Complex sentence: Two years earlier, the progenitor star produced a supernova imposter that was detected by Japanese amateur astronomer Koichi Itagaki, a high energy outburst that did not lead to stellar collapse.
Split into simpler sentences: Two years earlier, the progenitor star produced a supernova imposter that was detected by Japanese amateur astronomer Koichi Itagaki. The star produced a high energy outburst that did not lead to stellar collapse.