This brief animation shows the mechanism behind a light echo. A variable star known as RS Puppis is shown to the top right corner of the video, surrounded by clouds of gas and dust.
As the star expands and brightens, we see some of the light after it is reflected from progressively more distant shells of dust and gas surrounding the star — creating the illusion of gas moving outwards. This reflected light has further to travel, and so arrives at the Earth after light that travels straight from star to telescope. This is analogous to sound bouncing off surrounding objects, causing the listener to hear an audible echo.
More information and download options: http://www.spacetelescope.org/videos/heic1323h/
Credit: NASA, ESA, M. Kornmesser