Monday, September 20, 2010

How small stars in the sky are

For optical calculations, it is usual to approximate stars other than the sun as basically point sources of light.  How good an approximation is this?

Well, Alpha Centauri is about four light-years away, and a little bit bigger than the sun.  It is very easily seen, being the fifth brightest star in the sky.  How much apparent space in our naked-eye visual field would the size of Alpha Centauri's disk take up, if we didn't count diffraction, atmospheric effects, etc.?  Answer: Very, very little, about 0.01 seconds of arc.  In fact, it would take up approximately the same amount of space in our naked-eye visual field as an HIV virus seen from one foot away!  The stars take up tiny spaces in the sky.  But they're very bright for their size.

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