Now, if we had reason to think that
these stars were of much the same size and brilliance as our sun,
we should be able roughly to calculate their distance from their
faintness. We cannot do this, as they differ considerably in size
and intrinsic brilliance. Sirius is more than twice the size of
our sun and gives out twenty times as much light. Canopus emits
20,000 times as much light as the sun, but we cannot say, in this
case, how much larger it is than the sun. Arcturus, however,
belongs to the same class of stars as our sun, and astronomers
conclude that it must be thousands of times larger than the sun.
A few stars are known to be smaller than the sun. Some are,
intrinsically, far more brilliant; some far less brilliant.
Another method has been adopted, though this also must be
regarded with great reserve. The distance of the nearer stars can
be positively measured, and this has been done in a large number
of cases. The proportion of such cases to the whole is still very
small, but, as far as the results go, we find that stars of the
first magnitude are, on the average, nearly 200 billion miles
away; stars of the second magnitude nearly 300 billion; and stars
of the third magnitude 450 billion.
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