To see a nova is to witness a cataclysm. All involve close binary
stars where a tiny but extremely dense white dwarf star steals gas from its companion. The gas ultimately funnels down to the 150,000 degree surface of the dwarf where it's compacted by gravity and heated to
high temperature until it ignites in an explosive fireball. If you've ever wondered what a million nuclear warheads would look like detonated all at once, cast your eyes at a nova. Novae can rise in
brightness from 7 to 16 magnitudes (one magnitude equals 2.5 times brighter), the equivalent of 50,000 to 100,000 times brighter than the sun, in just a few days. Meanwhile the gas they expel in the blast
travels away from the binary at 700 to several thousand miles per second.
As described in the Spectroscopic Atlas for Amateur Astronomers by Richard Walker:
A supernova explosion totally destroys the star and forms the definitive
end point in its life. By this cataclysmic runaway reaction an unimaginable amount of energy is set free and almost the entire stellar mass, initially with> 10,000 km/s, is distributed to the surrounding
space. For comparison: the detonation velocity of our most rapid explosives just reaches ~ 8km / s (Nitropenta). As a result of such an explosion, the interstellar matter (ISM) is enriched with heavy
elements, which decisively influences the later formation of stars, planets and finally also of possible life. The diameter of old Supernova Remnants (SNR) may finally reach up to some 100 ly, so eg the
famous Cygnus Loop. Otherwise the diameter of the relatively young Crab Nebula M1 is just about 11 ly. The image (HST) shows the SN 1987A (SN type II) in the Large Magellanic Cloud (distance ~168'000 ly) –
about 20 years after the apparent explosion time.