What would happen if the star Sirius exploded into a supernova?
Since Sirius is about 8.6 Light years distant, it would take 8.6 years for the light from a supernova event to reach us. Although since Sirius is a main sequence star similar in mass and luminosity to the Sun, a supernova would be very unlikely, more probably a nova or red giant phase. Due to the mass, it is very unlikely that anything of that nature would happen in the near future, it would probably happen long after our own sun has died. Bear in mind that the Sirius system is estimated at only 200-300 million years old while the sun is 4.6 billion years old.
