A light echo is a physical phenomenon caused by light reflected off surfaces distant from the source, and arriving at the observer with a delay relative to this distance. The phenomenon is analogous to an echo of sound, but due to the much faster speed of light, it mostly manifests itself only over astronomical distances.
Images showing the expansion of the light echo of V838 Monocerotis. Credit: NASA/ESA.
An echo of ShaSS 073 galaxy's light detected by ESO's VLT Survey Telescope.
A Hubble Space Telescope image of NGC 5972, a quasar ionisation echo.
A supernova is a powerful and luminous explosion of a star. A supernova occurs during the last evolutionary stages of a massive star, or when a white dwarf is triggered into runaway nuclear fusion. The original object, called the progenitor, either collapses to a neutron star or black hole, or is completely destroyed to form a diffuse nebula. The peak optical luminosity of a supernova can be comparable to that of an entire galaxy before fading over several weeks or months.
SN 1994D (bright spot on the lower left), a type Ia supernova within its host galaxy, NGC 4526
The Crab Nebula is a pulsar wind nebula associated with the 1054 supernova.
A 1414 text cites a 1055 report: since "the baleful star appeared, a full year has passed and until now its brilliance has not faded".
2015 supernova in galaxy cluster RXC J0949.8+1707. In 2011, two supernovae were observed in the same face-on spiral galaxy.