Hubble Telescope Measures Diameters of Pulsating Stars

Don Savage
Headquarters, Washington, DC             July 8, 1996
(Phone:  202/358-1547)

Tammy Jones
Goddard Space Flight Center, Greenbelt, MD
(Phone:  301/286-5566)

Ray Villard
Space Telescope Science Institute, Baltimore, MD
(Phone:  410/338-4514)

RELEASE:  96-129

HUBBLE TELESCOPE MEASURES DIAMETERS OF PULSATING STARS

       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.

       The Space Telescope Science Institute is operated by the
Association of Universities for Research in Astronomy, Inc.,
for NASA, under contract with the Goddard Space Flight Center,
Greenbelt, MD.  The Hubble Space Telescope is a project of
international cooperation between NASA and the European Space
Agency (ESA).

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