A supernova is usually a huge explosion that destroys a star, leaving only a small remnant of material behind. But astronomers recently discovered a star that went supernova at least twice — meaning that the model for star explosions needs to be re-examined.
Stars shine from regular fusion of elements inside of the core; for example, our sun currently fuses hydrogen nuclei into helium. The sun isn't big enough to go supernova, but stars that are eight to 15 times the mass of the sun can. They either generate supernovas called Type II supernovas, or collapse into black holes.
If the stars are going supernova, they first fuse heavier and heavier elements up to carbon. Once the star's core passes a mass called the Chandrasekhar limit, the star implodes, the core grows even warmer and denser, and the implosion causes the core to rebound and explode. When the core bursts, it shoots the star's material into space, creating a supernova.
But that's not what happened in the case of iPTF14hls, an explosion that was found in 2014 by the Palomar Transient Factory, which is an automated camera survey that looks for events in the sky such as exploding supernovae or comets passing by. As usual, astronomers did a spectroscopic analysis of the explosion, yielding information about its chemical composition and the speed of the explosion.