen these colors here on Earth when different objects - often metals - glow when they are heated.

You may have seen an iron poker stuck into a fire. At first it gives off no visible light. But if one were to leave the poker in a hot fire for a longer time, you would notice it starts to glow red, then orange, then yellow. If you could heat up the metal in a very, very hot fire it would become “white hot.” Notice that these are all the same colors as stars. 

What you may not know is that if you could heat a substance up even hotter than that, it would begin to give off a bluish hue. 

Actually, these objects are giving off all the colors of the visible spectrum at once, it is just that they are giving off “mostly” red or orange or yellow, etc., so we see that predominant color as the overall “color” of that object.

Same with stars! The cooler stars are bleeding out mostly red, hotter stars mostly orange. The hottest, most dangerous stars pour out a lot of blue, hence their bluish hue. 

The colors of stars are giving us temperature clues, which in turn tell astronomers what is going on inside that hot gas ball trillions of miles away: how they burn, how fast they live, what they are made of. Stellar profiling based on color is an absolutely vital tool in the realm of astronomy.