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.
Okay, good question. How the heck do you find an object that emits no radiation? Astronomers find—and study—just about everything in the universe using the radiation it emits or reflects. So…what happens when the object we’re looking for has such a strong gravitational pull that even light can’t escape?
Well, that’s when we need to turn to the theoretical science behind black holes. What measurable effects do they have on objects in their vicinity? Can we detect them indirectly?
Of course, some of you might be screaming at me that we’ve already photographed a black hole—in visual wavelengths! Yes, astronomers did make that achievement—we now have visual proof that what we’ve been theorizing all along is indeed real.
But that black hole was so faint, it took an interferometer the size of the Earth to image. We had to know exactly where to look in order to get that picture.
So how the heck do we find one in the first place?
If you’re a sci-fi fan, you’ve probably seen these in movies. And I’m guessing you’ve heard a lot about them in pop culture. The problem is, pop culture and movies don’t do a very good job of describing black holes.
First off, let me clear up a common misconception: Black holes do not act like giant space vacuum cleaners, sucking in everything around them. Describing them as “gobbling up” anything is inaccurate.
The representation in movies that bugs me the most is in J.J. Abrams’ Star Trek reboot, when the bad guy falls into a black hole and the good guys almost get pulled in with him. First of all, please…black holes do not growl. And basically none of what happens in that scene is accurate.
For those of you who missed my last couple of posts, allow me to introduce the neutron star: a stellar remnant similar to a white dwarf, but much denser, so dense that its protons and electrons have combined to form a neutron soup.
A neutron star forms from the collapsing core of a star between 10 and 20 M☉ (solar masses). Its collapse produces powerful magnetic fields and extremely high temperatures, but because it becomes so small—less than the size of Los Angeles—it is very faint and radiates away its heat very slowly.
The exception to that rule comes in the form of two powerful beams of radiation that blast away from the object’s magnetic poles. As a neutron star spins—at around a hundred times per second—these radiation beams sweep across the sky like the the beams of a lighthouse.
If these beams happen to sweep over Earth, human observers see regular, rapid pulses of light. This visual phenomenon produced by neutron stars is called a pulsar.
Now that we have a basic understanding of neutron stars and pulsars, let’s explore some of the details of how these extreme objects work.
Imagine you’re observing the sky with a radio telescope. Observing the faintest, lowest-energy photons the universe has to offer is your specialty. You study interstellar dust clouds, protostars, and lots more.
One day, though, something interesting pops up in your data. You’re looking at raw data on a computer screen, not an eyepiece of a “typical” (optical) telescope—you get all your data from the giant dish above. Strangely enough, there’s a series of regular pulses.
At first, you think it’s just “noise” from sources on Earth—like static on your car radio. But then you see it, day after day, in the same place in the sky. It’s not static. It’s real.
You wonder if this is perhaps evidence of contact with a distant civilization. Personally, I’d hope for that one. Unfortunately, more research leads to the conclusion that it’s nothing of the sort—within weeks, you find that there are several other objects in completely different parts of the sky, all emitting similar (but different) pulses.
You’ve discovered a pulsar. But…what exactly is a pulsar?
Above is a theoretical rendering of a white dwarf, the collapsed husk of a low-mass or medium-mass star. Interestingly enough, these strange cosmic objects—which begin their existence as intensely hot balls of carbon the size of the Earth—may eventually cool off and crystalize into giant space diamonds.
White dwarfs are made up of free-floating hydrogen and helium nuclei and degenerate electrons—and their mass is supported by the nature of these electrons.
But degenerate electrons, like any other material, have a specific material strength. What happens if they’ve, well…just got too much stuff to support?
In the constellation of Perseus, there is a star named Algol that exists in a binary system. The binary consists of two stars: a massive main-sequence star and a less massive giant.
According to what we’ve explored so far…that doesn’t make any sense.
More massive stars evolve faster than less massive ones. They expand into giants before less massive stars do. In any one binary system out there, we should observe a more massive giant and a less massive main-sequence star, not the other way around.
But the Algol system is not alone in this peculiarity. Over half the stars in the universe are binaries, and in a number of those, the more massive star is still on the main sequence.
Now that we’re finally talking about white dwarfs, we’re getting into the really cool stuff.
In my last post, we explored planetary nebulae, and we left off with a question: where does the fast wind that forms planetary nebulae come from? Well, remember that planetary nebulae are formed from the atmospheres of medium-mass stars, but there’s still the stellar interior to worry about.
White dwarfs are objects comparable in size to our own Earth. They are the remains of medium-mass stars like our own sun. Often, you can see a white dwarf at the center of a planetary nebula with a large telescope. Together, they form what’s left of a star after it loses stability completely.
When we talk about star death, we’re not really talking about death. We’re talking about the end of a functioning star. Astronomers tend to personify cosmic objects like stars, saying that they are born and die, when it’s more like they transition into something new.
With stars in particular, there’s two main courses their “life cycles,” such that they are, can take: one for massive stars and one for low-mass stars.
We can further subdivide low-mass star “deaths” into those of red dwarfs—like our nearest stellar neighbor, Proxima Centauri—and those of medium-mass stars, like the sun.
But before we dive into the final stages of these stellar life cycles, let’s review what kinds of stars we’re talking about here…
If we were talking about people, I’d say there’s no such thing as a “normal” person. We’re all weird in our own way—that’s what makes us unique and ourselves.
However, there’s such a thing as a functional human—a human with a combination of functional organ systems and/or prosthetics that makes daily life navigable. And just as no star is exactly alike, there are functional stars.
Nature makes mistakes all the time. It is not intelligent—it doesn’t know the best way to do anything. It doesn’t know the path of least resistance or least effort. It just tries everything at random, and we get to observe what happens.
A “normal” star is what happens when nature stumbles upon the right conditions. But…what does that mean?