table star presented
itself.

THE GIANT BETELGEUSE

Russell, Shapley, and Eddington had pointed out Betelgeuse (Arabic
for "the giant's shoulder"), the bright red star in the constellation
of Orion (Fig. 25), as the most favorable of all stars for measurement,
and the last-named had given its angular diameter as 0.051 of a
second of arc. This deduction from theory appeared in his recent
presidential address before the British Association for the Advancement
of Science, in which Professor Eddington remarked: "Probably the
greatest need of stellar astronomy at the present day, in order
to make sure that our theoretical deductions are starting on the
right lines, is some means of measuring the apparent angular diameter
of stars." He then referred to the work already in progress on
Mount Wilson, but anticipated "that atmospheric disturbance will
ultimately set the limit to what can be accomplished."

[Illustration: Fig. 25. The giant Betelgeuse (within the circle),
familiar as the conspicuous red star in the right shoulder of Orion
(Hubble).

Measures with the interferometer show its angular diameter to be
0.047 of a second of arc, corresponding to a linear diameter of
215,000,000 miles, if the best available determination of its distance
can be relied upon. This determination shows Betelgeuse to be 160
light-years from the earth. Light travels at the rate of 186,000
miles per second, and yet spends 160 years on its journey to us
from this star.]

On December 13, 1920, Mr. Pease successfully measured the diameter
of Betelgeuse with the 20-foot interferometer. As the outer mirrors
were separated the interference fringes gradually became less distinct,
as theory requires, and as Doctor Merrill had previously seen when
observing Betelgeuse with the interferometer used for Capella. At
a separation of 10 feet the fringes disappeared completely, giving
the data required for calculating the diameter of the star. To
test the perfection of the adjustment, the telescope was turned to
other stars, of smaller angular diameter, which showed the fringes
with perfect clearness. Turning back to Betelgeuse, they were seen
beyond doubt to be absent. Assuming the mean wave-length of the
light of this star to be 5750/10000000 of a millimetre, its angular
diameter comes out 0.047 of a second of arc, thus falling between
the values--0.051 and 0.031 of a second--predicted by Eddington and
Russell from slightly different assumptions. Subse