Light echo

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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.
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]

[edit] Explanation

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.[2]

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.

[edit] Examples

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

Light echos were used to accurately determine the distance to the Cephid variable RS Puppis, according to an announcement in early 2008. [4]

Light echoes have been observed in connection with supernovae SN 1993J[5] and SN 1987A[6], the closest supernova in modern times. The first recorded instance of a light echo was 1936 [3], however it was not studied in detail.

[edit] 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. 
  2. ^ a b Britt, Robert Roy; Bond, Howard (2003-03-27). Hubble Chronicles Mysterious Ouburst with 'Eye-Popping' Pictures. Space.com. Retrieved on 2007-04-17.
  3. ^ a b Hubble watches light echo from mysterious erupting star. European Space Agency (March 26, 2007).
  4. ^ Kervella, Pierre: Light echoes whisper the distance to a star
  5. ^ Sugerman, Ben and Crotts, Arlin (November 8, 2002). "Multiple Light Echoes from Supernova 1993J". . Columbia University
  6. ^ Sugerman, Crotts, Kunkel, Heathcote and Lawrence (February 14, 2005). "A New View of the Circumstellar Environment of SN 1987A" (PDF).
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