A light echo is a phenomenon observed in
astronomy. Analogous to an echo of sound, a lightecho 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 some time after the initial flash. Due to their geometries, light echoes can produce the illusionof superluminalspeeds. [cite journal
last = Bond
first = Howard E.
coauthors = Henden, Arne; Levay, Zoltan G.; Panagia, Nino; Sparks, William B.; Starrfield, Sumner; Wagner, R. Mark; Corradi, R. L. M.; Munari, U.
title = An energetic stellar outburst accompanied by circumstellar light echoes
journal = Nature
volume = 422
issue = 6930
pages = 405–408
date = March 27, 2003
url = http://adsabs.harvard.edu/abs/2003Natur.422..405B
doi = 10.1038/nature01508
accessdate = 2006-08-10 ]
Light echoes are produced when the initial flash from a rapidly brightening object such as a
novais reflected off intervening interstellar dustwhich 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.cite web|last=Britt|first=Robert Roy|coauthors=Bond, Howard|title=Hubble Chronicles Mysterious Ouburst with 'Eye-Popping' Pictures|url=http://www.space.com/scienceastronomy/light_echo_030326.html|publisher=Space.com|date=2003-03-27|accessdate=2007-04-17]
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.
The variable star
V838 Monocerotisexperienced a significant outburst in 2002as 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 yearsin a matter of months. The expansion of the light echo is continuing and is expected to grow until 2010.cite web|url=http://www.esa.int/esaCP/SEMYZ09YFDD_index_0.html|title=Hubble watches light echo from mysterious erupting star|date=March 26, 2007|publisher= European Space Agency]
Light echos were used to accurately determine the distance to the
Cephid variable RS Puppis, according to an announcement in early 2008. [Kervella, Pierre: [http://www.eso.org/public/outreach/press-rel/pr-2008/pr-05-08.html Light echoes whisper the distance to a star] ]
Light echoes have been observed in connection with
supernovae SN 1993J[cite paper|title=Multiple Light Echoes from Supernova 1993J|url=http://arxiv.org/abs/astro-ph/0207497|author=Sugerman, Ben and Crotts, Arlin|date=November 8, 2002|publisher=Columbia University] and SN 1987A[cite paper|url=http://arxiv.org/abs/astro-ph/0502268v1|title=A New View of the Circumstellar Environment of SN 1987A|author=Sugerman, Crotts, Kunkel, Heathcote and Lawrence|date=February 14, 2005] , the closest supernova in modern times. The first recorded instance of a light echo was 1936, however it was not studied in detail.
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