They say there is quite a few astronomical events that could wipe out life on Earth. It's up to us if we believe them or not.
If your worried about big celestial bodies hitting us - fear this monster if it ever came our way its in this galaxy:-
http://www.youtube.com/watch?v=k7hsQA3wo3Q
http://en.wikipedia.org/wiki/VV_Cephei
On the Wiki page you can see its size in comparison to our own Sun.