The Caldwell Objects

he called "34th Tauri" proved to actually be of the
planet Uranus.)
REALM OF THE NEBULAE
Herschel is perhaps best known for his observations
of nebulae, which in his day were anything
appearing fuzzy and extended. Today we
know a few of these objects really are nebulae
(clouds of gas and/or dust), but most are actually
galaxies, which Herschel termed "milky" or
"detached" nebulae. The publication of Charles
Messier's short list of nebulae and clusters in the
Connaissance des Temps for the year 1783 at once
attracted Herschel's attention and he soon began
to examine them with his "small" 20-foot reflector
of 12 inches aperture. Herschel was not a skilled
artist and never attempted to make elaborate
sketches of his objects. His efforts were solely
intended to show the positions of nebulous
objects with respect to stars.
After observing Messier's objects, Herschel
came to believe that all "nebulae" could be
resolved into clusters of stars, if only they could
be viewed with a sufficiently large telescope. He
noted that many of Messier's nebulae yielded to
the superior force of his larger telescope and
were resolved into individual stars. Messier had
deemed 27 of his catalog's entries to be "nebula[e]
without stars," but Herschel found that these
objects "either plainly appeared to be nothing but
stars, or at least contained stars and gave every
other indication of consisting of stars entirely."
He concluded that Messier and Pierre Mechain
(1744-1805) — who also discovered several
objects in Messier's catalog — "saw only the more
luminous parts of the Nebula in their small
telescopes and the feeble shape of the remainder
for want of light escaped their notice."
Herschel decided to carry out a systematic
search for nebulae with his great telescopes.
462
In the next 20 years he discovered, cataloged, and
added to the science of astronomy no fewer than
2,508 new objects to the 100 or so that had been
known previously.
On June 17, 1784, Herschel wrote that he had
already discovered 466 new nebulae and clusters,
none of which had been seen before and most of
which were not within reach of the best common
telescopes then in use. His first Catalogue of One
Thousand new Nebulae and Clusters of Stars,
published on April 27, 1786, was extracted from a
series of sweeps with the large 20-foot telescope
that began on December 19, 1783, with an
observation of NGC 1055 in Cetus. He carefully
arranged his finds into eight classes according to
appearance and size. Using the same 20-foot
reflector as in the first survey, Herschel began his
second series of sweeps on April 28,1785, with an
observation of NGC 3813 in Ursa Major. Four
years later, on June 11, 1789, his Catalogue of a
Second Thousand of new Nebulae and Clusters of
Stars was published. His final series of sweeps
commenced on December 3, 1788, with an
observation of NGC 2787, and on July 1,1802, he
published his Catalogue of 500 new Nebulae,
nebulous Stars, planetary Nebulae, and Clusters of
Stars.
Upon completing the sweeps in 1802,
Herschel went back to his double-star studies
and reobserved many double stars for proper
motion. A colleague, John Michell, had a theory
that double stars must be physically connected
binary systems rather than chance, or optical,
double systems. Herschel was able to visually
confirm this in 1802 when he reexamined some
of his double stars and noted that their relative
positions had shifted. This observation provided
the first direct evidence that an attractive force
(namely, gravity) actually operated beyond the
confines of the solar system. From time to time
during the next 13
Deep-Sky Companions: The Caldwell Objects