Some astronomers are wondering if Betelgeuse is shrinking because it's used up hydrogen as its main fuel and is now "burning" from helium and other heavier elements. Which would mean that it's in the final stages of a star of its class. When it starts trying to fuse elements like iron and such, some positively hideous astrophysics come into play and Betelgeuse's internal energy is going to overwhelm its immense gravity...
...and we'll get to see a rilly big bang called a supernova.
The supernova of 1054 (the event that created the Crab Nebula) was recorded all over the world, including by Native Americans in what is now the southwestern United States. It was so bright that it could be seen during the daytime and reportedly even cast shadows at night.
If Betelgeuse goes supernova - and some are saying there's a likelihood that it might happen within the lifetimes of many people reading this - then it would very likely dwarf the brilliance of Supernova 1054.
I can't begin to imagine how awesome a sight that would be.
'Course, if Betelgeuse does go supernova, it's probably already happened since the star is around 600 light years away. In which case we're just now finding out about it... but it won't make it any less spectacular "when" it finally happens :-)