Sirius itself can't undergo a supernovae explosion, but it can induce one in it's companion. Type Ia supernovae are the collapse of white dwarfs, caused when they steal to much matter from a nearby star (only a few AU away of course, the other star in the binary). <br /><br />The reason for the collapse is the supporting force of the white dwarf, electron degeneracy pressure, can only support up to 1.4 solar masses. If it's tipped over that scale, it collapses and explodes in a supernova.<br /><br />Of course, I believe the white dwarf is to far from the companion stars to steal the matter, so it's no big deal.<br /><br />As for which form of radiation gets here first, vogon had it right. All forms of light will get here at the same time, unless delayed by part of the explosion process (absorbing materials being opaque basically delaying the escape and emission of a type of light), then Neutrinos, then cosmic rays (several days to weeks later).<br /><br /><br />If there was a supernova at that distance, earth would be pretty blasted and possibly sterilized. <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>