In 1967, Jocelyn Bell Burnell and Antony Hewish observed, in the night sky, strong radio pulses separated by 1.33 seconds, like some sort of cosmic alarm clock. While these radio bursts were all but certain to be natural, the source was named Little Green Men-1, or LGM-1. What Burnell and Hewish had, in fact, observed was what has come to be called a pulsar.
But what, exactly, is a pulsar? ...
Prior to this posts, I've discussed the history of the universe, as we currently can observe and understand it, up to present day (give or take a few million years). If you missed those, here's Part I and Part II, which will catch you up. But really, as cool as the history of the universe is (and it's pretty neat), I wrote those posts so that I could write this one, about what happens next. Fair warning, it's fairly bleak and existential, albeit fascinating. Now, with Halloween around the corner, I've got a scary story for you: how the universe will (likely) end.
So, a year and some change ago I did an absurdly wordy post that may have had slightly too many run-on sentances about the early history of the universe (If you didn't read it, here's A Brief History of the Universe, Part I). I then promised to follow it up with more history up until present day or so, and never did. Until now. And while you may think it took me a long time to follow up with a part II to that post, that time span is just peanuts to the universe. In fact, it's been around for about thirteen billion times longer than the wait between the first part of this series and this follow-up. So, cosmologically speaking, the delay was perfectly acceptable. Now, let's get on with it.
Let's talk a little bit about our galaxy. You probably know that it's a spiral galaxy, with several "arms" reaching out from a central bulge, and is shaped like a disk. Most of our galaxy's mass is centered in that central bulge, with is about 13,000 light-years from top to bottom (which is really, really big), and has a density of around 1,600 stars per cubic light-year. To put that in perspective, out where we are in the Milky Way it's only a few thousand light-years thick (which is still really big), and the stellar density is closer to 0.004 stars per cubic light-year.
Picture yourself on the surface of the Earth (or in a boat on a river). Now, if you were to toss something up into the air, it would eventually be pulled back down, due to the effects of gravity. Obviously, this force is not indomitable, as we've escaped the Earth's gravitational well many a time (most recently, when we sent something awesome to Mars—because orbit doesn't really count as escaping the Earth's gravity). In fact, if you were to toss something up with enough force, it would fly away forever, never to be seen again. The speed you instantaneously imparted on that object when you threw it is the Earth's escape velocity. Now, let's say we increase the mass of the Earth to that of Jupiter, but keep the size the same (we make the Earth much, much more dense). Now, in the instant before you were liquified by it, you'd notice the gravity has become much, much stronger than it was. This makes perfect sense, as gravity is a property of mass.