The Galaxy Cluster Abell 2218 is so massive that it magnifies and distorts images of faraway galaxies that appear as “arcs” throughout the picture. Copyright NASA/HST.
Comparison of the MOA-33 source oblateness with recent optical interferometry results for Achernar and Altair.
Fifty years after his death, Albert Einstein’s work still provides new tools for understanding our universe. An international team of astronomers has now used a phenomenon first predicted by Einstein in 1936, called gravitational lensing, to determine the shape of stars. This phenomenon, due to the effect of gravity on light rays, led to the development of gravitational optics techniques, among them gravitational microlensing. It is the first time that this well-known technique has been used to determine the shape of a star.
Most of the stars in the sky are point-like, making it very difficult to evaluate their shape. Recent progress in optical interferometry has made it possible to measure the shape of a few stars. In June 2003, for instance, the star Achernar (Alpha Eridani) was found to be the flattest star ever seen, using observations from the Very Large Telescope Interferometer (see ESO Press Release for details about this discovery). Until now, only a few measurements of stellar shape have been reported, partly due to the difficulty of carrying such measurements. It is important, however, to obtain further accurate determinations of stellar shape, as such measurements help to test theoretical stellar models.
For the first time, an international team of astronomers , led by N.J. Rattenbury (from Jodrell Bank Observatory, UK), applied gravitational lensing techniques to determine the shape of a star. These techniques rely on the gravitational bending of light rays. If light coming from a bright source passes close to a foreground massive object, the light rays will be bent, and the image of the bright source will be altered. If the foreground massive object (the “lens”) is point-like and perfectly aligned with the Earth and the bright source, the altered image as seen from the Earth will be a ring shape, the so-called “Einstein ring”. However, most real cases differ from this ideal situation, and the observed image is altered in a more complicated way. The image below shows an example of gravitational lensing by a massive galaxy cluster.
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