A light echo is a phenomenon observed in astronomy. Analogous to an echo of sound, a light echo is produced when a sudden flash or burst of light, such as that observed in novae, is reflected off a source and arrives at the viewer after a considerably longer duration than it otherwise would have taken with a direct path. Because of their geometries, light echoes can produce the illusion of superluminal speeds.
Light echoes are produced when the initial flash from a rapidly brightening object such as a nova is reflected off intervening interstellar dust which may or may not be associated with the source of the light. Light from the initial flash arrives at the viewer first, while light reflected from dust or other objects between the source and the viewer begins to arrive shortly afterward. Because this light has only traveled forward as well as away from the star, it produces the illusion of an echo expanding faster than the speed of light.
In the illustration to the right, 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 appears to the observer to expand at a rate faster than the speed of light.
All reflected light that originates from the flash will travel the same distance. When the ray of light is reflected, the possible paths between the source and the earth correspond to reflections on an ellipsoid, with the origin of the flash and the earth as its two foci (see animation to the right). This ellipsoid naturally expands over time.
The variable star V838 Monocerotis experienced a significant outburst in 2002 as observed by the Hubble Space Telescope. The outburst proved surprising to observers when the object appeared to expand at a rate far exceeding the speed of light as it grew from an apparent visual size of 4 to 7 light years in a matter of months.
Light echoes were used to determine the distance to the Cepheid variable RS Puppis to within 1% of its true value. This distance measurement is "the most accurate distance to a Cepheid" according to the lead author of the paper that reported the results.
Light echoes have been observed in connection with supernovae SN 1993J and SN 1987A, the closest supernova in modern times. The first recorded instance of a light echo was 1936, but it was not studied in detail.
By calculating the ellipsoid that has the Earth and a supernova remnant as its foci and finding places where the ellipsoid intersect with clouds of dust and gas, it is sometimes possible to see the faint reflections of historical supernovae. Using light echoes, astronomers can analyze the spectrum of supernovae whose light reached Earth long before the invention of the telescope. Astronomers can compare the explosion with its remnants, centuries or millennia old. One 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 can be identified by comparing photos of gas and dust clouds taken months or years apart and spotting changes in the light rippling across the clouds. If the source of the light is unknown, several such observations can be fitted to an ellipsoid to allow astronomers to pinpoint the origin.
Light echoes have 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.
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