sentences:  The progenitor could have been a massive star that expelled most of its outer layers, or one which lost most of its hydrogen envelope due to interactions with a companion in a binary system, leaving behind the core that consisted almost entirely of helium.As the ejecta of a Type IIb expands, the hydrogen layer quickly becomes more transparent and reveals the deeper layers.The classic example of a Type IIb supernova is SN 1993J, while another example is Cassiopeia A.