Popular Astronomy Monthly Publication 2
SURFACE OF GIANT STAR IMAGED
Georgia State University
Astronomers have produced the first detailed images of the surface of a
giant star, revealing a nearly spherical, dust-free atmosphere with complex
areas of moving material, known as convection cells or granules. The giant
star, named pi1 Gruis, is one of the stars in the southern constellation
Grus. An evolved star in the last major phase of its life, pi1 Gruis is
350 times the size of the Sun and resembles what our Sun will become at the
end of its life in five billion years. Studying that star gives scientists insight about the future activity, characteristics and appearance of the Sun. Convection, the transfer of heat by the bulk movement of gases and liquids, plays a major role in astrophysical processes, such as energy transport, pulsation and winds. The Sun has about two million convective
cells that are typically 2,000 km across, but theorists believe that giant
and supergiant stars should have only a few large convective cells because
of their low surface gravities. Determining the convection properties of
most evolved and supergiant stars, such as the sizes of granules, has been
challenging because their surfaces are frequently obscured by dust. In the
study summarized here, the researchers discovered that the surface of the
giant star pi1 Gruis had a complex convective pattern, and that a typical
granule measured 1.2 x 10^11 metres horizontally or 27% of the diameter of
the star.
This is the first time that astronomers have unambiguously imaged such a
giant star with that level of detail. The reason is that there is a limit
to the details that can be seen, related to the size of the telescope used
for the observations. The team used an interferometer, in which the light
from several telescopes is combined, achieving a resolution equivalent to
that of a much larger telescope. The star pi1 Gruis was observed with the
PIONIER instrument, which has four combined telescopes, in Chile in 2014
September. That study was also the first to confirm theories about the
characteristics of granules on giant stars. The images are important,
because the size and number of granules on the surface actually fitted very
well with models that predict what astronomers should be seeing. The
detailed images also showed different colours on the star's surface, which
correspond to varying temperatures. A star does not have the same surface
temperature throughout, and its surface provides our only clues to under-
stand its interior. As temperatures rise and fall, the hotter, more fluid
areas become brighter colours (whiter) and the cooler, denser areas become
darker (redder).
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Originally Posted by Dreamwoven
