The first systematic piece of practical astronomical work which John
Herschel undertook was connected with the measurement of what are
known as "Double Stars." It should be observed, that there are in
the heavens a number of instances in which two stars are seen in very
close association. In the case of those objects to which the
expression "Double Stars" is generally applied, the two luminous
points are so close together that even though they might each be
quite bright enough to be visible to the unaided eye, yet their
proximity is such that they cannot be distinguished as two separate
objects without optical aid. The two stars seem fused together into
one. In the telescope, however, the bodies may be discerned
separately, though they are frequently so close together that it
taxes the utmost power of the instrument to indicate the division
between them.
The appearance presented by a double star might arise from the
circumstance that the two stars, though really separated from each
other by prodigious distances, happened to lie nearly in the same
line of vision, as seen from our point of view. No doubt, many of
the so-called double stars could be accounted for on this
supposition. Indeed, in the early days when but few double stars
were known, and when telescopes were not powerful enough to exhibit
the numerous close doubles which have since been brought to light,
there seems to have been a tendency to regard all double stars as
merely such perspective effects.
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