gswork wrote:
I used the same calculations on nearby stars to fully appreciate why
we just can't see them as disks through telescopes, i roughly
estimated that seeing Sirius A as a disc roughly the size of the moon
would require x300,000 magnification (but i may have that wrong):

360 / 2Pi =57.3 (used later)

Moon degrees = 57.3 * arctan(diameter/distance) (thats roughly
3476/384000)
= just over 0.5 degrees

Sirius A degrees = 57.3 * arctan(1390000km/8.6ly)
(ly=9,460,530,000,000km)
= 0.000001645 degrees
multiply that by 300,000 to get just under .5 degrees!

Your calculations are more or less correct, but they don't take into
account the diffraction of light. This makes just about every star a
disc of noticeable size in most telescopes at reasonably high powers
(usually about 30x per inch of aperture or so, although it varies by
observer). This disc has just about nothing to do with the actual
disc of the star, which is ordinarily at least an order of magnitude
smaller.

The star with the largest known apparent diameter as seen from the Earth
(minus the diffraction effects) is R Doradus, with an apparent diameter
of about 57 milliarcseconds (mas). Betelgeuse, the previous record
holder, has a diameter of about 44 mas--a diameter that was measured as
long ago as the 1920s, by Michelson. (Yes, the same Michelson as in the
Michelson-Morley experiment, and who had an obsession with measuring the
speed of light to ever increasing precision.

R Doradus, by the way, appears about the same size as a golf ball seen
from a distance of 150 km.

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
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