Our sun is undoubtedly the star we know the best. It’s only 93 million miles away—which might seem far, but isn’t that large a distance when you realize that the nearest neighboring star is a whole 4.3 light-years away.
As in, it takes light—yeah, that same stuff that hits the ground from your flashlight in a split second—a whole 4.3 years to get here.
We’re pretty familiar with our star’s interior. We know it produces most of its energy in its core, a relatively small but very hot region at its center. We also know that energy then radiates outward until it hits the convective layer.
There, the energy gets stuck in circulation for a bit until it finally manages to leave the sun’s surface.
But…how normal is that? Is it the same for all stars, or just the sun?
The short answer is…yes and no.
Yes, all stars produce energy in their cores and have a convective layer—to our knowledge, at least. But no, it doesn’t work the same way in every case.
But first…what the heck is convection, and how else can energy be transported?
If you’ve ever burnt your hand on a pot handle or a clothing iron or maybe a hair curler, you’ve experienced conduction.
If a metal pot handle is hot, you can’t see it, but the atoms inside are jiggling about a bit faster than they would if it were cool. If you touch it, that energy of motion is transferred to your skin, and it hurts like heck.
If you’ve ever warmed your hands over a fire, or even cooked food, you’re familiar with radiation—and you’re probably quite familiar with it just from my frequent posts.
In this case, photons of energy are emitted by your heat source and absorbed by your hand or whatever you’re cooking—hopefully not the same thing!
Stars transport most of their energy by radiation.
From the moment a photon of energy is produced to the moment it gets backed up in the convective zone, it’s being radiated, absorbed, and reemitted throughout the star’s interior.
Oh, hey, there’s that word convection again.
The convective zone of a star operates on one main principle: that hotter gas is less dense than cooler gas, and therefore, hotter gas will rise as cooler gas sinks.
In general, the convective zone of a star isn’t particularly hot—compared to the core, that is. We’re still talking about temperatures in the thousands to ten thousands of Kelvins (which are just Celsius degrees + 273).
Anyway, a star’s convective zone is relatively cool. That means it’s also less transparent to radiation than the rest of the star, so energy will get backed up there in its journey to the surface.
But as those photons try to make their way outward, they’ll heat up the bottom layer of the convective zone. And that’s what gives the zone its name.
As its bottom layer is heated up, its density will decrease, and it’ll begin to rise…but as those gases move farther from the core of the star, they’ll cool down and sink again. As they sink, they’ll near the warmer interior of the star and heat up again, and they’ll begin to rise once more.
This process will continue as long as the star produces energy to heat its convective layer.
But…the convective zone isn’t actually always near the surface.
Stars that are similar to our sun in mass—roughly medium-sized stars—have an internal structure that’s also similar to the sun: an inner radiative zone and an outer convective zone.
But for massive stars, convection actually occurs just outside the core, with an outer radiative zone extending all the way to the surface. And very low-mass stars don’t have radiative zones at all.
The answer has to do with how stars produce their energy.
Stars like the sun mostly use the proton-proton chain, which fuses two hydrogen nuclei—protons—into one helium nucleus. Fusing lighter atoms into a heavier atom releases energy.
The proton-proton chain requires very little energy to operate. Protons have positive charges and repel one another, so a star does need to be hot enough to overcome that barrier. But comparatively speaking, it doesn’t take much to smash four single protons together.
Wait…comparatively speaking? What does that even mean?
Well, in a much larger star with a much higher internal temperature, the CNO cycle can produce the bulk of the energy—meaning, fusing hydrogen, carbon, nitrogen, and oxygen.
Fusing these heavier elements is much harder. Because—you guessed it—they have lots more protons, and as a result, a much greater repulsive force to overcome.
The CNO cycle is, as its name indicates, a cycle, meaning that its end products become its beginning products for the next cycle. But we can generally say that it starts with the fusion of a hydrogen nucleus and a carbon-12 nucleus (the step at the top middle).
The products of this reaction are a gamma ray (a high energy photon) and a nitrogen-13 nucleus, which is unstable and decays to a carbon-13 nucleus, and releases a positron and a neutrino in the process.
The carbon-13 nucleus then fuses with yet another proton, to create another gamma ray and a nitrogen-14 nucleus—which, again, fuses with a proton, and results in the release of another gamma ray and an oxygen-15 nucleus.
Oxygen-15 decays to nitrogen-15, releasing a positron and neutrino. Last but not least, nitrogen-15 fuses with a final proton, which results in—lo and behold—a carbon-12 nucleus and a helium nucleus.
Recognize that carbon-12 nucleus from the “beginning” of the cycle? Yep, told you the end product and beginning product were the same.
The CNO cycle also manages to accomplish the exact same task as the proton-proton chain. It does ultimately fuse protons into a single helium nucleus, which releases energy.
But…why? Why would a star use such a complicated process, and fuse heavier, more repulsive nuclei, if it didn’t have to?
Because—in a phrase—you get out of something what you put into something. If you pay for a cheap product, you get your money’s worth. If you do a crap job on a homework assignment, you get a bad grade.
But if you invest well in a high-quality product, or put time and effort into your homework, chances are you’ll get what you paid for.
The proton-proton chain is like a low-income budget or a low-effort project. It does the job, but you only get out as much energy as you put in.
The CNO cycle, on the other hand, is like a high-income budget or a project you work on diligently for weeks or months. It takes a lot of energy, but it gives a crap ton back. And stars that have the energy to use it are extremely powerful.
Only the central two percent of a massive star’s mass produces significant amounts of energy with the CNO cycle—but that central two percent generates about fifty percent of the star’s energy. So it’s like hundreds of cars accelerating onto a stopped freeway.
That’s how massive stars end up with convective zones right around their cores, instead of near their surfaces. Energy gets backed up there, and transport of energy by radiation just can’t carry it away fast enough. And so it churns away.
Farther from the core, the traffic jam isn’t as bad, and the energy can radiate away—making the rest of the star, from the outside of the convective zone to the surface, a radiative zone.
But why don’t stars less massive than the sun have radiative zones at all?
Simply put, they’re too cool for that.
No, seriously. They’re actually much cooler—comparatively—than stars closer to the sun in mass. And remember, our sun has no central traffic jam, just an easy flow of radiation to the cooler convective layer above where photons get backed up by the more opaque material.
Low-mass stars are like our sun, except they’re so cool throughout that there’s just no easy flow of radiation whatsoever. Energy churns away in convective currents throughout their interiors.
In the future, we’ll take a look at how low-mass stars’ convective structure helps them burn for billions upon billions of years, longer than any more massive stars—and how, despite the power of a massive star’s core, that power ends up sabotaging their life spans.