Stellar Cop
I like being a Stellar Cop. Most of the time it’s an easy job. Occasionally, however, something
interesting happens.
All is peaceful in the Universe…until it’s not. One day all the stars are happily coexisting in
equilibrium, and then, BAM, there’s a dead star floating in the void. Was this just the end of a
long happy life, or is there mischief involved? Did someone upset someone else’s hydrostatic
equilibrium to the point that nefarious things happened?
It is true that stars take their hydrostatic equilibrium seriously. They like their lives to remain
finely balanced between the gravitational force of their mass, and the gas pressure due to the
energy generated in their cores. Most of the time they regulate themselves, doing no harm.
Occasionally if the energy generation in the core slows down, gravity causes them to contract a
bit, but this usually leads to an increase in temperature and pressure, which leads to higher
energy generation rates and…well, things return to normal.
“Nothing here to see, folks. Everybody just move along now and give a star its privacy.”
So they generally police themselves with this and return to their equilibrium. Sometimes
though, somebody doesn’t make it, and that’s when we have a dead star on our hands.
It’s said that dead men tell no tales, but dead stars are another matter altogether. If we were to
apply forensics to the death of a star, we would recognize three distinct corpses. First, the
white dwarf, which is the remnant of a low mass star. Next, we have the neutron star, which is
the core of a higher mass star. Finally, we have Gravity’s ultimate victory, the black hole, which
is the remnant of a particularly massive star who tried to take on the King of the Universe,
Gravity, and lost. Here follows the Nick Charles/Perry Mason denouement of how we ended up
with each of our victims.
Our first victim, the regular-Joe white dwarf, started life as a nebula, like any other star, but this
little guy just lived a simple life (once he developed his full potential) as a cool red star. No highroller here, just an average stable, main-sequence guy with hydrogen fusion as his only energy
source, living a good, long simple life. What internal heat he was able to produce is probably all
that kept the little guy from collapsing under his own gravity. He never got hot enough or got
high enough pressure together to fuse helium, though, and eventually, he just burned up most
of his fuel, and his fusion just stopped. Poor little guy.
He shrinks as he dies: same mass taking up less space. Some of these guys can be the same size
as that little blue planet called Earth, but their densities can be a million times the density of
water. See, his gravity increases and pushes the electrons as close together as possible until
they kind of start to bump into each other. In the business, we call this degenerate electron
pressure, just to give it a name you see, not to be fancy or anything.
The more the electrons are squeezed together into a very small space, the faster they move. If
the white dwarf’s mass approaches 1.4Msun , his electrons have to be getting close to moving at
the speed of light. This is about as massive as this little guy can get because, hey, nothing can
move faster than the speed of light. I mean, it just doesn’t happen, so don’t even try to go
there, Star Wars or no Star Wars. Now, in the business, we call this the Chandrasekhar limit.
Typically, that’s all there is to it. The little guy glows for a while and then just fades away as a
black dwarf. Okay, if you put part of him on a teaspoon it would weigh as much as a Sherman
tank, but otherwise, end of story.
Okay, let’s move on. This next victim is a high-roller wanna-be. Funny that he ends up being the
smallest of the small white dwarfs…See the smaller the star was, the larger a white dwarf he
ends up as, and the bigger the star, up to a point (we’re talking just under 3MSUN here) then the
smaller a white dwarf he becomes. This guy had big plans, but he still ended up a white dwarf.
Eventually, this guy becomes a puffed-up, bloated mass of red, calling himself a Red Giant
because he isn’t big enough to hang out with those high-rollers, the Red Supergiants…just
pathetic and sad, in my opinion, but hey, he’s doing it to himself, and every star has a right to
be how he wants, right? Except now, he’s fusing hydrogen and helium like it’s going to last
forever, just using it up fast, fast, fast until, eventually, he’s done. His fuel is exhausted and he
just collapses. Now this guy started bigger, so he ends up smaller because he had more mass to
make more gravity. Everything pulls in a little tighter, you see. If a teaspoon of a regular-Joe
white dwarf was like a Sherman Tank, a teaspoon of this guy would be, like, a million Sherman
tanks…well you catch my drift…really, really dense.
Now, guys like this do sort of go out in style; I’ll give them that. As they start to collapse from
their Red Giant phase, these guys do something really classy: they cast off their outer
atmosphere, and give themselves a kind of “halo,” you know, the gas cloud that comes off
becomes a sort of halo. When they turn into this hot dense white dwarf in the middle of this
halo, they heat up that halo, and it glows. Like I said, classy.
Now we get to the really high rollers, the massive stars that thought they could beat Gravity.
Well, Gravity rules the Universe, and nobody beats Gravity.
These guys start out all high and mighty. I mean, these guys are big stars to begin with, but
then they just go crazy and turn into this HUGE thing called a Red Supergiant.
At this point, you want to stay out of this guy’s way because once he gets to this stage, he’s not
going to last long. Once he’s used himself up, the electrons are forced to combine with protons
to produce neutrons, never a good idea. In the business we call this degenerate neutron
pressure. That’s why we call them neutron stars, get it?
If you thought the second guy (the guy who ended up as the smaller white dwarf, remember?)
was going through his fuel fast, well this guy has him beat. Eventually this guy will also run out
of fuel, but does he just quietly collapse into a nice little white dwarf? Does he do something
classy like make a nice halo for himself? No, this guy starts creating iron, getting more and more
dense, until finally he’s do heavy he IMPLODES! What a mess!
See without his heat engine to keep everything in balance, to generate the gas pressure to keep
his huge gravitational field in check, he collapses…but this guy doesn’t just collapse, he
implodes and the rebounding shock wave from his insides is ENORMOUS. Holy crap-on-toast!
What a mess! This guy goes supernova. I haven’t seen many of these in my time, but it does
happen. Now, he’s still in there, but with all the gas debris, and with all his fuel burned up, you
can’t really see him. The guy is just a sad shadow of his former self. Occasionally, we get pulses
of stuff off of him, but he’ll never recover from that implosion. Poor sap!
Okay, now, if this guy is say, only a bit larger than 3 solar masses, once the supernova is over,
we get a nice light show, and everything’s fine. Nobody really gets hurt. We get the bejabbers
scared out of us when he goes off, but otherwise, we’re all okay.
Sometimes though, one of the really big boys (now I’m talking big…like 8 solar masses big)
decides to take on the King…well, he loses, badly, and Gravity uses this guy to show us all just
how much of a King of the Universe he really is. When one of the big ones blows, Gravity gets
his ultimate victory and turns the guy into (dare I say it?) a BLACK HOLE.
The guy started out big, so Gravity has a lot to work with, turning this guy into a real tool. He
collapses this dude into something so small and so dense that the guy’s surface gravity is strong
enough to pull in light. Yeah, LIGHT! (Hence the name, Black Hole, get it?) Gravity doesn’t let
ANYTHING out of that. Get too close and you get swept up by Gravity, never to be seen or
heard from again, so far as we know anyway. Gravity. Don’t. Play…and he takes his Rule of the
Universe very seriously. I mean, dude, Gravity can change the passage of time. TIME!
So, there you have it. The life of a stellar cop can be lonely, I mean, we’ve got a lot of open
territory to cover. Most of the time, you just enjoy the view, and go about your business
keeping the constellations in line, watching the planets rotate and orbit, watching a young
nebula take its first steps into a protostar, then watching that little protostar grow up. Just gets
ya right…there…you know?
Every once in a while though, you have something “interesting” to contend with. On those
days, you just stick to the Laws of Physics, keep your nose clean, and stay out of Gravity’s way.
Resources
Tarbuck, E., Lutgens, F., & Tasa, D. (2014). Earth science (14th Ed.). Upper Saddle River, NJ:
Prentice hall.
Copyright: Donna Matthews, Galena, AK November, 2016