The Size, Structure, and Variability of Late-Type Stars Measured ...

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Chapter 2
Diameter Measurements
Performed with the ISI Compared
with Other Instruments
Other than the sun, stars are too small to resolve through standard single aperture
telescopes 1 . Stellar size is a fundamental property which, for all stars, is accessible only
indirectly, through a handful of techniques.
The method of lunar occultation involves
recording the light curve of a star as it is eclipsed by the moon thus measuring its profile.
In aperture synthesis, the aperture of an existing large telescope is masked, and the fringe
pattern inverted to form an image. It is possible for angular diameters to be crudely derived
through photometry by measuring both the total flux from a star and its color temperature.
In at least one case, the limb-darkening and size of a K-giant was probed through a microlensing
event! (See Albrow et al. (1999) [1].) Finally, interferometry re-constructs the small
angular scale image of a star by exploiting the wavelike properties of light to achieve a
resolution higher than its component telescopes could obtain alone.
Lunar occultation has been successful at measuring the diameters of a number of
stars. (See White and Feierman, 1987 [110].) However, this technique is inherently limited
in its inability to achieve resolution greater than about 1 mas (di Giacomo et al. (1991) [24])
and its inflexibility with regard to sky coverage and position angle. Aperture synthesis has
provided some of the best diameter measurements of giant stars at visible and near-IR
1 The Hubble Space Telescope has reportedly barely resolved the chromosphere of α Ori (one of the largest
stars) in the ultraviolet. See Gilliland and Dupree, 1996 [34]