Supernovas are a rare occurrence for astronomers to observe; not only do they happen very far away, they also last for a few days at most. However, an international team of astronomers observed one over a billion light years away take over 50 days to reach its maximum brightness, then slowly dissipate over a similar time period. This event, named OGLE-2014-SN-131, was likely caused by a very massive (40 to 60 times the Sun’s mass) star interacting with a helium gas cloud and creating shock waves. This interaction is what supposedly caused this supernova to last far longer than any others.
Supernovas occur when a star much more massive than our sun uses up its Hydrogen fuel, then resorts to Helium fusion, then to Carbon fusion, until it is no longer capable of fusing any more elements and implodes. Stars can fuse any element up to Iron, any heavier elements are created in the few milliseconds of the implosion. The article states that the supernova was likely enhanced by a Helium cloud that was being produced by the star, this implies that the star was in a metal-poor (part of a) galaxy, and that the star likely produced several heavy elements.