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ESO 05/08 - Associated Images and Animations

11 February 2008
Under embargo till 11 February 2008, 00:01 GMT

Light echoes whisper the distance to a star

Astronomers calibrate the distance scale of the Universe

The nebula around the Cepheid star RS Pup undoubtedly holds a wealth of information about the mass-loss history of this star. It will thus be instrumental to understand the evolution of Cepheids. Less than five years away from the centenary of the discovery of the Period-Luminosity relation by Leavitt & Pickering, RS Pup could well become a 'Rosetta stone' for this important class of stars. This colour composite image is based on observations made with the 3.6-m ESO New Technology Telescope (NTT) installed at the La Silla Observatory (Chile) and equipped with the ESO Multi-Mode Instrument (EMMI). The image is based on data obtained through B, V, and R-filters. In order to avoid a heavy saturation of the detector, RS Pup was positioned in the gap between the detectors.

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The nebula around RS Pup seen at 6 different epochs, corresponding to different phases of its 41.4 day cycle, as indicated in the top left part of each image. By monitoring both the light variation of the star (not visible on these images as it is positioned in the gap between the detectors) and the the light echoes from several features in the nebula, it is possible to determine the distance of the star. The changes in the nebula are for example very easy to detect in the middle part, just above and right to the two close foreground stars. The astronomers looked at the echoes produced by 7 nebular features, some of which are indicated on this image. The exposures were obtained randomly between 28 October 2006 and 24 March 2007 with EMMI on ESO's NTT at La Silla.

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In order to better vizualise the light echo phenomenon, the astronomers computed a continuous movie of the progression of the light echoes in the nebula (using a spline interpolation of the measured epochs for each pixel). The variation of the luminosity of RS Pup is visible on these images from the changing extension of the wings of its 'Point Spread Function' (PSF). The morphological changes of the nebula with time are considerable. In these images, the astronomers could monitor at all times the propagation of more than seven echoes, allowing them to determine the distance of the star.

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The determination of the distance to RS Pup, following the method of the American astronomer Robert Havlen, is based on the measurement of the phase difference between the variation of the star and the variation of isolated nebular features. Because the luminosity of the star changes in a very distinctive pattern, the presence of the nebula allows the astronomers to see light echoes and use them to measure the distance of the star. The light that travelled from the star to a dust grain and then to the telescope arrives a bit later than the light that comes directly from the star to the telescope. As a consequence, if we measure the brightness of a particular, isolated dust blob in the nebula, we will obtain a brightness curve that has the same shape as the variation of the Cepheid, but shifted in time. This delay is called a 'light echo', by analogy with the more traditional echo, the reflection of sound by, for example, the bottom of a well. By monitoring the evolution of the brightness of the blobs in the nebula, the astronomers can derive their distance from the star: it is simply the measured delay in time, multiplied by the velocity of light (300 000 km/s). Knowing this distance and the apparent separation on the sky between the star and the blob, one can compute the distance of RS Pup. This artist's illustration is not to scale.

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This short animation illustrates the principle of the light echo phenomenon used to determine the distance of the Cepheid star RS Pup (see PR Photo 05c/08).

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This artist's impression shows the location of the Cepheid star RS Pup in our Galaxy, as determined by the astronomers using data from ESO's NTT. From the observations of the echoes on several nebular features, the distance of RS Pup was found to be 6500 light years, with an uncertainty of 90 light years. RS Pup is distant by about a quarter of the distance between the Sun and the Centre of the Milky Way. RS Pup is located within the Galactic plane, in a very populated region of our Galaxy.