Light echo

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.

For example, a light echo is produced when a sudden flash from a nova is reflected off a cosmic dust cloud, and arrives at the viewer after a longer duration than it otherwise would have taken with a direct path.

The variable star V838 Monocerotis experienced a significant outburst in 2002 as observed by the Hubble Space Telescope.

Astronomers calculate the ellipsoid which has Earth and a supernova remnant at its focal points to locate clouds of dust and gas at its boundary.

Identification can be done using laborious comparisons of photos taken months or years apart, and spotting changes in the light rippling across the interstellar medium.

Its light echoes have aided in mapping the morphology of the immediate vicinity [5] as well as in characterizing dust clouds lying further away but close to the line of sight from Earth.

[6] Another example is the SN 1572 supernova observed on Earth in 1572, where in 2008, faint light-echoes were seen on dust in the northern part of the Milky Way.

[16] Within this study, Couderc published the derivation of echo locations and time delays in the paraboloid, rather than ellipsoid, approximation of infinite distance.

"[26] The analysis of HsV in turn has led to the study of objects called Voorwerpjes and Green bean galaxies.

Reflected light following path B arrives shortly after the direct flash following path A but before light following path C. B and C have the same apparent distance from the star as seen from Earth .
The distance traveled from one focus to another, via some point on the ellipse, is the same regardless of the point selected.
Direct light from a stellar outburst (white spot) reaches the observer (path 0) followed by light reflected off particles on progressively wider paraboloids (1–5): the observed disc apparently initially expands faster than light but the illusion is due to light reflecting off different unrelated particles [ 2 ] (ANIMATION)
Images showing the expansion of the light echo of V838 Monocerotis . Credit: NASA / ESA .
Light echoes from RS Puppis propagate through its reflection nebula.
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.