A “light echo” is *not* a stellar explosion per se. What is happening is that there is dust or gas near a star (spanning distances of lightyears) that is normally too dimly lit to be seen. But if a star suddenly gets much brighter, the dust is suddenly lit up. Think of it as smoke or fog on a pitch black night except for stars overhead; normally you couldn’t see the fog, but if someone nearby lights up a flashlight, suddenly to can see the fog. But the thing that throws people about light echoes is that the speed of light, which means that the fog surrounding you is effectively *instantly* lit up, is slllloooooowwwww on the scale of interstellar dust clouds. And thus the cloud is not all lit up at once, but instead you can watch the outward crawl of light from the newly bright star as it goes through the cloud.
If the star is bright for only a brief period – perhaps it blew up in a supernova, say – then the light echo will take the form of a hollow spherical shell, with ta constant wall thickness but a constantly expanding outer diameter. If the star brightens and stays bright for a span of years, then you’d get a growing solid sphere of glowing gas or dust.
Hubble managed to catch such a light echo around the star red variable star V838 Monocerotis from 2002 to 2006. In 2002 the star (20,000 light years away) suddenly grew 600,000 times brighter than it had been, becoming one of the brightest and largest stars in the galaxy, and then began to cool and shrink. A few years ago the images were put together into a substantially awesome video:
[youtube U1fvMSs9cps]
