Young Galactic supernova remnants are unique laboratories for supernova physics. Due
to their proximity they provide us with the most detailed view of the outcome of a
supernova. However, the exact spectroscopic types of their original explosions have
been undetermined so far - hindering to link the wealth of multi-wavelength knowledge
about their remnants with the diverse population of supernovae.
Light echoes, reflections of the brilliant supernova burst of light by interstellar
dust, provide a unique opportunity to re-observe today - with powerful scientific
instruments of the 21st century - historic supernova explosions even after hundreds
of years and to conclude on their nature. We report on optical light-echo
spectroscopy of two Galactic supernovae, in particular the youngest known CC SN Cas
A. These observations finally recovered the missing spectroscopic classifications
and provide new constraints on explosion models for future studies.