How can you be so hot and still be so blue?

I posted a picture of Alberio the other day and it got me thinking about why stars are different colors, that one being a good example of how different individuals can be.

On the surface, it’s not all that hard to understand why these stars vary so drastically in appearance. We’ve all learned by now that stars are just big balls of hydrogen that, under their own gravity, start smashing hydrogen atoms together making helium and booming out massive amounts of energy. My thinking point for today is diversity in the stellar population and the (kinda) simple reason their appearance varies: temperature.

Oh Anteres, you're so cool.

Take a ball bearing, in your mind please (for safety), and start heating it up. At first you don’t see much happening, but eventually you would start to notice a reddish glow. As it continues to get hotter, red turns to orange, then to yellow and eventually white. If you could keep going without melting your mental BB and making a dangerous mess that would light your brain on fire, you can imagine the white eventually glowing brightly with a bluish hue. That’s an analogy for a star’s color, sort of.

Naturally, things are a bit more complicated when you get into the nuts and bolts of it, but fortunately, there’s the venerable Hertzsprung-Russell Diagram to help you see how things compare.

Hmmm, that's very cool, but something seems to be missing.

Notice the lack of green? There are no green stars, even the ones (like our sun) that put out more green-ish light than anything else.  The color is directly tied to temperature and it’s easy to understand the lack of green in that context, you never hear about anything being “green-hot”. Additionally, as I said in an earlier post, all colors are our brain’s interpretation of a small slice of light wavelengths, tuned to see the light from our star as “white”. Maybe to some alien that evolved around a hotter or cooler star would see it differently. Maybe to them our sun looks green, and your brain looks delicious.

The thing I find most enjoyable is that both hot and cool stars can be so bright. The processes that make stars shine are INTENSE, and when we talk about “cooler” stars, we’re still talking about seething balls of nuclear fusion. They differ in longevity by huge margins too and many change dramatically as they go through various stages of their lives. It’s all terribly interesting stuff.

For now though, it’s enough to say that now when you look at stars like Betelgeuse and Sirius, you’ll have a better idea of why they look so different.


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