  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.
