Image: Light echo principle

Description: Simplified principle of a light echo, drawn by CMG Lee based on http://esahubble.org/images/heic0304f . Coloured spots denote stationary dust particles around a dying star, denoted by the white spot. When the star explodes, light rays are emitted in every direction. Coloured arrows denote rays reflected by each particle towards the observer, who is so distant that incoming rays are effectively parallel. (Light is actually reflected in multiple directions but only the part towards the observer is shown.) All paths have the same length as it is a snapshot of a moment in time. It can be seen that the all rays arriving at a given time are reflected from particles in a paraboloid. Rays arriving later come from a wider paraboloid with the same focus. The explosion appears to initially expand faster than the speed of light despite the particles being at rest. In the SVG file, hover over a region to highlight it.
Author: Cmglee
Usage Terms: Creative Commons Attribution-Share Alike 3.0
License: CC-BY-SA-3.0
License Link: http://creativecommons.org/licenses/by-sa/3.0/
Attribution Required?: Yes