How close a supernova would have to be to wipe out all life on Earth

[ahlam1399]

http://static-global-s-msn-com.akamaized.net/img-resizer/tenant/amp/entityid/BBGmQoQ.img?h=100&w=100&m=6&q=60&o=f&l=fAt a certain distance, even a small supernova represents a threat to life on Earth.

أكثر... (http://www.msn.com/en-ca/video/wonder/how-close-a-supernova-would-have-to-be-to-wipe-out-all-life-on-earth/vi-BBGnOnb?srcref=rss)