Thanks to Brilliant for helping support this episode. Hey Crazies. There are a lot of stars in the universe. Somewhere around 10-to-the-24. That’s a 1 followed by 24 zeros. I’m not even going to try to makes sense of that. It’s big. And those stars come in all the basic colors except green and purple. This episode was made possible by generous supporters on Patreon. See. A star’s color comes from one of its outermost layers. A layer called the chromosphere, or color sphere. Because, you know, science names. The light emitted from the chromosphere is based on its temperature, which means we can tell a stars surface temperature just by its color. How cool is that?! Red stars are the coolest and blue stars are the hottest. I know this is backward from the labels you see on sink handles and stuff. Those colors are thermodynamically inaccurate. Where was I again? Right. Stars. In temperature order, they go from red to blue. But, where we'd expect to have a green star, we have a white star instead. Why though? Statistical mechanics! An object’s temperature comes the wiggling and jiggling of its little particles. In fact, we relate that temperature directly to the kinetic energy of those particles. That’s the energy of their motion, their wiggling and jiggling. As the particles jiggle around, they bump into each other and lose some of that energy, which is carried away as light. But each individual particle is jiggling a different amount. This is only the average kinetic energy of the particles. The actual kinetic energy of any given particle probably isn’t that number. What did you expect? The universe to be simple? Pffft! When they bump into each other, different particles will lose different amounts of energy and, therefore, release different light. Statistically, the distribution looks something like this. We call it a black body curve. This is what the curve looks like for the Sun and it matches up well with experimental data. But, we can’t actually see all of that light. We only see a tiny fraction of it, what we call visible light. Because, you know, science names. They’re boring, but descriptive. Seriously though, the scale on this graphic isn’t linear. It’s a logarithmic scale. Some people call it exponential scale, but don’t get me started on another rant. On a chart that goes from a trillionth of a meter up to 10 kilometers. The visible range only spans 300 nanometers. If you were to stretch the full spectrum between Los Angeles and New York, the visible part would only be as wide as this single piece of thread. Human eyes suck. Anyway, let’s get back to that black body curve. We don’t need the entire curve to understand color. We only need the part inside the visible range. We can’t see the rest anyway. Since different colors are emitted simultaneously, they’ll mix together and we have to worry about what that’s going to look like to human eyes. We saw in a previous video how human vision actually works. Our eyes have red, blue, and green sensors that send signals to the brain. Our brain combines those signals to interpret the entire visible spectrum. But it has an interpretation for every possible signal it could receive. Including signals that don’t correspond to a single wavelength of light. To understand color, we need chromaticity graph. This assigns a color to every possible signal the human brain could receive from the eyes. Actually, an RGB screen can’t display all those colors. He’s right. I’m lying a little bit. Mmhmm. Technically, an RGB screen can only accurately display colors inside this triangle. Outside of that triangle, it’s faking it. It’s the best I can do, ok?! So, chromaticity. This outer curved edge is all the colors that corresponds to a single wavelength of light, what we call monochromatic colors or single-color colors. Again, boring name, but descriptive. Anyway, that curve is the entire visible spectrum. From red all the way up to violet or what you might call purple. There isn’t much purple though, only a tiny bit at the very end. Everything else on the chart is a combination of multiple colors. That’s where you’ll find the rest of the purple. So let’s see what happens with the light coming from a star. Red stars aren’t too complicated. Almost all the light is red, so the star appears red. As we look at hotter stars, the color shifts as we’d expect; orange and then yellow, but something weird happens when that peak shifts to green. Even though it’s peaking in green, there are a lot of other colors coming from that star. There might be more green than anything else, but there’s a significant amount of all visible wavelengths. There’s also a roughly equal amount coming from the blue side as there is from the red side. The monochromatic colors are mixing together. Back to the chromaticity graph! Star colors aren’t along the monochromatic curve. They’re along this curve. We call it color temperature. You'll find the coolest stars in the red corner. I’m ready coach! Oh, no, I didn’t mean that kind of red corner. Man! It’s been forever since I’ve been in the ring! I know. I’m sorry. I'll find a use for you soon. I promise. Just put me in! Ok, so the coolest stars are in the red spot. They’re nearly monochromatic. As we look at hotter stars, we get orange and yellow. But they’re not monochromatic orange and yellow. Color mixing gradually gets more extreme. By the time we expect to see green, there are so many other colors around that the star looks white instead. All colors mixed together in roughly equal amounts looks like white to us, so we don’t get any green stars. We get white ones like our Sun. Yes, the Sun is white. This balance between the colors disappears when we look at even hotter stars. As the temperature goes up, the peak continues moving to the left. That means more blue light and less red light will be emitted from the star. If the star is hot enough, that imbalance will be large enough for the star to look blue. So how hot to be a purple star? Never going to happen. Seriously. Never. That black body curve might be shifting to the left, but it's also going up. It might be peaking in ultraviolet or x-rays or something, but there’s still a lot of visible light. and that intense mix of colors keeps the star blue, no matter what the surface temperature is. The color temperature on the chromaticity graph dead ends in blue. I mean, stars can’t get anywhere near infinite surface temperature. The hottest stars only get up to about 200 thousand kelvin, but you get the point. So why aren’t there green or purple stars? Because color is more complicated than the visible spectrum. Stars emit multiple wavelengths of light at the same time. When colors mix, we have to consider how all those signals are going to be interpreted by the human brain. Stars emit light based on their surface temperature, which only generates colors along this curve. Green and purple aren’t on that curve. So how cool would it be to see green and purple stars anyway? Let us know in the comments. Thanks for liking and sharing this video. Don’t forget to subscribe if you’d like to keep up with us. And until next time, remember, it’s ok to be a little crazy. Brilliant's mission is to help people achieve their learning goals, so whether you're a student or professional, brushing up or learning cutting-edge topics, or someone who just wants to understand the world better, you should check out Brilliant. They make that easy with interactive explorations and a mobile app that you can use on the go. In their new course “Programming with Python,” you can actually test your code right in the browser. With Brilliant Premium, you can learn something new every day. If you liked this video, you might like their course on astronomy. It has quizzes on how we explore the universe using light. Brilliant helps you achieve your goals in STEM, one small commitment-to-learning at a time. If this sounds like a service you’d like to use, go to brilliant dot org slash Science Asylum today. The first 200 subscribers will get 20% off an annual subscription. So why is the Martian sky red? Technically, Mars’s sky isn’t red. It’s pink. The Martian atmosphere is 95% carbon dioxide and filled with iron oxide dust. So it’s the same explanation as Earth’s blue sky, it just has a different composition. Anyway, thanks for watching.
