page_title: Light echo
section_title: Cepheids
hierarchical_section_title: Light echo / Examples / Cepheids
caption_attribution_description: English: This unique image from ESO’s VLT Survey Telescope (VST) reveals two galaxies at the very beginning of the merging process. The interactions between the duo have created a rare effect known as a light echo, where light reverberates around the material within each galaxy. This is analogous to the acoustic echo where the reflected sound arrives at the listener with a delay after the direct sound. This is the first case of a light echo observed between two galaxies. The larger galaxy, seen here in yellow, is ShaSS 073 — an active galaxy with an extremely luminous core. Its less massive companion, in blue, is named ShaSS 622, and together the pair make up the intriguing ShaSS 622-073 system. The bright core of ShaSS 073 is exciting a region of gas within the disc of its blue companion: it bombards the material there with radiation, causing it to glow brightly as it absorbs and then re-emits this light. This glowing region extends across 1.8 billions of square light-years. However, while studying this merger, astronomers found the luminosity of the large central galaxy to be 20 times lower than needed to excite the gas in this way. This indicates that the centre of ShaSS 073 has dramatically faded over the last 30 000 years or so — but the highly-ionised region between the two galaxies still retains the memory of its former glory.
context_page_description: 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 only manifests itself over astronomical distances.
  For example, a light echo is produced when a sudden flash from a nova is reflected off a cosmic dust cloud, and arrives at the viewer after a longer duration than it otherwise would have taken with a direct path. Because of their geometries, light echoes can produce the illusion of superluminal motion.
context_section_description: Light echoes were used to determine the distance to the Cepheid variable RS Puppis to an accuracy of 1%. Pierre Kavella at the European Southern Observatory described this measurement as so far "the most accurate distance to a Cepheid".
caption: An echo of light detected by ESO's VLT Survey Telescope .[12]
exif: {"Image ImageDescription": "This unique image from ESO\u2019s VLT Survey Telescope (VST) reveals two galaxies at the very beginning of the merging process. The interactions between the duo have created a rare effect known as a light echo, where light reverberates around the material within each galaxy. This is analogous to the acoustic echo where the reflected sound arrives at the listener with a delay after the direct sound. This is the first case of a light echo observed between two galaxies.  The larger galaxy, seen here in yellow, is ShaSS 073 \u2014 an active galaxy with an extremely luminous core. Its less massive companion, in blue, is named ShaSS 622, and together the pair make up the intriguing ShaSS 622-073 system. The bright core of ShaSS\u00a0073 is exciting a region of gas within the disc of its blue companion: it bombards the material there with radiation, causing it to glow brightly as it absorbs and then re-emits this light. This glowing region extends across 1.8 billions of square light-years. However, while studying this merger, astronomers found the luminosity of the large central galaxy to be 20 times lower than needed to excite the gas in this way. This indicates that the centre of ShaSS\u00a0073 has dramatically faded over the last 30\u00a0000 years or so \u2014 but the highly-ionised region between the two galaxies still retains the memory of its former glory."}