A supernova has gone off in the nearby galaxy M82, which is located in Ursa Major, well placed for viewing right now in the Northern Hemisphere. The supernova itself is what we call a Type Ia, a dwarf explosion. Astronomers are still trying to figure out exactly what happens in a Type Ia explosion, but there are three competing scenarios. Each involves a white dwarf, the small, dense, hot core left over after a star turns into a red giant, blows off its outer layers, and essentially “dies.” One scenario is that the white dwarf is orbiting a second star. It siphons off material from the star and accumulates it on its surface. Eventually this material gets so compressed by the huge gravity of the white dwarf that it fuses, creating a catastrophic explosion that tears the star apart.
Another is that two white dwarfs orbit each other. Eventually they spiral in, merge, and explode. The third, which is a recent idea, is that there are actually three stars in the system, a normal star and two white dwarfs. Due to the complex dance of gravity, the third star warps the orbits of the two dwarfs, and at some point they collide head-on! This too would result in a supernova explosion. All three scenarios involve very old stars, since it can take billions of years for a normal star to turn into a white dwarf.
The galaxy M82 is undergoing a huge burst of star formation right now, and that means lots of massive stars are born. These live short lives and also explode as supernovae (called Type II, or core collapse) though the mechanism is very different from that of the white dwarf explosions. You’d expect M82 to have more core collapse supernovae, but this new one is a Type Ia.
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