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Points of LightWeek of February 2, 1998 There was an interesting discussion recently on the USENET bulletin boards about the sizes of stars. When you look up in the sky all the stars look like little points, with no discernible disk (except for one, of course: the Sun). But this does not mean the stars are infinitely small points, just too small for the eye to resolve. Sometimes, though, it does look like the stars have some sort of width to them, but this is not because the stars are big, but instead because the Earth's atmosphere distorts the stars' images. Tiny cells of air in the Earth's atmosphere are constantly dancing about, and they act like tiny lenses, bending the stars' light. We see this dance as twinkling, which smears out the point of light from the star into a small disk. The more turbulent the atmosphere, the bigger the star looks.
There are ways around this. One, which seems kind of obvious now,
is to get a telescope above the Earth's atmosphere. We have done
this with the Hubble Space Telescope
and with many other telescopes as well. But this is terribly
expensive. Another method, used many years ago and still
producing excellent data today, is to take extremely short
exposures. This snapshot method freezes the turbulence
as far as the telescope is concerned, so the turbulence doesn't
affect the picture. This only works with very bright stars of course,
since a dim star won't show up well on such a short exposure.
but some of the brightest stars are also the biggest, so you
can use this method to see if they have disks. (I am grossly
oversimplifying the technique used, but if you'd like to know more
details, there are
plenty of websites about speckle interferometry).
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