Binary Star

Two small closely mounted bulbs simulate resolution problems.

What it shows:

The ability to resolve two closely separated stars depends upon the aperture size of the observing instrument. Here two tiny bulbs represent stars that are barely resolvable by human eyes across the lecture hall.

How it works:

The light collected from two stars by the eye (or by a telescope mirror) are themselves geometric point sources but are circular diffraction fringes

The light from a distant star is not detected as a geometric point source, but as a circular diffraction pattern (see the Light and Optics section on Resolution). Thus for two stars to be resolved, the Rayleigh criterion requires that the their patterns be separated such that the central maximum of one source lies on the first zero of the second. In terms of angular distance, the minimum resolvable separation θ is

θ = 1.22 λ/b in radians

θ = 70 λ/b in degrees

Here the observing aperture has a diameter b with the source emitting light of wavelength λ. For the human eye, the pupil has a diameter of about 3mm with a peak response to light at 550nm. This gives a resolution limit of 1/80°, although in reality is closer to 1/60° or 1 arc minute. 1 For the dimensions of a lecture hall, small bulbs will approximate as point sources, and a separation of about 3mm will be resolvable at 10m distance.


This demo could be enhanced by having different colored bulbs representing different spectral type stars, or by mounting the bulbs on a revolving platform so that they pass in and out of the observer's resolution limit.


D.S. Falk, D. R. Brill, D. G. Stork, 1986 Seeing the Light (Harper and Row), p.337

1 about the width of a finger at 30m