The Cassiopeia A Supernova was of Type IIB
  Cassiopeia A is one of the youngest supernova remnants known in the Milky Way and a unique laboratory for supernova physics. We present an optical spectrum of the Cassiopeia A supernova near maximum brightness, obtained from observations of a scattered light echo - more than three centuries after the direct light of the explosion swept past Earth. The spectrum shows that Cassiopeia A was a type IIb supernova and originated from the collapse of the helium core of a red supergiant that had lost most of its hydrogen envelope prior to exploding. Our finding concludes a longstanding debate on the Cassiopeia A progenitor and provides new insight into supernova physics by linking the properties of the explosion to the wealth of knowledge about its remnant. 
Oliver Krause, Stephan M. Birkmann, Tomonori Usuda, Takashi Hattori,   Miwa Goto, George H. Rieke and Karl A. Misselt