The Distance of the Stars
Just about this time, however, a great optician came
to the aid of the astronomers. Joseph Fraunhofer perfected
the refracting telescope, as Herschel had perfected
the reflector, and invented a wonderfully accurate
“heliometer,” or sun-measurer. With the aid of
these instruments the old and almost infinitely difficult
problem of star distance was solved. In 1838 Bessel
announced from the Königsberg observatory that he
had succeeded, after months of effort, in detecting and
measuring the parallax of a star. Similar claims had
been made often enough before, always to prove fallacious
when put to further test; but this time the announcement
carried the authority of one of the greatest
astronomers of the age, and scepticism was silenced.
Nor did Bessel's achievement long await corroboration.
Indeed, as so often happens in fields of discovery,
two other workers had almost simultaneously
solved the same problem—Struve at Pulkowa, where
the great Russian observatory, which so long held the
palm over all others, had now been established; and
Thomas Henderson, then working at the Cape of Good
Hope, but afterwards the Astronomer Royal of Scotland.
Henderson's observations had actual precedence
in point of time, but Bessel's measurements were so
much more numerous and authoritative that he has
been uniformly considered as deserving the chief credit
of the discovery, which priority of publication secured
him.
By an odd chance, the star on which Henderson's observations
were made, and consequently the first star
the parallax of which was ever measured, is our nearest
neighbor in sidereal space, being, indeed, some ten billions
of miles nearer than the one next beyond. Yet
even this nearest star is more than two hundred thousand
times as remote from us as the sun. The sun's
light flashes to the earth in eight minutes, and to Neptune
in about three and a half hours, but it requires
three and a half years to signal Alpha Centauri. And
as for the great majority of the stars, had they been
blotted out of existence before the Christian era, we of
to-day should still receive their light and seem to see
them just as we do. When we look up to the sky, we
study ancient history; we do not see the stars as they
are, but as they were
years, centuries, even millennia
ago.
The information derived from the parallax of a star
by no means halts with the disclosure of the distance of
that body. Distance known, the proper motion of the
star, hitherto only to be reckoned as so many seconds of
arc, may readily be translated into actual speed of prog-
ress; relative brightness becomes absolute lustre, as
compared with the sun; and in the case of the double
stars the absolute mass of the components may be computed
from the laws of gravitation. It is found that
stars differ enormously among themselves in all these
regards. As to speed, some, like our sun, barely creep
through space—compassing ten or twenty miles a second,
it is true, yet even at that rate only passing
through the equivalent of their own diameter in a day.
At the other extreme, among measured stars, is one
that moves two hundred miles a second; yet even this
“flying star,” as seen from the earth, seems to change
its place by only about three and a half lunar diameters
in a thousand years. In brightness, some stars yield to
the sun, while others surpass him as the arc-light surpasses
a candle. Arcturus, the brightest measured star,
shines like two hundred suns; and even this giant orb
is dim beside those other stars which are so distant that
their parallax cannot be measured, yet which greet our
eyes at first magnitude. As to actual bulk, of which
apparent lustre furnishes no adequate test, some stars
are smaller than the sun, while others exceed him hundreds
or perhaps thousands of times. Yet one and all,
so distant are they, remain mere disklike points of light
before the utmost powers of the modern telescope.
