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A light echo is a physical phenomenon caused by light reflected off surfaces distant from the source, and arriving at the observer with a delay relative to this distance. 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, and arrives at the viewer after a longer duration than it otherwise would have taken with a direct path. Because of their geometry, light echoes can produce the illusion of superluminal motion.
In the first illustration above, light following path A is emitted from the original source and arrives at the observer first. Light which follows path B is reflected off a part of the gas cloud at a point between the source and the observer, and light following path C is reflected off a part of the gas cloud perpendicular to the direct path. Although light following paths B and C appear to come from the same point in the sky to the observer, B is actually significantly closer. As a result, the echo of the event in an evenly distributed (spherical) cloud for example will appear to the observer to expand at a rate approaching or faster than the speed of light, because the observer may assume the light from B is actually the light from C.
All reflected light rays that originate from the flash and arrive at Earth together will have traveled the same distance. When the rays of light are reflected, the possible paths between the source and Earth that arrive at the same time correspond to reflections on an ellipsoid, with the origin of the flash and Earth as its two foci (see animation to the right). This ellipsoid naturally expands over time.
An example is supernova SN 1987A, the closest supernova in modern times. Its light echoes have aided in mapping the morphology of the immediate vicinity as well as in characterizing dust clouds lying further away but close to the line of sight from Earth.
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.
Light echoes have also been used to study the supernova that produced the supernova remnant Cassiopeia A. The light from Cassiopeia A would have been visible on Earth around 1660, but went unnoticed, probably because dust obscured the direct view. Reflections from different directions allow astronomers to determine if a supernova was asymmetrical and shone more brightly in some directions than in others. The progenitor of Cassiopeia A has been suspected as being asymmetric, and looking at the light echoes of Cassiopeia A allowed for the first detection of supernova asymmetry in 2010.
Yet other examples are supernovae SN 1993J and SN 2014J.
Light echo from the 1838-1858 Great Eruption of Eta Carinae were used to study this supernova imposter. A study from 2012, which used light echo spectra from the Great Eruption, found that the eruption was colder compared to other supernova imposters.
HsV is made entirely of gas so hotabout 10,000 degrees astronomers felt it had to be illuminated by something powerful.
After several studies of light and ionisation echoes, it is thought they are likely caused by the 'echo' of a previously-active AGN that has shut down. Kevin Schawinski, a co-founder of the website Galaxy Zoo, stated: "We think that in the recent past the galaxy IC 2497 hosted an enormously bright quasar. Because of the vast scale of the galaxy and the Voorwerp, light from that past still lights up the nearby Voorwerp even though the quasar shut down sometime in the past 100,000 years, and the galaxy's black hole itself has gone quiet." Chris Lintott, also a co-founder of Galaxy Zoo, stated: "From the point of view of the Voorwerp, the galaxy looks as bright as it would have before the black hole turned offit's this light echo that has been frozen in time for us to observe." The analysis of HsV in turn has led to the study of objects called Voorwerpjes and Green bean galaxies.
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