Learning! — How Stars Are Classified

So, there’s a lot of different kinds of stars out there. We have our normal Sun, but then we hear about the stars that are thousands of times larger, the white ones that are smaller, and the blue-ish ones that are really hot. You may not know this, but hundreds of thousands of stars have been cataloged and classified, and when we graphed them, we found this crazy pattern.

It’s called the HR diagram, and from all the data we’ve gathered, the vast majority of the stars fit on one cohesive line when you graph them based on surface temperature and brightness. Today I’m just going to talk about this one picture and explain it so that you understand what it means and how cool it is.

The simple explanation is this: Bright, hot stars are at the top left, and dim, cool stars are on the bottom right. (You can see our own sun in the middle with its yellow buddies.) You see, since pretty much every star functions the same way and holds the same fundamental properties, they show similar results. The diagonal lines going through the diagram describe the size of the star. “1 Solar Radius” means its the size of the sun. “10 Solar Radii” means it’s radius is ten times larger than our sun, and so on.

So this interesting pattern we see here is that most stars are (relatively speaking) pretty similar in size. But why are the brighter and hotter ones larger than their red, small counterparts? Well, it has to do with the amount of energy it emits, but there’s more.

Let me hit you with this equation:

 

What does it mean? Well, it’s simple, really. “Stellar lifetime is proportional to the mass over the luminosity of the star.” In other words, “fuel over the rate in which it is burned”. This equations mean that bright, massive stars burn out extremely quickly compared to red dwarfs, and it’s why there are so few examples in the HR diagram above: there is only one blue giant for every ten thousand stars you look at. It’s just because they die out within a few hundred million years.

But if you look at the other side of the spectrum, the dim stars are extremely efficient at burning their fuel. In fact, as far as we know, not a single red dwarf star has ever died. They are so efficient it takes trillions of years for them to burn out, and that amount of time simply hasn’t passed yet. Our own star, by comparison, is five billion years old, and is scheduled for a permanent departure in the next five billion years.

So, given a star’s luminosity and temperature (which we can discover through parallax and spectroscopic measurements, respectively, being an entire can of worms I won’t get into today), we can tell pretty much everything about a star: how large it is, what it’s mass is, how long it’s lifetime is, and based on the stars around it we can also guess how old it is, since nebulas tend to form stars in clusters.

So in the HR diagram, that nice, even line of stars is called “the Main Sequence”. Pretty much every star you know about will fir somewhere on that line. And as for the outliers, that’ll have to wait for another time.

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