Arcipluvian Darkness

Buckle up, because it’s time for either everyone’s favorite, or least-favorite subject.

See that picture? See the bright light near the bottom left corner? That’s a star’s death 55 million light years away frozen in time, a single frame of one of the most powerful events in the entire universe. Discovered in the mid 90’s, SN1994D is a very common way for stars to breathe their last, going supernova and expelling energy at an unimaginable rate.

One of my favorite definitions of supernova comes from Wikipedia: “A supernova is a stellar explosion that briefly outshines an entire galaxy.” This particular explosion is what is known as a Type Ia Supernova: when one of a pair of stars orbiting each other ceases nuclear fusion. The two stars work together to eventually increase the mass of the dead star to such an unstable degree that it explodes in a mighty nuclear blast.

Now there’s a lot of work behind the scenes of this terrifying release of power. Not all stars have the capacity to do it. In fact, you never have to worry about it happening to our sun at all because, hard as it may be to believe, it’s far too small. Our sun has a mass of 1.989 × 10^30 kg, and is over 800,000 miles in diameter. That’s just ridiculous, but it’s also fairly common knowledge. What you might not know is this: Our sun is a pathetically tiny star in comparison to the vast majority out there and it would need a mass of over 1.4 times what it currently has to explode. We know this thanks to the work of a man named Subrahmanyan Chandrasekhar, who discovered the upper limit of mass that a star can have and remain stable after it has run out of energy. Unsurprisingly, the scientific community named this discovery after him, and thanks to the Chandrasekhar Limit we have discovered that the Sun will suffer the fate of all minor stars: It will use all its fuel, die out, and cool down. Not that humanity will have to worry about that, we’ll all be long gone by then.

But what about the stars that are big enough? How do they suddenly go crazy and violently explode? What kind of damage does it do? What remains after they’ve been blown to smithereens? Well, I’m glad you asked.

As I mentioned before, the Type Ia supernova is the most common way for a star to not go gently into that good night. The resulting explosion is pretty epic, but it’s just totally underwhelming after the bright flash of light is over. The star just separates into stardust, and the entropy of the universe continues.

NOT.

You see, sometimes there’s more to the story of a star’s death. Sometimes, if you’re say, a star with a whopping 25 times the mass of our sun, you’ve got a very special death and subsequent existence. Welcome to the wonderful world of black holes, the fun little creatures that everyone is afraid of but no one understands. Quick, scary picture!

This is how most people think of black holes. They know that a star has gone supernova, but it leaves something behind. A nasty, collapsed core that becomes a vortex that sucks in everything around it. It’s so powerful a vortex that not even light is fast enough to escape its clutches.

Well… that’s all technically true, but before you run in fear that a black hole will soon suck up Earth and everything we know, let me tell you something. There’s one at the very center of our galaxy! Now you can run!

Orrr you can learn a little more about these dark beasts. They’re scary in concept… and I suppose in reality too, but that doesn’t change the fact that they have a very important and necessary effect on our universe.

As I said before, you have to be a very special star if you want to one day become a black hole. Our Sun just doesn’t make the grade, I’m afraid, being just a little too small. Since it can’t even enter a state of supernova, which is (usually) a prerequisite to becoming a black hole, there’s no chance it will have any purpose outside of giving gingers nightmares and beach-goers unappealing tan lines.

Anyway, let me get back on track. So as I mentioned before, a star that’s around 25 times bigger than our Sun goes supernova. In fact, this is the second kind of supernova: Type II supernova (there are more kinds of course, but I’ll save them for a later date). That explosion casts off much of the star’s mass leaving behind only a super-dense core. This core is constantly collapsing into itself, becoming ever more dense as it shrinks. The degeneration of the core eventually becomes so intense that it becomes what is called a neutron star. But wait! It’s not done. This neutron star is still collapsing, and after a long time every single part of the core’s matter is compressed into an “infinitely small, infinitely dense point called a singularity.” (source: physlink.com) This, my friends, is the center of a black hole.

Now can anything in the physical world be truly infinite? Science and all our math classes tell us “no, not really,” but it’s still a very important concept to have a working understanding of. A better term for a singularity would be “a point so dense it constantly increases its own density to an infinite limit” or something along those lines. Don’t hate, I’m no astronomer.

So this core that’s known as a singularity has a sphere of variable size surrounding it defined by another mathematical limit, coincidentally also named after the scientist who discovered it. The Schwarzchild Radius is the radius of the sphere around the singularity, and can be calculated using a complex formula that I can’t make heads or tails out of. Still, you might have heard of this sphere that it calculates: the Event Horizon of a black hole.  Now the event horizon is sort of imaginary, you can’t really see it with your own eyes, but it’s the absolute closest to the singularity anything can get and still have the possibility to escape the gravitational pull. Once you’ve gone over the event horizon, there is nothing that can save you from being stretched, squeezed, and crushed into a nearly infinitely tiny point.

Here’s an interesting question: if you were watching a spaceship getting pulled into a black hole, would it speed up or slow down as it neared the event horizon? Well, as you no doubt have figured out, gravity is an insane force. It does some weeeird stuff. Well that’s not quite right; the things gravity does are perfectly natural but can certainly seem bizarre to us. In this case, the spaceship would look like it was slowing down the closer it got to the black hole, and the moment it reached the event horizon it would seem to stop, and we’d never see it move again.

What?

As we know, even light cannot escape the event horizon. The reason for this is of course the gravitational pull is too strong even for particles of light (aka photons), meaning that the escape velocity of a singularity is greater than the speed of light. So an object moving away from the black hole, like light, would be getting slowed down by the gravitational pull. This means that the photons are moving far slower than normal, and if you’re watching from a safe distance, it takes those photons longer and longer to reach you the closer they get to the event horizon. As we’ve previously defined, the event horizon is the point where nothing escapes the pull, so as soon as we saw the spaceship reach it, the speed of the photons given off by the craft would become zero and we’d just see it as if it was frozen in time. Now that’s only from an observer’s perspective. The spaceship wouldn’t even notice passing through the event horizon and would continue on its merry way into the singularity until its destruction at the hands of gravity.

So now that we understand a little more about black holes, let’s revisit a statement I made earlier in this article. You remember, when I said there was a black hole in the middle of our galaxy! Wanna see it? Of course you do.

Meet the lovely Sagittarius A*, the supermassive black hole in the center of the Milky Way, only 26,000 light years from where you sit. At a mass four million times greater than our sun’s, it’s… small. Kind of a trend when it comes to Earth’s surroundings, isn’t it? Everything scaled to the perfect size for humanity’s existence. Thank your lucky stars (heh heh). Now as always, when I say small I’m talking  in relative terms. This baby is big, but not as big as other supermassive (and yes, that is the technical term) black holes that reside at the center of other galaxies. Using NASA’s website description of these photos (which is also where I got the image), let me break them down for you.

“The brightest white dot is the hottest material located closest to the black hole, and the surrounding pinkish blob is hot gas, likely belonging to a nearby supernova remnant. The time series at right shows a flare caught by NuSTAR [Nuclear Spectroscopic Telescope Array, NASA’s black hole hunter] over an observing period of two days in July, 2012; the middle panel shows the peak of the flare, when the black hole was consuming and heating matter to temperatures up to 180 million degrees Fahrenheit (100 million degrees Celsius).

The main image is composed of light seen at four different X-ray energies. Blue light represents energies of 10 to 30 kiloelectron volts (keV); green is 7 to 10 keV; and red is 3 to 7 keV. The time series shows light with energies of 3 to 30 keV.”

Now before I go in-depth into Sagittarius A* (you can just call her Sagittarius A-star), I’d like to touch on the subject of supermassive black holes. As far as we know, they’re at the center of every galaxy; in fact the very existence of a galaxy is directly tied to the size of the supermassive black hole at its center!

I feel that at this point I should mention that black holes aren’t always in a constant state of “eating” everything that falls into their gravitational pull. They can pull in too much for them to handle all at one time. Furthermore, like anything with a strong gravitational pull, an object can orbit them without fear of ever getting dragged to the core. Did I blow your mind? Asteroids, planets, even other stars can orbit black holes much the same way satellites orbit the Earth. After all, they aren’t that special. They have extraordinarily strong gravitational pulls, yes, but they’re still beholden to every law concerning gravity we’ve discovered.

One more thing: black holes expel material. Yes, contrary to everything you’ve ever learned about them, they push stuff away from them as well as suck things in. In fac-Hold on! you say. You’re telling me that everything about singularities being inescapable is wrong? Why’d you teach it to me in the first place?!?!

Ah, well you see while it is indeed true that nothing can escape the event horizon of a black hole, matter that gets very close to it, but remains outside, has a variety of odd things that can affect it. As an object begins to fall towards the black hole, it heats up. There is an incredibly dense cloud of gas and particles surrounding a black hole, all fighting with each other and getting hotter and hotter. Because of all the energy being released, reused, and otherwise transformed, massive explosions and other phenomena can often send particles of stardust far away from the black hole, where they eventually cool and become asteroids or other planetary bodies. This is how scientist believe the universe was created, and would be a very good solution as to why there are supermassive black holes at the center of most galaxies. Interesting side note: A current theory that’s being examined is that the very oldest black holes are in fact these supermassive types, forming right around the time of the “Big Bang.”

So to bring all this back to our topic of Sgr A* (a common abreviation for our very own black hole, since scientists are pretty lazy), it appears that the swirling mass of stardust surrounding it is what eventually formed the entire Milky Way galaxy. This is the very same galaxy that you can see from Earth on occasion, if that gives you a sense of how vast it is.

A single galaxy is practically impossible for us to comprehend, for our very own solar system is inside that very same galaxy pictured above. It looks far away, but we are on the edge of the whole thing. Trillions and trillions of galaxies exist in the ever expanding Universe, and we’re pretty sure black holes are at the center of each one, as well as popping up from the corpses of gigantic stars to boot. Fortunately, one will never suck us up, unless we discover a portal to one like in the crazy movie Interstellar (very good one too). There’s still a thousand things that we don’t know about them though, but I look forward to the scientific community learning more about how they operate and form. And who knows, maybe they are portals to other worlds, like some scientists actually claim! But that’s a post for another time. Until then- stay curious.