Light echo

A sudden brightening or explosion sends a pulse of light outward. Dust grains can scatter some of that light toward Earth; in infrared echoes, dust heated by the pulse emits radiation of its own. The observer receives the direct flash first and then light that has travelled a longer route via surrounding material. As the pulse illuminates different locations, the visible pattern changes over time. The dust itself need not be moving outward.

A changing echo can look like a shell expanding across the sky, even when the apparent expansion is only the moving boundary of illumination. Echo geometry depends on the location of the source, the observer and the scattering material; it is not generally a spherical surface of matter expanding from the star. Repeated images at different dates can therefore map otherwise faint circumstellar or interstellar material. In suitable cases, analyzing the echo can also constrain properties of dust and the illuminating event.

The unusual 2002 eruption of V838 Monocerotis produced a prominent Hubble-observed echo: successive images showed light from the outburst illuminating surrounding dust. NASA notes that the apparent growth in the images was not the dust racing outward. Light echoes have also been observed around supernovae. Spitzer observations of Cassiopeia A showed infrared-bright dust features appearing successively as an outburst's light reached different clouds; the shifting pattern created an optical illusion of rapid motion.

Light echoes offer a way to study the distribution and scattering behavior of dust, and can carry delayed information about a transient source. They are uncommon as clearly resolved features: the source, echoing material and viewing geometry must be favorable, and the event must be observable with suitable instruments over time. Interpreting them requires distinguishing true material motion from the propagation of illumination and accounting for the geometry and wavelength-dependent behavior of dust.