This whole amount of light would have to be
multiplied by 90,000,000 to make a light equal to that of the sun.
It is, therefore, not at all necessary to consider how far the
system must extend in order that the heavens should blaze like the
sun. Adopting Lord Kelvin's hypothesis, we shall find that, in
order that we may receive from the stars the amount of light we
have designated, this system need not extend beyond some 5000
light-years. But this hypothesis probably overestimates the
thickness of the stars in space. It does not seem probable that
there are as many as 1,000,000,000 stars within the sphere of 3300
light-years. Nor is it at all certain that the light of the
average star is equal to that of the sun. It is impossible, in the
present state of our knowledge, to assign any definite value to
this average. To do so is a problem similar to that of assigning
an average weight to each component of the animal creation, from
the microscopic insects which destroy our plants up to the
elephant. What we can say with a fair approximation to confidence
is that, if we could fly out in any direction to a distance of
20,000, perhaps even of 10,000, light-years, we should find that
we had left a large fraction of our system behind us.
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