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.

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 some time after the initial flash. Due to their geometries, light echoes can produce the illusion of superluminal speeds.^[1] Adjectives: Terrestrial, Terran, Telluric, Tellurian, Earthly Atmosphere Surface pressure: 101. ... A giant Hubble mosaic of the Crab Nebula, a supernova remnant Astronomy is the science of celestial objects (such as stars, planets, comets, and galaxies) and phenomena that originate outside the Earths atmosphere (such as auroras and cosmic background radiation). ... In audio signal processing and acoustics, an echo (plural echoes) is a reflection of sound, arriving at the listener some time after the direct sound. ... Sound is a disturbance of mechanical energy that propagates through matter as a longitudinal wave, and therefore is a mechanical wave. ... A nova is a cataclysmic nuclear explosion caused by the accretion of hydrogen onto the surface of a white dwarf star. ... The reflection of a bridge in Indianapolis, Indianas Central Canal. ... Table of Geometry, from the 1728 Cyclopaedia. ... For other uses, see illusion (disambiguation). ... Faster-than-light (also superluminal or FTL) communications and travel are staples of the science fiction genre. ...

Explanation

Light echoes are produced when the initial flash from a rapidly brightening object such as a nova are 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.^[2] Interstellar cloud is the generic name given to accumulations of gas and dust in our galaxy. ... A line showing the speed of light on a scale model of Earth and the Moon The speed of light in a vacuum is an important physical constant denoted by the letter c for constant or the Latin word celeritas meaning swiftness. It is the speed of all electromagnetic radiation...

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 obesrver to expand at a rate faster than the speed of light.

Examples

Images showing the expansion of the light echo of V838 Monocerotis. Credit: NASA/ESA.

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.^[2] The expansion of the light echo is continuing and is expected to grow until 2010.^[3] Image File history File links Size of this preview: 800 × 400 pixel Image in higher resolution (1000 × 500 pixel, file size: 75 KB, MIME type: image/jpeg) From http://hubblesite. ... Image File history File links Size of this preview: 800 × 400 pixel Image in higher resolution (1000 × 500 pixel, file size: 75 KB, MIME type: image/jpeg) From http://hubblesite. ... The National Aeronautics and Space Administration (NASA) is an agency of the United States Government, responsible for the nations public space program. ... This article is about the European Space Agency. ... V838 Monocerotis (V838 Mon) is an enigmatic variable star in the constellation Monoceros about 20,000 light years (6 kpc)[1] from the Sun. ... For album titles with the same name, see 2002 (album). ... The Hubble Space Telescope (HST) is a telescope in orbit around the Earth, named after astronomer Edwin Hubble. ... A light year, abbreviated ly, is the distance light travels in one year: roughly 9. ... This article or section does not adequately cite its references or sources. ...

V838 Monocerotis is the only confirmed example of a light echo observed in recent years, with the last recorded instance being in 1936 ^[3] and was not studied in detail. 1936 (MCMXXXVI) was a leap year starting on Wednesday (link will take you to calendar). ...

References

1. ^ Bond, Howard E.; Henden, Arne; Levay, Zoltan G.; Panagia, Nino; Sparks, William B.; Starrfield, Sumner; Wagner, R. Mark; Corradi, R. L. M.; Munari, U. (March 27, 2003). "An energetic stellar outburst accompanied by circumstellar light echoes". Nature 422 (6930): 405–408. DOI:10.1038/nature01508. Retrieved on 2006-08-10. 
2. ^ ^{a b} Britt, Robert Roy; Bond, Howard (March 27, 2003). Hubble Chronicles Mysterious Ouburst with 'Eye-Popping' Pictures. Space.com. Retrieved on 2007 April 17.
3. ^ ^{a b} Hubble watches light echo from mysterious erupting star. European Space Agency (March 26, 2007).