Stellar Evolution Demystified


Whaddya know…after what seems like a geological age, we’re finally done with stellar evolution! And we’ve covered a truly ridiculous amount of information.

We’ve covered a star’s relatively gentle, humble beginnings within the collapsing cores of giant molecular clouds (or GMCs). We’ve explored how stars begin fusing hydrogen nuclei for fuel and how their interiors work.

We’ve covered how they evolve across the main sequence, and how they eventually exhaust their fuel, lose stability, and expand into giants.

We’ve delved into the way low- and medium-mass stars quietly expel their atmospheres and shrink into inert balls of carbon called white dwarfs. And we’ve watched as massive stars burst apart in brilliant supernova explosions and then collapse into some of the most extreme objects in the universe, neutron stars and black holes.

Those three end states–white dwarfs, neutron stars, and black holes–are known as compact objects, and we’ve explored them too.

If it all seems super complicated…I understand. But now, just as I did once with types of stars, I’m going to give you an overview to put it all together.

Collapse of a Molecular Cloud

Stars are born from dense clouds of dust and gas in the interstellar medium, the space between the stars. This space is filled with diffuse clouds. We just don’t see it, most of the time, because it’s not hot enough to produce its own light.

(We do see it when nearby stars light up regions of it–in the form of nebulae, like the Eagle Nebula, Swan Nebula, Orion Nebula, Trifid Nebula, etc.)

The interstellar medium maintains a careful equilibrium between the different types of clouds. But now and then, a shockwave from a powerful explosion elsewhere in the galaxy drives the denser clouds–specifically, the giant molecular clouds, or GMCs–to collapse.

The GMCs fragment into many dense cores, each of which will collapse into a newborn star if it has enough material. The amount of material the cloud cores start out with will determine how massive the star turns out to be.

Contraction of a Protostar

Each star begins shrouded within a dusty cocoon, invisible at visible wavelengths of light. It’s invisible for two reasons: one, it’s too cool to produce anything but the longest wavelengths of radiation, and two, wavelengths other than the longest ones can’t get through the cocoon to reach our eyes.

At the center of the cocoon is a slowly contracting object. Gravity drives its material to fall inward, building up the star’s mass and generating kinetic energy. Slowly, the object begins to heat up.

As it becomes hotter, it emits shorter wavelengths of light. Those shorter wavelengths can’t get through the surrounding cocoon–so they get stuck inside. They instead create radiation pressure on the inside, which slows the contraction of the cloud.

Soon enough, the object stops accumulating mass. Now, we call it a protostar.

On the other hand, it might not quite make it–an object that forms this way, but was never massive enough to ignite hydrogen fusion, becomes a brown dwarf. Brown dwarfs are strikingly similar in both size and temperature to our own Jupiter. But they don’t count as planets, because they formed like a star.

Formation of a Young Stellar Object

A protostar is an object that is not gaining any more mass from its surrounding cocoon, because it’s hot enough to push the infalling material away. But it’s still contracting.

(In fact, if the protostar didn’t continue to contract, it would never become a “true” star.)

Conservation of angular momentum forces contracting objects to spin faster. So as the protostar contracts, its surrounding cocoon flattens into a disk. The protostar also grows hotter and more powerful as the density in its core increases, particles collide more violently, and kinetic energy is generated.

Eventually, the protostar becomes hot enough to blast its remaining cocoon out of the way. Now it’s producing light at visible wavelengths, and it doesn’t have that pesky cocoon in the way of us seeing it.

Now we call it a young stellar object (or a YSO)–or, if you prefer, a pre-main-sequence star.

Birth of a Main-Sequence Star

A YSO is an object that is detectable at visible wavelengths, but it’s not considered a “true” star yet because it hasn’t ignited hydrogen fusion.

Until hydrogen fusion ignites, the object–presuming it formed with enough mass to become a star–just can’t produce enough energy to support its mass. So it’ll continue to contract under its own gravity.

When the core becomes dense enough…BOOM!

The core is already made up of mostly hydrogen nuclei–that is, free-floating protons. A YSO generates its energy due to those protons colliding like crazy as contraction provides them with less and less space to move around. And soon, they’re going to start colliding violently enough to fuse.

Now the object can produce enough energy to support its mass–and we call it a main-sequence star.

Wait…what is the main sequence, anyway?

Well…here we are again, back to the good ol’ Hertzsprung-Russel diagram (or H-R diagram for short).

I really can’t stress enough how important this diagram is to stellar astronomy. It’s so important that the last time I wrote an overview post like this, it was to unite the many, many posts I’d written detailing this diagram in a nutshell. This is the single most useful diagram for classifying, studying, and analyzing stars. It’s not just the astronomer’s best tool–it’s the whole dang toolbox.

You can see the main sequence in the diagram above–it’s that long chain of stars that cuts a swath straight through the middle, from upper left corner to lower right corner.

But what’s so special about it?

Note the axes of the diagram. The H-R diagram sorts stars by their surface temperature and luminosity. The luminosity is the total radiation the star emits, including all wavelengths across the electromagnetic spectrum.

That means that stars along the “main sequence” follow a specific pattern of surface temperature and luminosity. And those stars are the stable, “adult” ones. These are “normal” stars. Essentially, main-sequence stars operate within optimal parameters. They’re “healthy,” so to speak.

Stellar Structure & Stability

The key to understanding the internal structure of a star is hydrostatic equilibrium. The mechanism for maintaining that equilibrium is the pressure-temperature thermostat.

Stars wage a constant battle between their internal pressure and their own gravity. The pressure exerted by energy production drives them to expand; their gravity drives them to contract. These forces must be balanced throughout the star’s interior for it to continue functioning.

The arrows up above represent the increasing magnitudes of these forces as we get closer to a star’s center. The center bears the weight of all the mass above and therefore endures the strongest gravitational force. But that’s what keeps the pressure high–high enough to balance gravity with nuclear power!

Conversely, the force of gravity at the surface is minimal, but so is energy production. Again, forces are balanced.

The star is good at maintaining this balance almost like a lifeform maintains homeostasis.

If energy production falls, the surrounding mass collapses, which increases the internal pressure and drives up energy production again. And if energy production goes into overdrive, the surrounding mass expands, which lessens the pressure and lets nuclear reactions cool off.

You’ll see this response to changes in energy production–collapse and expansion–as the star burns through its fuel and evolves into an aging giant star. The difference then is that, as the star runs out of fuel, its interior loses stability, and it can’t regulate hydrostatic equilibrium anymore.

Hydrostatic equilibrium looks to me almost like a beating heart, maintaining a regular pulse…right up until the star ages, and the pulse becomes erratic. The star then reaches the end of its life.

But I’m getting ahead of myself…

The “Adult” Life of a Star

When I first began learning about the main sequence, I thought it was sort of like a train track, a line that stars follow over the course of their “lives.” But that’s not quite how it works.

When a YSO finishes contracting and becomes a star, it starts out somewhere along the main sequence line, depending on how massive it is. For example, the sun started out where “Initial Sun” is labeled. A star three times as massive (3 solar masses, or 3 M) would begin its main sequence life where “3 M” is labeled, and a 15 M star would begin its life where “15 M” is labeled.

This is because the H-R diagram relates temperature to luminosity, and both temperature and luminosity are related to a star’s mass.

So how does a star evolve along the main sequence, then?

It crosses the main sequence–like crossing a river.

Look at the red line labeled “ZAMS.” This refers to the zero-age main sequence, or the moment when a star newly becomes a main-sequence star. That red line hugs the lower edge of the main-sequence “band.” As the star ages, its temperature and luminosity relation will move across that band to the upper edge.

Why?

The lower edge of the main-sequence band indicates higher surface temperature and lower luminosity: in other words, a healthily compact star. But as a star ages, its outer layers expand and cool off. That expansion means that it becomes physically larger, even though its mass hasn’t increased. Greater surface area equals higher luminosity.

So as a star’s outer layers expand and cool off, the star’s data point on the H-R diagram will move up and to the right, indicating lower temperature and higher luminosity. Note the direction the little arrows curve for each different mass of star.

Speaking of a star’s expansion…

Expansion into a Giant

This is where the life cycles for different masses of stars diverge. I’ll cover low-mass stars first.

Low-Mass Stars (like Proxima Centauri)

The key to understanding the evolution of low-mass stars–also called red dwarfs–is their internal structure. They differ from other stars in that they have no radiative zone. Their interiors are entirely convection.

That means that their nuclear fuel gets constantly stirred throughout their interiors. They’re not limited to the hydrogen nuclei in their cores–they can use all the hydrogen they contain.

The result? They can live a really long time.

As in, according to stellar models…since the formation of the universe roughly 14 billion years ago, not a single low-mass star should have had time to reach the end of its lifespan.

And you know what that means?

Yup, you guessed it–we can model their deaths, and we do predict that they will end up in a state similar to medium-mass stars’ end states, but there’s no proof yet. We just don’t know what they’ll end up looking like.

What we do know is that they have very quiet deaths. They probably burn through their fuel extremely slowly and gently, and then just wink out, like a light turning off.

Medium-Mass Stars (like our sun)

Medium-mass stars have a convective layer, but between that and their cores is a radiative zone. That means their fuel doesn’t get mixed up.

As a result, once the hydrogen in their cores is used up, stability pretty much goes out the window.

The main problem that medium-mass stars face is that they’re not hot enough to immediately start fusing helium, the byproduct of hydrogen fusion. So their cores begin an uncontrolled collapse as energy production stalls.

That collapse generates gravitational energy, which heats the surrounding layers like a stovetop–and actually ignites hydrogen fusion outside the core, where it was never hot enough before.

We call the hydrogen-fusing layer a hydrogen-burning “shell.” That shell burns outward through the stars layers like a brushfire, and the energy production forces the star’s envelope to expand.

Here, you can see what the star’s doing on the H-R diagram, along with a few spoilers for the rest of the process…

A star with a hydrogen-fusing shell and an inert helium core is called a subgiant. As you can see from the diagram, it’s moved up and to the right from its spot on the main sequence–meaning it’s gotten cooler and brighter. Once again, that’s because its atmosphere has been forced to expand–this time because of the energy from the hydrogen shell.

At this point, the core is still in a free-fall collapse, getting hotter and hotter by the minute. But medium-mass stars on the lower end of the mass scale still can’t ignite helium fusion.

The core will just continue to collapse, and collapse, and collapse. Nothing’s stopping it. That’s nuclear fusion’s job, and there’s no hydrogen left–no usable nuclear fuel.

There’s only two ways to stop the collapse: get hot enough to ignite helium fusion, or rely on the material strength of the infalling particles themselves. And medium-mass stars are not hot enough to ignite helium fusion yet.

These stars will become degenerate.

In a nutshell, degenerate mass relies on the material strength of compressed electrons–which triggers laws of nature from the crazy world of quantum mechanics. The long story short is that the electrons in the core get stuck at extremely high pressure. A healthy adult star would be able to rely on hydrostatic equilibrium to regulate that pressure, but a degenerate core can’t do this.

And to make matters worse, the free-floating helium nuclei in the core control the temperature–which just keeps rising like a bullet train.

Actually, faster. Metaphors from human civilization don’t quite do this chaos justice.

Anyway, eventually–thanks to the nuclei–the core does get hot enough to ignite helium fusion. Nuclear energy production starts up with a vengeance.

Except thanks to the runaway core temperature, it’s way too much of a vengeance.

The degenerate core still can’t maintain equilibrium. So the result is the helium flash: a runaway explosion so violent that, for a split second, the core generates more than 1,000,000,000,000 times as much energy per second as the sun. That’s comparable to the combined luminosity of all the stars in our galaxy.

But after that, equilibrium is restored, and it’s business as usual–except with a helium-fusing core and a hydrogen-burning shell.

The helium core will continue to generate nuclear energy until it runs out of helium to fuse. It’ll then be left with the “ashes” of the helium fusion reaction: carbon and oxygen.

Carbon and oxygen would be easy for a high-mass star to fuse. But for a star like the sun, it’s the end of the line.

So what do we have at this point?

Let’s look at the star from two different standpoints: the internal goings-on and what we see going on from the outside.

On the inside, we have a multi-layered star. At the center is an inert carbon-oxygen core that will never ignite nuclear fusion. Surrounding that we have a layer of helium fusion–a “shell” just like the hydrogen shell layer. Then we have an inert helium layer, which is the yet-to-be-ignited ashes of the hydrogen fusion shell. And then we have the surrounding hydrogen envelope, which includes the star’s atmosphere.

But from the outside…

We already covered the “subgiant” phase. At the helium flash, the uncontrolled rise in temperature from the degenerate core drives the star’s atmosphere to expand and cool–hence a sharp peak of high luminosity and low surface temperature.

The star temporarily expands into a red giant, but as helium fusion begins and the core stabilizes, its atmosphere shrinks again and heats up. That’s the sharp trough where the star is represented as a smaller yellow circle.

But as the core runs out of helium and becomes inert once more, it collapses and generates gravitational energy, the same energy we first encountered at the beginning of the star’s lifespan when we observed contraction of the protostar. This energy drives the atmosphere to, once again, expand.

Now you have the red giant at the top of the diagram: a giant, luminous, but very cool (on the surface) star.

I’ll get to what becomes of the carbon-oxygen core in just a bit.

Massive Stars (like Betelgeuse)

Massive stars lead very short, explosive lives.

While still on the main sequence, they are sometimes referred to as “blue dwarfs”–this refers to the fact that, relative to the expansion they’ll undergo as they evolve, they are still young, small stars. But make no mistake–they are massive.

And they pay the price.

While low-mass stars can live longer than the universe has been around, and medium-mass stars like the sun can live billions of years, massive stars can only live for a few million years. That’s because, in order to support the weight of their own mass, they must rely on a method of nuclear fusion that generates wild amounts of energy–and they explode through their fuel in the blink of an eye.

Well. Astronomically speaking. I guess it doesn’t quite take a few million years for a human to blink.

Once massive stars exhaust their hydrogen fuel, they just move straight on to the helium ashes in the core without pausing for breath. They go on to immediately fuse carbon, oxygen, neon, magnesium, and finally silicon. Each of these elements is the “ashes” of the previous nuclear fuel. Each takes a shorter time to burn through–silicon can take as little as a few days!

The result is a ridiculously layered interior that looks a lot like a red giant…but with way more fusion “shells.”

But, more importantly, massive stars end up with a core full of inert iron ash.

That’s a problem. Iron can’t actually be fused or split for energy.

It’s an interesting quirk of nuclear physics. All elements lighter than iron can at least yield some energy when fused; all elements heavier can produce energy through nuclear fission. But iron can’t. Split it or fuse it, there’s no nuclear energy to be had.

So the iron core is a dead end.

More than that, this dead-end inert core is currently supporting the weight of a ridiculously massive star.

It’s a little bit like the degeneracy that precedes the helium flash. The core collapses. But unlike the inert helium core of a medium-mass star, there’s no sudden surge of nuclear production to save an iron core.

So instead of recovering its equilibrium, the star implodes like quicksand–and then blasts apart in a brilliant supernova explosion.

So what does the star look like on the outside?

The best way to talk about that is with another peek at the H-R diagram.

The evolutionary tracks above are all for the most massive stars. Here, we can see that the stars initially cool off and get more luminous (as their atmospheres puff up with the transition to helium fusion), but then settle down and zigzag back to being hot and a bit less bright.

Later in their life, however–as the arrows turn orange–the stars do a much more pronounced zigzag.

As massive stars explode through their fuel, their interiors destabilize and force their atmospheres to puff up again. This is what causes expansion into a giant.

But because they’re just that massive, these stars don’t become “red giants”–they become supergiants.

Star Death

Again, star death is a bit different for each mass of star–so I’ll divide it up. But I’ll skip low-mass stars this time, because like I mentioned above, we won’t really know for a few billion more years!

Medium-Mass Stars: White Dwarfs & Planetary Nebulae

The Ring Nebula, cataloged as Messier 57, is one of my favorite nebulae–one of my go-tos for star parties, when it’s visible that night!

When a medium-mass star reaches a carbon-oxygen core, degeneracy makes a comeback. But this time, there’s no rebound. Medium-mass stars just don’t have the mass to drive temperatures high enough to ignite nuclear fusion this time. Temperatures can skyrocket all they want as the core collapses, but it’ll never be enough to start fusing carbon.

The medium-mass star is having trouble holding onto its atmosphere right now. It’s expanded into quite the giant–and its size is exceeding its gravitational reach. When skyrocketing temperatures create radiation pressure that pushes at the star’s atmosphere, those gases blow away into space.

The result: a planetary nebula like the one above. This phenomenon is, physically, the remains of the star’s atmosphere.

At the center of the nebula, with a powerful enough telescope, we can spot the faint core that’s left behind. Since it’s not the core of anything anymore–it’s just a standalone ball of degenerate carbon–we call it a white dwarf.

Fun thing is, carbon is what diamonds are made out of. And models predict that, over time, these objects crystalize. So medium-mass stars die very gentle deaths–and essentially become giant space diamonds.

Well. Giant to us…ridiculously tiny, for a star. This thing used to be the core of a star like our sun. More than a million Earths could fit into our sun. The core is a fraction of a star, but it’s not that small…the white dwarf, though, is about the size of the Earth.

Massive Stars: Supernovae, Neutron Stars, & Black Holes

And here’s the Crab Nebula, aka Messier 1. Another one of my star party favorites! This one is actually a supernova remnant.

Supernovae themselves are extremely rare, because they account for a tiny blip of a massive star’s lifespan and massive stars themselves are already exceedingly rare. So there are a few historical accounts of supernovae that humans have observed–the Crab Nebula in particular is the aftermath of one such explosion–but there aren’t really modern-day photographs of them.

But they produce a parallel phenomenon to the planetary nebula. Supernova remnants are a type of nebula, and–like planetary nebulae–they are the remains of a massive star’s atmosphere.

Here’s the fun bit, though. Remember waaaaay up above, when I was talking about the shockwaves that trigger star formation out of a giant molecular cloud? Well, one type of such shockwave is a supernova explosion!

This is why stars’ lives are often referred to as life cycles. They don’t really “reproduce,” but there’s something poetic about the way their death throws can trigger new “life.”

But that’s not all a massive star leaves behind.

The supernova shockwave blasts the star’s atmosphere apart. But that shockwave was the result of an internal implosion–the collapse of the core. So what happens to the core, anyway?

That’s where compact objects come in…

Compact Objects

Compact objects are essentially objects with insane densities and gravitational fields. The three types in astronomy all share one thing in common: nuclear energy failed, and nothing but the material strength of subatomic particles could stand in the way of the object’s collapse.

We’ve already covered one type, white dwarfs. So let’s move on to sum up neutron stars and black holes.

Neutron Stars

What would happen if a star was so massive that degenerate electrons couldn’t stand up to its gravity?

That’s a neutron star for ya.

Massive stars are just too heavy for degenerate electrons. When the iron core collapses, it skips right over electron degeneracy and moves right on to neutron degeneracy. By some quirk of quantum mechanics, neutrons follow similar laws to electrons–but their material strength is a bit stronger.

If a massive star is on the, well, less massive end of the “massive” scale, then the material strength of degenerate neutrons can cut it. The core collapses into a neutron star, an object with a diameter about as wide as Los Angeles.

Yeah.

It’s still got all the mass of the core. It’s just squeezed ridiculously tight.

That creates some crazy phenomena–insane gravity, wild magnetic fields, intense radiation beams, you name it. But star death can get even crazier…

Black Holes

Yeah, yeah, you guys all knew this bit was coming, right? And not just because you read my last few posts, I’m guessing. Black holes are just the best bit of stellar evolution. They’re straight out of science fiction. But you know what they say…sometimes truth is even stranger than fiction.

Black holes are insane. And right now, we’re just talking about the babies. In future posts, we’re going to be covering supermassive black holes and other crazy phenomena that goes on in active galaxies.

But I’m getting ahead of myself…

This is one of my favorite black hole diagrams–it illustrates their most basic features very simply.

At the heart of a black hole is a singularity, an object of zero radius but infinite density. Which honestly breaks my brain a bit. Practically, how do you fit the core of a massive star–at least 3 times the mass of our sun–into zero space whatsoever?

Mathematically and theoretically, though, it makes sense. Black holes are what happens when even the material strength of degenerate neutrons isn’t enough to stand against gravity. There isn’t any more material strength to step in after that. So if a black hole occupied even a sliver of an inch of space, it would have room to collapse more, and, well…it would. It already has. That’s how it became a singularity.

As you can imagine, the theoretical implications of a singularity are pretty insane. Black holes are surrounded by accretion disks–pretty standard for a compact object. But that’s where the “standardness” stops. They’ve also got event horizons and relativistic jets.

At the singularity, gravity is so strong that the escape velocity exceeds the speed of light. And that’s called the “universal speed limit” for a reason–nothing can go faster. The event horizon denotes the distance from a black hole where escape velocity is physically possible to reach.

Doesn’t mean it’s easy, though…

And as for the relativistic jets…honestly, I’m not familiar with those yet. But as soon as I am, I’ll fill you in.


So there you have it: the life cycle of a star, from space dust to compact object! I’ll leave you with that diagram I put up top, just to sum it all up.

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