Black Hole Sun

Greetings Squaddies! Welcome back! Today we journey into the inky blackness of space to determine just how we're all going to buy it in the end, provided we all live to be several billion years old.

This week's question, is again from Andy. You too can send me your questions either via the comments, or in an email to suburbanjoe - at- gmail -dot- com. I don't want you to think that every week he sends me a question like some kind of slavering fanboy. He sent me a bunch all at a time. That's not to say I wouldn't want slavering fanboys, or fangirls, hanging on my every word and starting massive discussion board flame wars over some perceived inconsistancy in my writings. That would be very cool. No, Andy saw that I needed questions, and he sent some to me, him being a nice guy and all, unlike the rest of you worthless wretches. Um, I mean, unlike the rest of you very nice people (wretches). Did I say wretches? I meant wretches. OK, moving on now. This week I tackle my favorite question from all the ones sent in, specifically:

What if the Sun became a black hole?

Ah, the black hole. Is there nothing more exciting and mysterious in all of astronomy? A gaping maw in space, sucking in anything that dares get near to it, enveloping its prey in its crushing darkness as it sends you on a journey...to where? Good times. Most people associate black holes with stars and as we orbit a star, and depend on it for our existence, it's a common concern that one day it will collapse upon itself, turn into a black hole and suck us all into eternity. Well, maybe not common, but common enough to be easily researched on da Intraweb.

Before we talk about what would happen specifically to our little corner of the galaxy should good old Sol go all collapsoidal on us, lets talk about what a black hole is, and why it's so durn hard to get away from them. A black hole is an object who's mass has been compacted into such a small size that it's gravitational pull is irresistable. There are no special characterstics that an object has to have to be a black hole. Technically speaking, you could become a black hole, although I wouldn't recommend it, as it would require compacting your mass into a particle a hojillion times smaller than you are at the moment.

Every object that has mass exerts a gravitational pull on nearby objects. You do, your stapler does, the bus outside does, Mars does, our Sun does and on and on and on. For objects with relatively small mass like you and your stapler, the gravitational pull on objects is so ridiculously small, that nearby objects aren't affected. For big objects such as planets and the Sun, the force is much greater, and large enough to keep us from hurtling out into space both as individuals on our planet and as our planet in this solar system. That doesn't mean that you can't escape the grip of gravity, as Buzz Aldrin and the gang can attest to. In order to do that, you have to reach escape velocity. Escape velocity is the velocity at which you have to be traveling to escape an object's gravitational pull. It's a factor of an object's mass, and how far from the center of the object that you're calculating the velocity. For planets, we usually use the planet's radius for that last value, as usually rockets are launched from the planet's surface and not from some orbital platform. For Earth, the escape velocity is about 7 miles a second.

The important thing about the formula used to calculate escape velocity, is that the calculated escape velocity is inversely proportional to the distance from the center of the mass to where you're calculating the velocity. In other words, if this distance is small, the escape velocity is big. If the distance is big, the escape velocity is small. Remember we said that a black hole is an object who's mass has been compacted. Not lessened, but compacted. All that mass which had been in a very, very, very large object is now in a very, very, very small (relatively speaking) object, but the mass remains unchanged. If we keep the mass constant (which we are) for an object, and just decrease the size, our escape velocity for this object gets bigger. For black holes, the escape velocity gets so large, that it exceeds the speed of light. The gravitational pull of these objects is so large, that nothing, not even light, the fastest thing in the known universe, can achieve an escape velocity sufficient to escape it's gravitational pull. No, not even the Millenium Falcon. Yes, I know it did the Kessel run in under 12 parsecs, but the parsec is a unit of distance. Look it up.

Now, before you get your panties all in a bunch thinking that somewhere out there is a black hole that's sucking the entire universe into it's greedy jaws, remember that the calculated escape velocity is dependent on the distance between where you're calculating the velocity and the center of the object. What that means is that there are values for this distance where the escape velocity exceeds the speed of light, and values where they don't. The radius where the escape velocity equals the speed of light is called the Schwarzschild radius. Yes, yes, I know. Your Schwarzschild is as big as mine. For any distance from the center of the object greater than the Schwarzschild radius, you're A-OK. No sucking for you. Anything less than that, and well, lets just say that your calendar has been cleared for the rest of eternity. For the record, should you be pulled into a black hole, you're not going to end up in some alternate reality. You're going to be ripped apart by gravitational waves in about 8 seconds. So, maybe you will end up in an alternate reality, it just happens to be whatever version of the afterlife you cling to

Now, getting back to our good buddy, Mr Sun. As stars go, the Sun is on the smaller size, which is a good thing, because if it were bigger, it'd be hotter and we wouldn't be here. Well, not in this current, non-flameproof version anyway. Given the mass of the sun, were it to become a black hole, it's Schwarzschild radius would be about 3km, or a little under 2 miles. Given that we're 93,000,000 miles from the Sun, I think you can scratch "crushed to death by a black hole's gravitational pull" off of your ways to die list. So, from a gravitational perspective, what would happen if the Sun became a black hole is absosmurfly nothing. You'd stay where you are, it'd stay where it is, and never the twain shall meet.

That's not to say that life would be all happy-go-lucky should the Sun turn into a black hole. Stars turn into black holes when they run out of fuel and their mass is so large it begins to collapse upon itself, the important part of that sentence being "run out of fuel". No fuel means no light and no heat, which means that the Earth becomes a frozen, lightless world and we all die. Yay! But, don't worry. The Sun doesn't have enough mass to become a black hole. Instead, in about 5 billion years it'll get bigger, become a Red Giant and turn this world into a lifeless cinder. As my good buddy Robert Frost said:

"Some say the world will end in fire,
Some say in ice."

Neither one sounds particularly appealing, but at least we have all of our bases covered. Well, except for monkeys. I think the world will end in monkeys.

Sources:
The Space Place @ NASA - Phone Dr. Marc Archives
Black Holes FAQ List - Ted Bunn
NASA's Imagine the Universe - Black Holes
Wikipedia - Escape Velocity
"Fire and Ice" - Robert Frost, Harper's Magazine, December 1920