The Sizes and Masses of the Stars 75
Sun, and most of them are between twice and four times
its diameter. As they are of Classes B and A, this con-
firms our estimates, when it is remembered that the stars
in Orion’s belt are unusually luminous, even for B-stars,
and therefore probably larger than the average. The one
star which is smaller than the Sun (about two-thirds its
diameter) is a dwarf (W Ursae Majoris) of spectrum
F8, and again supporting our conclusion.
But until last year, there was no direct confirmation of
the great diameters computed for the redder giant stars.
This has been furnished by one of the most notable ad-
vances in observational astronomy in the present genera-
tion—the direct measurement of the angular diameters of
stars by Michelson and his associates at Mount Wilson.
The great difficulty in the way of such measurements lies
partly in the enormous distances of the stars, but mainly
in the properties of the waves of light themselves. Though
these travel in straight lines in empty space, yet, when
they pass a sharp obstacle they have a slight tendency to
bend inwards into the shadow: just as sea-waves passing
the end of a long breakwater would spread sidewise into
<rhe calm space behind it. This spreading, called diffrac-
tion, is greater the narrower the aperture through which
the light is admitted. With a very narrow slit, it becomes
conspicuous, as may be seen by looking at a distant object
through the gap below two lead-pencils held close in front
of the eye, and pressed so close that light barely gets be-
tween them. With larger apertures, it is evident only
under considerable magnifying power. The “diffraction
pattern” produced by the circular aperture of a telescope,
when sufficiently magnified, appears as a circular disk of
light, brightest at the centre, and fading off gradually
toward the edge, surrounded by a succession of concentric