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17 Jun 2002
The Hubble Space Telescope has measured the diameters of a special class of pulsating star called Mira variables, which rhythmically change size. Astronomers used Hubble's Fine Guidance Sensors (FGS) to achieve visible light observations of the angular diameters (a measure of apparent width) of two Mira variables, R Leonis and W Hydrae. The results suggest these gigantic, old stars aren't round but egg-shaped.
These unique observations were made by Dr. Mario G. Lattanzi of Turin Observatory (Italy), Dr. M. Feast of Cape Town University (South Africa), Dr. U. Munari of Padova Observatory (Italy), and Dr. P. Whitelock with the South African Astronomical Observatory. The results are being submitted to the Astrophysical Journal Letters for publication.
Hubble's Fine Guidance Sensors are normally used for tracking astronomical targets observed using other scientific instruments aboard Hubble. Instead of taking pictures, the FGSs make an interference pattern from incoming starlight. The resulting bright and dark zones created by the interference pattern, which resemble ripples in a pond, can be used to measure extremely small angles on the sky of only 1/100 of an arcsecond across (the apparent width of a dime at about 200 miles away). The FGS measurements show with unprecedented clarity that the atmospheres of the two stars aren't perfectly round, but rather slightly elongated, like an egg. The unusual shape might be produced in a number of ways, according to researchers. One possibility is that as the stars pulsate they do not expand equally in all directions. Alternatively, there might be large dark spots on the star's visible disk that give the illusion of a non-spherical shape.
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