Back in August—sorry I took so long!—we talked about the helium flash, an explosion that occurs within stars when helium nuclei begin to fuse within a degenerate core.

So…this is not what the helium flash would look like.

Even though it’s a powerful explosion, it happens in such a small region in the center of the star that we wouldn’t see it at all, and the star’s outer layers absorb most of the energy from the explosion. I just thought it was a cool picture 🙂

In any case…what happens after the helium flash?

First, let’s review what’s actually going on inside the star right now.

Even though the sizes in this diagram aren’t exactly to scale, they’re a good representation of what’s going on, physically, in the star.

We started out with a core made up of hydrogen, fusing to create helium nuclei that couldn’t be fused further under the current conditions. These helium “ashes” were dumped into the center of the core.

As the core ran out of hydrogen to fuse, it began to contract, generating gravitational energy which acted like a stovetop, exciting the layers of hydrogen above to begin fusion in a hydrogen-burning “shell.” More helium ashes were dumped into the core.

The core was then composed entirely of helium nuclei…which still couldn’t fuse under the current pressures. The pressure-temperature thermostat regulating the star’s stability broke down as the core became degenerate.

Then, finally, the degenerate core managed to heat enough that it could begin fusing helium. What followed was a runaway explosion as bright as all the stars in our galaxy—the helium flash.

Let’s take a look at what’s been going on with the star on the H-R diagram.

Remember that the spectral class axis of the H-R diagram refers to a star’s temperature, and the magnitude and luminosity scales on the left and right of the diagram refer to the star’s brightness.

Let’s look specifically at the evolutionary track for the sun. See the part that’s labeled “H→He”? That’s where the hydrogen shell is burning outward through the star’s layers like a brush fire, dumping helium into the core.

As you can see, the star is getting brighter but cooler—and this means that it’s expanding.

But look at the helium flash. See that dramatic shift? One minute the star is expanding, and the next it’s getting hotter and fainter again—meaning it’s contracting.

That’s because when the helium flash happens, the star’s pressure-temperature thermostat—that is, its way of maintaining internal homeostasis—starts working again.

When the core was degenerate, it had contracted until it physically couldn’t contract anymore. But when the pressure-temperature thermostat kicks back in, the core begins to expand—and to do so, it absorbs energy that had previously supported the star’s vastly expanded outer envelope.

So, after the helium flash, the core stabilizes and expands, while the outer envelope contracts. You can see this represented in the diagram I showed you up above:

(Notice that this diagram also follows the vague shape of the star’s path along the H-R diagram.)

Now here’s the question of the day. When hydrogen began fusing, it dumped helium ashes into the star’s core. Now helium is fusing. What ashes are being dumped into the star’s core now?

The answer is something quite extraordinary.

The thing is, helium is the end product of hydrogen fusion, but it’s also an element that makes up about about 28% of the star to begin with. But the end product of helium fusion? That’s something that exists in this universe because it was initially created by stars—and its name is carbon.

Sound familiar?

It should—you are made up of carbon, same as all life on Earth.

Pretty cool, huh?

Carbon isn’t the only element that stars initially created. Most elements heavier than helium, including carbon, nitrogen, oxygen, calcium, and iron, were created in stars by nucleosynthesis.

So…now we’re fusing helium into carbon and oxygen, the two products of helium fusion. What happens next for a star?

Well, for a medium-mass star, helium fusion and the creation of carbon and oxygen are kind of the end of the line. And something very familiar happens…

When the core runs out of helium, it is an inert ball of carbon-oxygen ashes.

Remember the hydrogen-fusing shell that’s still burning outward through the star? Well, now we see a helium-fusing layer appear and burn outward in the same way, dumping more carbon-oxygen ashes behind it.

What we end up with is a many-layered star. At the center is the inert carbon-oxygen core, which we’ll explore in a future post. Around it is a helium-fusing shell, just like the hydrogen-fusing shell.

Then, between the helium-fusing shell and the hydrogen-fusing shell, we have a layer of helium that has yet to be ignited by the “stovetop” that is the helium-fusing shell.

And lastly, just outside the hydrogen-fusing shell, we have the hydrogen envelope and the atmosphere of the star.

What’s happening on the H-R diagram right now?

After the helium flash, the star’s temperature initially rises as its brightness falls. It soon stabilizes as the pressure-temperature thermostat kicks back in, and the star begins to regulate its internal pressure once more.

But after a medium-mass star runs out of helium in its core, the inert carbon-oxygen core begins to contract under its own gravity, and the two energy producing shells—the hydrogen one and the helium one—force the star’s outer layers to expand.

As the outer envelope expands, the star cools, but its luminosity increases because luminosity is directly related to surface area. So the star moves back up and to the right a bit in the H-R diagram.

But…wait a second. This whole time, we’ve mainly been talking about medium-mass stars. What about high-mass stars?

Well, for them, helium fusion is very much not the end of the line.

As you can see, there’s no simple process for the fusing of elements heavier than helium, but the end result—no matter how stars get there—is a silicon nucleus.

By the way, that’s the same stuff found in rocks. It’s also been suggested as a potential basis for alien life in a lot of science fiction.

As complicated as fusion reactions can be for elements heavier than helium, the pattern of how it changes the star’s interior is beautifully simple—and it looks a whole lot like the pattern we’ve seen for hydrogen shell fusion and helium shell fusion.

The layers you see here are our “shells.” Near the surface, you can see our old friend, the hydrogen fusion shell, and our new friend, the helium fusion shell.

The difference between a medium-mass star and a high-mass star—about 8 solar masses, or 8 times the mass of the sun—is that massive stars can begin to fuse carbon.

When they run out of carbon in their cores, the core contracts under its own gravity, and the energy generated from the collapse ignites a carbon-fusing shell that burns outward behind the helium- and hydrogen-fusing shells.

Then oxygen fusion ignites in the core, and eventually the core runs out of oxygen, too. So the inert neon core—the product of oxygen fusion—ignites an oxygen-fusion shell as it collapses…and so on, all the way down to silicon.

Wait…what about the iron-ash core we see in the image up above?

Well…for now, I think I’ll just leave you to wonder about that one 🙂 We’ll explore what happens to the iron core soon enough, but for now, let’s move on to our best evidence of the paths of stellar evolution: star clusters.