At peak light output, supernova explosions can outshine a galaxy. The outer layers of the exploding star are blasted out in a radioactive cloud. This expanding cloud, visible long after the initial explosion fades from view, forms a supernova remnant . Because of their similar appearance, the distinction between supernovae and the unrelated and intrinsically much fainter novae events were not appreciated at the time of the Curtis-Shapley debate. Stretching of time produced by relativity.
Time dilation is a predicted effect of the cosmological paradigm.
The existence and degree of time-dilation effects in GRB data is controversial.