Basically, the sun is expected to use up all it's fuel in something like 5 billion years time. At that point it will become what they call a "red giant" star, when it goes it to one of the final stages of it's life. When it gets to this stage the processes that keep it "alive" change and it expands to the point where earth will become uninhabitable due to the extreme temperatures. We may even be swallowed up by the sun and burnt to a crisp.
From everything I've read or watched, this is the greatest threat to the earth as a planet.
When you're having this discussion, you also have to make the distinction between threat to "life" and threat to "earth" as a planet in the solar system.
I haven't read or seen anything about this Yellowstone volcano, but I'm gonna take a look at some of these docos as it seems pretty interesting and it's the sort of stuff fascinates me. But, it appears it's more of a threat to life on earth rather than earth itself. In which case you can add it to the list of theories which includes things like stray asteroids, mega earthquakes, solar storms eroding our magnetic fields, gamma-ray bursts... the list goes on. The one thing they all have in common is the chances are pretty remote.
