Light echoes in RS Pup

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.