A Text-Book of AstronomyComstock, George C. (George Cary)
Science
A Text-Book of Astronomy
Comstock, George C. (George Cary)
Astronomy
The magnitudes of the telescopic stars have not yet been measured
completely, and their exact number is unknown; but if we apply our
principle of a threefold increase for each successive magnitude, we
shall find for the fainter stars--those of the tenth and twelfth
magnitudes--prodigious numbers which run up into the millions, and even
these are probably too small, since down to the ninth or tenth magnitude
it is certain that the number of the telescopic stars increases from
magnitude to magnitude in more than a threefold ratio. This is balanced
in some degree by the less rapid increase which is known to exist in
magnitudes still fainter; and applying our formula without regard to
these variations in the rate of increase, we obtain as a rude
approximation to the total number of stars down to the fifteenth
magnitude, 86,000,000. The Herschels, father and son, actually counted
the number of stars visible in nearly 8,000 sample regions of the sky,
and, inferring the character of the whole sky from these samples, we
find it to contain 58,500,000 stars; but the magnitude of the faintest
star visible in their telescope, and included in their count, is rather
uncertain.
How many first-magnitude stars would be needed to give as much light as
do the 2,843 stars of magnitude 5.0 to 6.0? How many tenth-magnitude
stars are required to give the same amount of light?
To the modern man it seems natural to ascribe the different brilliancies
of the stars to their different distances from us; but such was not the
case 2,000 years ago, when each fixed star was commonly thought to be
fastened to a "crystal sphere," which carried them with it, all at the
same distance from us, as it turned about the earth. In breaking away
from this erroneous idea and learning to think of the sky itself as only
an atmospheric illusion through which we look to stars at very different
distances beyond, it was easy to fall into the opposite error and to
think of the stars as being much alike one with another, and, like
pebbles on the beach, scattered throughout space with some rough degree
of uniformity, so that in every direction there should be found in equal
measure stars near at hand and stars far off, each shining with a luster
proportioned to its remoteness.
Public-domain text, read in full here on John Shaqi.
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