ly due to the
very considerable eccentricity of the orbit, long impeded its
satisfactory determination. The mean distance apart of the component
stars, as now ascertained, is just twenty-two million miles, and their
joint mass quadruples that of the sun. But these are minimum estimates.
For if the orbital plane be inclined, much or little, to the line of
sight, the dimensions and mass of the system should be proportionately
increased.

An analogous discovery was made by Miss Maury in 1889. But in the
spectrum of Beta Aurigae, the lines open out and close up on
alternate days, indicating a relative orbit[1442] with a radius of less
than eight million miles, traversed in about four days. This implies a
rate of travel for each star of sixty-five miles a second, and a
combined mass 4.7 times that of the sun. The components are
approximately equal, both in mass and light,[1443] and the system formed
by them is transported towards us with a speed of some sixteen miles a
second. The line-shiftings so singularly communicative proceed, in this
star, with perfect regularity.

This new class of "spectroscopic binaries" could never have been
visually disclosed. The distance of Beta Aurigae from the earth, as
determined, somewhat doubtfully, by Professor Pritchard, is nearly three
and a third million times that of the earth from the sun (parallax =
0.06"); whence it has been calculated that the greatest angular
separation of the revolving stars is only five-thousandths of a second
of arc.[1444] To make this evanescent interval perceptible, a telescope
eighty feet in aperture would be required.

The zodiacal star, Spica (Alpha Virginis), was announced by Dr.
Vogel, April 24, 1890,[1445] to belong to the novel category, with the
difference, however, of possessing a nearly dark, instead of a
brilliantly lustrous companion. In this case, accordingly, the tell-tale
spectroscopic variations consist merely in a slight swinging to and fro
of single lines. No second spectrum leaves a legible trace on the plate.
Spica revolves in four days at the rate of fifty-seven miles a
second,[1446] or quicker, in proportion as its orbit is more inclined to
the line of sight, round a centre at a minimum distance of three
millions of miles. But the position of the second star being unknown,
the mass of the system remains indeterminate. The lesser component of
the splendid, slowly revolving binary, Castor, is also closely double.
Its spectral lines wer