The Caldwell Objects

fluid, but added that ages of observations probably
would be required to test the notion. The
object we know as NGC 6543 (Caldwell 6) was
one example.
Herschel reasoned that if there really were a
connection between a star and its surrounding
nebulosity, then the star would have to be very
distant and uncommonly luminous if the
surrounding nebulosity were in fact a multitude
of unresolved stars. On the other hand, the star
could be ordinary in its properties, but the
surrounding luminosity then had to be "a shining
fluid of a nature totally unknown to us. I can
adopt no other sentiment than the latter," he
decided, "since the probability is certainly not for
the existence of so enormous a body as would be
required to shine like a star of the 8th magnitude
at a distance sufficiently great to cause a vast
system of stars to put on the appearance of a very
diluted milky nebulosity."
Planetary nebulae genuinely puzzled Herschel,
as they were nebulous in appearance yet
seemed to have planetlike disks.
Some look so much like planets that [I] can hardly
suppose them to be nebula[e]. If they were singly
large stars with large diameters then they should be
brighter. They might be comets at aphelion, however
they must consist of stars that are compressed and
accumulated in the highest degree.
Herschel eventually decided that plane-taries
were highly condensed, highly evolved nebulae,
and he surmised that there might be a connection
between planetary nebulae, nebulous stars, and
stellar regeneration.
Herschel considered globular clusters to be
the most interesting objects in the heavens. He
noted that some of them bordered what we call
dark nebulae and speculated that the stars in the
globular clusters somehow had been gath-
ered from adjacent parts of the heavens, which
thereafter were completely devoid of matter.
The 80th object of the Connaisance des Temps [M80, a
globular cluster in Scorpius] is one of the richest
clusters of small stars and is located on the western
border of an opening or hole [today called a dark
nebula] as if the stars were collected from that place
and had left a vacancy.
While he did not understand the true nature
of the objects we now call galaxies, many of
Herschel's thoughts about them were remarkably
prescient. For example, he was the first to note
their tendency to cluster. When he came to one,
he noted, generally he would find several others
nearby on the dome of the sky.
Herschel finally put together a construction
plan of sorts for the heavens by stating that stars
condensed out of a surrounding fluid, the true
nature of which was unknown. Diffuse clouds of
nebulosity would collect into smaller
concentrated clouds, which then condensed into
scattered stars. These individual stars then were
attracted into clusters, and the light given off
from these clusters eventually would collect into
diffuse nebulous clouds, which then would begin
the cycle all over again. Perhaps, Herschel
thought, he had too hastily surmised that all
milky nebulosity was made up of unresolved
gatherings of stars. In fact, it has been postulated
that one reason Herschel quit using the 40-foot
telescope was his realization that larger
telescopes would not resolve all nebulae into
individual stars.
THE GENIUS OF WILLIAM HERSCHEL
While nobody has ever questioned William
Herschel's observational skills or telescopebuilding
abilities, he has been criticized by some
people for his resistance to advice and
commitment to speculation. His common-
Appendix C 465