A baffling, long-lived supernova

The vast majority of the thousands of supernovae observed so far are easily categorized based on their brightness, duration, and spectrum. At first the supernova iPTF14hls, which was detected in sky survey images from the Intermediate Palomar Transient Factory telescope in September 2014, seemed to check all the boxes for qualifying as a type II-P supernova. In particular, its optical spectrum contained hydrogen absorption lines, suggestive of an expanding shockwave triggered by the collapse of a massive star into a neutron star. But then iPTF14hls did something strange: It kept shining. Type II-P supernovae typically exhibit constant luminosity for about 100 days before fading, as shown in blue on the graph; iPTF14hls glowed for more than 600 days and, as the graph illustrates, peaked in brightness several times. Also unlike the behavior of other type II-P explosions, the velocity and temperature of the light-emitting region remained relatively constant.