The Hubble Space Telescope has been used successfully to measure the diameters of a special class of pulsating star called Mira variables, which rhythmically change size. The results suggest these gigantic, old stars aren't round but egg-shaped.
Knowing more about these enigmatic stars is crucial to understanding how stars evolve, and may preview the fate of our Sun, five billion years from now.
Due to their distance, the stars are too small for their disks to be resolved in conventional pictures (taken in visible light), so 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.
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 that are observed with the 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. 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.
"This is probably an indirect confirmation of earlier research that suggested the photospheres of Mira stars are not perfectly spherical," says Lattanzi. "Such evidence is going to be fundamental to a better understanding of how the photospheres of these stars pulsate and interact with the surrounding environment."
FGS measurements show that R Leonis' apparent diameter (in visible light) is 70 x 78 milliarcseconds (eight by nine hundred million miles at the star's distance of about 390 light-years) along the star's long and short axis, respectively, and 76 by 91 milliarcseconds (with linear dimensions similar to those of R Leonis) for W Hydrae. If placed within our solar system, both of these stars would extend well beyond the orbit of the Earth and almost to that of Jupiter.
Stars like our Sun eventually evolve into red-giant stars. A crucial stage in this process, during which the star ejects its outer layers into space to become a planetary nebula, is occupied by the Mira variables. Their year-long pulsation periods, and their large ranges in brightness (by factors of more than 10,000), set the Miras apart from other stars and make them important indicators of stellar evolution process.
These new Hubble observations, supplemented by coordinated ground-based optical and infrared photometry, are the first of a year-long program to monitor the changes in the diameters and shape of these stars as they pulsate.
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