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BETELGEUSE IS SHRINKING
The red supergiant star Betelgeuse, in the constellation Orion, has
steadily shrunk over the past 15 years, according to University of
California, Berkeley, researchers. Long-term monitoring by an
interferometer on Mount Wilson in California shows that Betelgeuse,
which is so big that in our Solar System it would reach to the orbit
of Jupiter, has shrunk in diameter by more than 15% since 1993.
Despite the star's diminished size, its visual brightness has shown no
significant decrease during that period.
Betelgeuse was the first star (after the Sun) to have its angular
size directly measured, and even today it is one of only a handful of
stars that can be identified by the Hubble telescope as being slightly
extended rather than an unresolved point of light. In 1921, Francis
Pease and Albert Michelson used optical interferometry to estimate
that its diameter was equivalent to the orbit of Mars. Last year, new
ideas of the distance to Betelgeuse raised it from 430 light-years to
640, which increased the diameter derived from it from about 3.7 to
about 5.5 astronomical units (Earth--Sun distances). Since 1921, its
size has been re-measured by many different interferometer systems.
At a given wavelength the variation has been scarcely beyond the
measurement uncertainties, but different wavelengths give diameters
differing by up to 30%, because the tenuous gas in the outer regions
of the star emits light at some wavelengths and absorbs it at others
-- the star looks bigger in the light of an emission line than it does
in an absorption line.
The interferometer built at Berkeley in the early 1990s sidesteps the
confounding emission and absorption lines by observing in the mid-
infrared with a narrow bandwidth that can be tuned between spectral
lines. The interferometer combines signals from pairs of telescopes
in order to determine path-length differences between light that
originates at the star's centre and light that originates at the
star's edge. Its observing wavelength of about 11 microns, in the
mid-infrared, penetrates the dust, and the narrow bandwidth avoids
any spectral lines, so the star is seen relatively undistorted.
The first measurements showed the size to be quite close to Pease and
Michelson's result, but over 15 years it has decreased about 15%,
changing smoothly but faster as the years have progressed.