On November 11, 1572, the Danish astronomer Tycho Brahe saw what he thought was a new star. Outshining the planet Venus, it was visible for several months until it faded. What Brahe actually saw was a supernova - the titanic explosion of a dying massive star. Its designation "Cassiopeia A" is derived from the fact that it lies in the direction of the constellation Cassiopeia, about 11,000 light-years from Earth.

Interestingly, astronomers have been able to do spectral analysis on the supernova not just by using the light that reaches us today, but by using the light that originated not long after the event itself. How? They measure it's light echo. A ‘light echo’ is light from the original supernova event that bounces off dust particles in surrounding and background interstellar clouds and reaches Earth many years after the direct light passes by.