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.

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Stars: The Limits of “Normal”

How big—or small—can a star get?

As with most questions in astronomy, the answer to that is not definitive. But stellar models can give us a pretty good idea.

Mathematical models of stars tell us that their life—or, to use a less personifying term, function—depends on the balance between two opposing forces: internal pressure and gravity.

Stars produce energy to function. They don’t just do this to light up our skies and provide for life on their orbiting worlds. They need to produce energy to constantly support the weight of their own mass.

The more massive stars are, the more energy they need to produce—and the reverse is true too. There has to be a balance.

But is there a limit? Is there a point where balance is impossible?

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What is Contagious Star Formation?


Recognize this nebula?

Yeah…we’re talking about the Orion Nebula again. I know, we already took a tour through the Orion constellation in my last post…but there’s still more to cover about how stars come to life, and Orion is still the best case study I know.

So…hold up a second. Contagious star formation? What’s that supposed to mean? I mean, usually, when you think about “contagion,” you think of catching diseases from others around you. So…can stars get sick?

Well, no. Stars are pretty good at maintaining their own homeostasis, something I’ll explain in a later post. By “contagious” star formation, I mean that star formation can trigger more star formation.

Basically…forming stars is contagious.

But how the heck does that happen?

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How are Stars Born?


Astronomers have discovered that the clouds of gas and dust—the interstellar medium (ISM)—found between the stars are made of the same materials as the stars themselves. In fact, hydrogen is the most common element in both stars and the ISM, followed closely by helium.

But it would be more accurate to say that stars are made of the same material as the ISM, not the other way around.

This is because all of the stars formed out of material in the ISM at some point millions to hundreds of billions of years ago. And when they die, they return that material—what’s left of it—to the ISM.

Specifically, stars form out of the giant molecular clouds (GMCs) of the ISM. But how?

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Where Did the Interstellar Medium Come From?


Contrary to popular belief, space is not empty. The space between the stars is filled with clouds of dust and gas. And this space—the interstellar medium—is incredibly beautiful and fascinating.

I often refer to the interstellar medium as the galaxy’s “backstage.” Why? Because it’s not the part of the universe that astronomy enthusiasts usually think about. And yet, there are whole studies devoted to studying this natural wonder of the universe.

Also, the interstellar medium is largely hidden from us. There are ways we can detect it—when light from a distant star passes through it, for example. And with our eyes, we can see nebulae, the visible evidence of this interstellar expanse.

The backstage of a theater is similar—it’s not the main part of the show, but you sometimes see evidence of it in the forms of new costumes donned as the play progresses and new props brought into play. The audience often forgets about it entirely.

Nevertheless, it’s beautiful. Stars are born out of giant molecular clouds, triggered by compression from expanding bubbles of coronal gas. The interstellar medium spells our beginning.

But how did it get there?

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