The Caldwell Objects

wrongly) that the nebulosity around the 7thmagnitude
star in the heart of nearby M43 had
steadily decreased in intensity and size until
nothing could be seen around it in 1811, even with
the 40-foot telescope. This led him to believe that
some of the nebulous matter must be in motion
with respect to the star, which was behind it, and
that the nebulous matter scattered the starlight.
To Herschel, these subtle changes in the
nebula's shape and luster meant that the nebula
was composed not of stars but of an unknown type
of luminous matter. In 1802 he wrote, "To attempt
even a guess at what this light may be would be
presumptuous. If it is surmised to be of the nature
of the zodiacal light, what is the cause?" Other
observations of the "Lucid Spot in Orion"
convinced him of its intrinsic faintness; he figured
it could be only two to three times as far as Sirius,
while the dim 6th- or 7th-magnitude stars had to
be far more distant. These nebulous objects, Herschel
reasoned, had to be closer than the stars in
order to be seen at all. This was a complete reversal
of his previous ideas regarding the distances to,
and the composition of, nebulae.
After many years and thousands of observations
Herschel gradually settled on the idea of a
universe in transition. Nebulae and star systems
slowly developed over time under the constant
action of gravity, and the source of their
luminosity was unknown. A nebula that was a
"little brighter in the middle" than on its periphery
had not undergone much central attraction and
therefore was not very advanced. A nebula
"gradually brighter in the middle" was in a more
advanced evolutionary state, while one that appeared
"gradually much brighter in the middle"
was even more evolved. Here gravity had had a
longer time, possibly millions of years, to act.
Herschel's "Round Nebula[e]" with nuclei
were special cases:
464
When round nebulae have a nucleus, it is an
indication that they have already undergone a
high degree of condensation. I suppose there are
not less than one thousand of these round objects.
These clusters of stars are more condensed and
brighter in the middle and the central brightness
must be the result of central powers. [A] Nebula
that [is] composed of a thousand stars must arrive
at the perfection of its form sooner than another
which takes in a range of a million years. A
cluster or nebula [that] is very gradually more
compressed and brighter to the middle may be in
the perfection of its growth while another with a
more equal compression, which I have called
Planetary, may be looked upon as very aged.
Nebulae that enveloped stars within diffuse
material represented a further stage of stellar
development and became an important analytical
tool. On October 16, 1784, Herschel discovered
the nebula in Monoceros now known as NGC
2170 and described it thus:
A star of about 9th magnitude, surrounded by a
milky nebulosity or chevelure [luminous
atmosphere], of about 3 minutes in diameter. The
nebulosity is very faint and a little extended or
elliptical. The chevelure involves a small star,
which is about 1½ minute north of the cloudy star;
other stars of equal magnitude are perfectly free
from this appearance.
It seemed reasonable that the nebulosity
belonged to the star in its center (or vice versa).
Herschel's discovery of "Nebulous Stars," or
stars with atmospheres — which previously had
been mentioned by other astronomers —
compelled him, again, to conclude that there
must be some kind of luminous fluid in space,
the nature of which was not yet understood. He
surmised the star was condensing out of the
Deep-Sky Companions: The Caldwell Objects