Watch the Crab Nebula expand!

Things in astronomy tend to happen slowly.  After all, space is a big place!  It can take a long time to travel the distances between just about anything, and most of the Universe is empty enough that chance encounters are rare.  One million years is a pretty short timescale; one hundred thousand years is downright quick.  Unfortunately for us, even these "short" lengths of time dwarf the human lifespan.  That means that we rarely get to experience an astronomical event in realtime.  Instead, we're forced to hunt around the sky for examples of a phenomenon at different stages of development.

This makes this pair of images even more remarkable.  They depict the Crab Nebula as seen by the Very Large Array in 1999 and 2012.  When you flip between them, it's easy to see the nebula expanding, even over a period of just thirteen years.

So why is this object evolving so quickly?  It's a matter of fortune, really.  The Crab Nebula is an example of a planetary nebula.  Planetary nebulae are the remnants of the outer layers of an exploded star.   When a supernova occurs, these outer layers are violently thrown out into space.  The core of the star, which is extraordinarily hot, heats up the expanding gas and causes the planetary nebula to glow.

Chinese astronomers recorded a supernova in the location of the Crab Nebula in 1054.  As we've seen, that's practically yesterday in astronomical terms.  Thus the gas is still expanding very rapidly.  Eventually, it will begin to slow and cool and the nebula will fade from view.  Until then, the Crab Nebula will remain one of the most fascinating astronomical objects.  Check out the video above to see it in action.