Stars are like cars. They need fuel to go. And also like cars, they don’t have an infinite supply.

But here’s where the metaphor breaks down. They can never refuel.

Yup. That’s right. For their entire lives, stars are stuck with only the amount of fuel they formed with. They can’t get more.

What happens when you’re driving, and you run out of gas?

Well, if you can’t refuel, you’re gonna have to call a tow truck. But stars don’t have tow trucks, and for them, it’s not a matter of moving or not—it’s a matter of life and death, such as it is.

But how does that work?

Remember our old friend, the H-R diagram?

The H-R diagram is a fundamental graph of stellar astronomy that sorts stars by their surface temperature and their total luminosity. Surface temperature is measured in Kelvins; luminosity is measured in solar luminosities, L_☉.

This diagram shows all known star types—as well as a few that aren’t actually stars. White dwarfs are more like the dead husks of smaller stars. But for our purposes right now, all we really need to look at is the main sequence.

The main sequence refers to the relation between surface temperature and total luminosity that makes a star stable. These are “successful” stars—stars that accidentally formed with the right balance between internal pressure and their own gravity.

When I first started studying star life cycles, I thought that the main sequence actually represented a single life cycle. I thought that stars began as the massive blue stars at the top of the main sequence and evolved to become cool red dwarfs.

That’s not true. Red dwarfs remain red dwarfs for as long as they remain stable, and massive blue stars remain the way they are as well. Our sun, a medium-sized yellowish star—or type G2, if we want to be specific—did not evolve from a massive blue main-sequence star, and will not one day become a red dwarf.

Stars do not evolve along the main sequence. They evolve across the main sequence.

Wait…what?

This version of the H-R diagram eliminates all star types but the main sequence, and shows what these stars’ life cycles look like.

Notice the red line that runs along the main sequence? Every star begins somewhere along that line when it forms out of a giant molecular cloud. As soon as it ignites hydrogen fusion and begins to blow away its dusty cocoon, it’s on that lower edge of the main sequence.

This red line represents the zero-age main sequence—or ZAMS.

As each star evolves, it moves across the band of the main sequence. Notice the little arrows that represent the star’s evolution.

So what’s that dotted blue line?

That’s where the star leaves the main sequence.

Remember, we’re not talking about the star’s actual motion in space—the data point that represents the star on this diagram is moving. When the data point reaches that blue line, the star loses stability.

But why?

Let’s take a small step backwards and explore how stars produce their energy.

Remember the proton-proton chain?

I know, it’s been a while since I really went over it. The proton-proton chain is essentially a chain reaction that fuses four protons into one helium nucleus. It’s the nuclear reaction that powers low- and medium-mass stars.

The CNO cycle, on the other hand, is a nuclear reaction that does the same thing—fuses four protons into one helium nucleus—but uses carbon, nitrogen, and oxygen as catalysts.

Only the hottest stars can use the CNO cycle, and it affords them a lot more energy than they could get from just the proton-proton chain.

No matter if they primarily use the proton-proton chain or the CNO cycle, all “true” stars fuse four protons into one helium nucleus. And here’s the key: each nucleus exerts the same pressure on its surroundings.

That goes for hydrogen nuclei—aka the free-floating protons—and helium nuclei. They both exert the same pressure.

But…did anyone notice that we just converted four nuclei into one?

That one helium nucleus, the product of the reaction that keeps a star stable, exerts only a quarter of the pressure that the “reactants”—the four protons—did.

Stars fuse nuclei to generate energy, which exerts the pressure necessary to support their own weight. But the very process of fusing those nuclei gradually reduces the pressure of the core.

This won’t spell the end for the star right away, though. It’ll still take a while for the star to lose the perfect balance between pressure and gravity.

As a main-sequence star ages, its core will contract under its own gravity, get hotter, and produce more energy. But the entire star doesn’t contract. The excess energy flowing from the core will actually push the star’s outer layers away, forcing them to expand and cool.

The result is a larger star with a cooler surface temperature—but its mass has stayed the same. The same amount of stuff exists inside it, that stuff is just exerting less pressure due to being arranged in fewer nuclei.

Here’s the fun part, though. The star has a cooler surface temperature, but since its surface area has increased, it’s actually going to glow a bit brighter. Why? Check out my post explaining luminosity.

So…how long does all this take?

That depends on a star’s mass.

Oh hey, look, we’re back to this diagram.

This time, I want to point out the lifespans labeled on the main sequence. See the arrow that represents the sun’s lifespan?

When our sun was born, it was on the zero-age main sequence. It’s now less than halfway through its lifespan. Within 10 billion years, it’ll finally exhaust its fuel to the point that its internal thermostat will fail.

That’s quite a difference from the 15 M_☉ star at the top of the main sequence. As the diagram indicates, it’ll only take 10 million years for that much more massive star to exhaust its fuel supply.

But…why? I thought having more mass meant having more fuel to burn? Stars are literally made of their fuel, after all.

Well…that’s the trouble with the CNO cycle. Massive stars have enough energy to use it, and that’s a major plus for them as far as generating totally wild amounts of energy. But the problem is…they’re so good at producing energy, they exhaust their fuel quickly—even though they’ve got more of it.

Stars that are mostly restricted to the proton-proton chain can’t generate nearly as much energy from their nuclear reactions. But they’ll also retain their fuel for billions of years to come.

Let’s think about what that means for a moment. If massive stars both form rarely and don’t last very long, and low-mass stars are quite easy for nature to form by accident and don’t use up their fuel for billions of years…shouldn’t that mean there are way more low-mass stars in the sky than the massive ones?

Yup!

Here’s a graph that shows you how common different types of stars are in the sky. This is an H-R diagram laid flat as if on a desk surface, with bars rising up from its data points to indicate how many of that type of star there are.

Most of the brightest stars in the sky are actually giants and supergiants—star types we’ll explore very soon. But as you can see, there are very few of them, not even enough to warrant a bar of their own. That’s because they’re so bright, the ones we see are very far away.

Our perception of low-mass stars is skewed, too. It’s hard even to see the nearest ones because they’re so faint. And the distant ones are nearly impossible to detect. But theory predicts that they are there, and sky surveys have managed to find them.

The evidence is clear: nature favors low-mass, low-energy, low-cost stars. And why wouldn’t it? It takes so much more work to generate a powerhouse. In a universe that operates at random, we are left to depend on chance alone to create those more massive stars.

We’ve covered how a middle-aged star evolves, but what happens next?

That’s what we’ll explore coming up.