46 Recent Advances in Stellar Astronomy
still awaiting observation. Our direct meaures of parallax
are fairly good up to thirty, or even to fifty parsecs.
When we can get a star’s distance, it is very easy to
calculate its real brightness, and the range in the values is
enormous. The faintest star so far known, is an eleventh
magnitude star, discovered by Innes, which is a distant
companion of the first magnitude star Alpha Centauri—
two degrees away from it, but moving in the sky at the
same rate, so that the two probably form one great system.
Its parallax is slightly greater than that of the bright star,
which makes it the nearest known object in the heavens
(1.3 parsecs) and its real brightness is but one ten-
thousandth part that of the Sun. It is only visible because
it happens to be so near us. Alpha Centauri itself is just
about as bright as the Sun, but our next nearest neighbor
in space—another faint star discovered by Barnard—gives
out about 1/2700 of the Sun’s light.
Sirius (also a near neighbor, at 2.7 parsecs) is 25
times as bright as the Sun—and so our list might go on
almost indefinitely, for there are more than a thousand
stars for which direct measures of parallax have been
made, but many of the brightest stars in the sky have
parallaxes too small to measure in this way. All that our
measures indicate is that their distances are greater than
50 parsecs, and probably than 100, and that their real
brightness in some cases must be more than a thousand
times the Sun’s.
With such an enormous range of values to deal with, it
is obviously desirable to have some simple method of
expressing the real brightness of a star, as convenient as
the stellar magnitude is for the apparent brightness.
Kapteyn met the need by inventing the “absolute magni-
tude” which is simply the magnitude that the star would