The Astronomy of the Invisible
But by far the strongest support of such a forecast as
this is furnished by those stellar bodies which even now
appear to have cooled to the final stage of star development
and ceased to shine. Of this class examples in
miniature are furnished by the earth and the smaller of
its companion planets. But there are larger bodies of
the same type out in stellar space—veritable “dark
stars”—invisible, of course, yet nowadays clearly recognized.
The opening up of this “astronomy of the invisible”
is another of the great achievements of the nineteenth
century, and again it is Bessel to whom the honor of
discovery is due. While testing his stars for parallax;
that astute observer was led to infer, from certain
unexplained aberrations of motion, that various stars,
Sirius himself among the number, are accompanied by
invisible companions, and in 1840 he definitely predicated
the existence of such “dark stars.” The correctness
of the inference was shown twenty years
later, when Alvan Clark, Jr., the American optician,
while testing a new lens, discovered the companion of
Sirius, which proved thus to be faintly luminous.
Since then the existence of other and quite invisible
star companions has been proved incontestably, not
merely by renewed telescopic observations, but by the
curious testimony of the ubiquitous spectroscope.
One of the most surprising accomplishments of that
instrument is the power to record the flight of a luminous
object directly in the line of vision. If the luminous
body approaches swiftly, its Fraunhofer lines are
shifted from their normal position towards the violet
end of the spectrum; if it recedes, the lines shift in the
opposite direction. The actual motion of stars whose
distance is unknown may be measured in this way.
But in certain cases the light lines are seen to oscillate
on the spectrum at regular intervals. Obviously the
star sending such light is alternately approaching and
receding, and the inference that it is revolving about a
companion is unavoidable. From this extraordinary
test the orbital distance, relative mass, and actual
speed of revolution of the absolutely invisible body
may be determined. Thus the spectroscope, which
deals only with light, makes paradoxical excursions
into the realm of the invisible. What secrets may the
stars hope to conceal when questioned by an instrument
of such necromantic power?
But the spectroscope is not alone in this audacious
assault upon the strongholds of nature. It has a worthy
companion and assistant in the photographic film,
whose efficient aid has been invoked by the astronomer
even more recently. Pioneer work in celestial
photography was, indeed, done by Arago in France and
by the elder Draper in America in 1839, but the results
then achieved were only tentative, and it was not till
forty years later that the method assumed really important
proportions. In 1880, Dr. Henry Draper, at
Hastings-on-the-Hudson, made the first successful
photograph of a nebula. Soon after, Dr. David Gill,
at the Cape observatory, made fine photographs of a
comet, and the flecks of starlight on his plates first
suggested the possibilities of this method in charting
the heavens.
Since then star-charting with the film has come virtually
to supersede the old method. A concerted effort
is being made by astronomers in various parts of the
world to make a complete chart of the heavens, and
before the close of our century this work will be accomplished,
some fifty or sixty millions of visible stars be-
ing placed on record with a degree of accuracy hitherto
unapproachable. Moreover, other millions of stars
are brought to light by the negative, which are too distant
or dim to be visible with any telescopic powers
yet attained—a fact which wholly discredits all previous
inferences as to the limits of our sidereal system.
Hence, notwithstanding the wonderful instrumental
advances of the nineteenth century, knowledge of the
exact form and extent of our universe seems more
unattainable than it seemed a century ago.
