A supernova is the cataclysmic end to a massive star’s life. The entire process is (of course) somewhat complicated, but in a nutshell it runs out of fuel in its core, the core collapses, a vast amount of energy is released, the outer layers explode, and all that matter and energy gets blasted out into space.
The core can become a neutron star or a black hole (it’s even possible it gets torn apart in the blast), and the expanding gas cloud can do all sorts of interesting things, but for the star itself that’s the end of the story. It’s gone.
But up until that final cataclysm, the star can still undergo some pretty violent, near-supernova-level outbursts. Astronomers have found such a supernova, with a least one epic paroxysm or two before the last explosion destroyed it… and the supernova event may be the most powerful ever seen.
The supernova in question was in an unnamed galaxy about 500 million light years from Earth (which is a fair distance away). It was found using the Intermediate Palomar Transient Factory (iPTF), a very large camera that is mounted on a relatively small 1.2-meter telescope. It is a fully automatic survey that scans the sky looking for things that change — asteroids, various variable stars, and the like. If a supernova goes off, a spot on the sky gets brighter, and if iPTF is looking that way and the explosion is bright enough, iPTF will find it.
The software flagged the supernova, and it was quickly determined to be what’s called a Type II: a massive star exploding. Furthermore, it’s a Type II-P: Instead of simply getting brighter and then fading as most Type IIs do, the fading plateaued (hence the P), staying at a constant brightness for some time. These are not uncommon, and usually start to fade again after a few weeks or so. This happens because the huge mass of exploded material is opaque to radiation, so we only see the surface of it. As it expands and cools the outermost parts become transparent, and we see light emitted from further inside the expanding material. The expansion outward and the depth into which we see the hot material inward kinda sorta balance, and the total light emitted stays constant. At some point the expanding material becomes wholly see-through, and the light fades.
But after a little while astronomers saw that this new supernova, called iPTF14hls, wasn’t playing by the rules. Instead of fading after a few weeks, it kept…