Hey, SciShow viewers, it's John Green.

So over on Crash Course, we just launched a limited series podcast called The Universe, where the astrophysicist Dr. Katie Mack walks me through the history of the entire universe, including the parts that haven't been written yet.

Take a look. So today we are going to pause from our cosmic timeline, where we've been going through the first seconds and then millions of years of the universe, and we're going to take a quick gander at something I have genuinely, deeply no understanding of, which is black holes. I mean, you are painting onto a blank canvas right now, Doctor Mack. (Katie Mack) Okay, there’s a lot of weird stuff about black holes. (John Green) So how come they don't suck in everything? (Katie Mack) Well, because not everything is already falling toward the black hole. (John Green) Oh, okay.

So it's only things that are already falling toward the black hole. It doesn't, like, expand infinitely. (Katie Mack) Yeah, it can't, like, reach out to distant things. (John Green) But if you get - if I got near it - (Katie Mack) You get near it, it’s a problem. (John Green) If I got in a spaceship and got close to it that would be a big problem. (Katie Mack) Yeah. The way that gravity works, like, let's say that you're in, like, I don't know, like a giant cloud of gas or something, and it's, there's a whole lot density at the center, but you're kind of toward the outer edges.

You're going to feel the gravity of everything within, like closer to the center than you. Like, you can draw, like, a sphere, and you're at the edge of that sphere and everything close to the center, you're going to feel the gravity of that. If you get closer and closer to the middle, you're just going to feel the gravity of the stuff interior to you.

And so if you get really far away from that gas cloud, you're going to feel as much gravity as if all of that whole glass cloud was compressed to a single point in the center. The amount of gravity that you'll feel is the same, because you're still just feeling all of the gravity of the stuff interior to you. (John Green) Okay. Okay. (Katie Mack) So what matters is how close you are to the center of, like, where the matter is concentrated. (John Green) Okay, so if you made the earth half the size that it is now with the same density. (Katie Mack) Well, the same amount of stuff. (John Green) Same overall amount of stuff, but in half the size, I would still walk around the earth feeling the way I feel now. (Katie Mack) Let me put it this way: If you could keep the outer layer of the earth, but compress all of the rest of it into a smaller space, you would feel the same. (John Green) Right, right, right.

Okay. (Katie Mack) But if you actually compressed it all and then you were closer - you were down there, right? If you compressed everything and then you were down there, then it would feel - you would feel more gravity because you're closer to the center of where all that gravity is. (John Green) Okay, okay. (Katie Mack) However you redistribute the matter, as long as it's still kind of interior to you in that sort of sphere. It's how far you are from, I mean, to first approximation, you can - there are things that can change, but like, you know, how far away you are from the center.

So like a neutron star, a white dwarf, you know, the sun, they can all be around the same mass, like around a solar mass. There's some variation. Neutron stars have to be a little bit more massive, but, you know, they can be in the sort of in the same sort of range of masses.

But it would kill you a lot more to be... The gravity would kill you a lot more to be on a white dwarf or a neutron star than it would on something as massive as the sun, because all that matter is much more compact. You'd be closer to the center of it.

So it's the compactness of matter that really affects how much gravity you feel. (John Green) And because these black holes are really, really, really compact. (Katie Mack) Then if you get close enough to them, you get really screwed up. But if you're far away, like if you have a black hole that has the same mass as the sun, if you're far enough away from it, you don't feel any different gravitationally than if it were just a regular star. So the same reason the sun is not sucking up all the matter around it, a black hole won't, either. (John Green) Got it.

Okay. But, if you get close enough, it will. (Katie Mack) If you get close enough, then things get really weird. (John Green) There's a lot more where that came from. The Universe is available now, both on the Crash Course YouTube channel and wherever you get your podcasts. [♪Music♪]