The Caldwell Objects

years, Herschel would observe some of the more
interesting of the bright Messier objects with
different instruments, from the 40-foot reflector
downward. His advancing age gradually put a
stop to the marvelous activity he had shown as
an observer.
COSMIC INTERPRETATIONS
Herschel carefully analyzed everything he saw in
the night sky, and he tried to understand what all
astronomical objects were composed of, and how
and why they acquired their diverse forms.
Herschel believed that stars were all about as
bright as the Sun, and that first-magnitude stars
were the nearest to us, while 2nd-magni-tude
ones were twice as far away. Mathematically he
figured that his large 20-foot telescope showed
stars up to 497 times the distance of Sirius, and
that a larger telescope would reach even farther.
With a telescope, he reasoned, he could not
only see objects at great distances; he could also
look back in time:
A telescope with a power of penetrating into
space like my 40 feet has also a power of
penetrating into time past. To explain this we
must consider that from the known velocity of
light it may be proved that when we look at Sirius
the rays that enter the eye cannot have been less
than 6 years and 4½ months coming from that star
to the observer. Hence it follows that when we see
one of these remote nebulae the rays of light
which convey its image to the eye must have been
almost 2 million years on the way and that so
many years ago this object must already have had
an existence in the sidereal heavens in order to
send out those rays by which we now perceive it.
William Herschel's thinking was limited to
multiples of millions of miles, and therefore it
is quite obvious that he had no idea how large
the universe really was. Nevertheless, he remarkably
proposed a nearly correct distance for
Sirius, and he concluded correctly that the
Andromeda Nebula (M31) was "the farthest
object that can make an impression on the eye
when not assisted by telescopes."
Herschel saw the Milky Way as an extensive
stratum of scattered stars, not unlike the many
objects he had discovered through his telescope,
and concluded:
It is probable that the Sun is placed inside the Milky
Way though perhaps not in the very center of its
thickness. We gather this from the appearance of the
galaxy which seems to encompass the whole heavens
as it must do if the Sun is within the same.
Nebulae, he long reasoned, were composed
of bodies very much like our planetary system,
with the Sun, planets, moons, and comets.
Planets like the Earth could not be seen around
other stars since "bodies shining only with
borrowed light can never be seen at very great
distances." Since nebulae were often seen in beds
of stars — and since the stars must have been
nearer to us than the nebulae — Herschel
concluded that the stars we see one by one in the
heavens belonged to a detached system of which
we are a part, while the Milky Way itself was
composed of an unresolved background of
planetary systems at much greater distances.
As stated previously, Herschel initially
thought that all nebulae could eventually be
resolved into individual stars if only a large
enough telescope could be applied to the task.
However, a series of detailed observations, from
1774 to 1811, of the Orion Nebula — which
Herschel considered the nearest in the heavens
— led him to reconsider this belief. His
observations led him to believe (rightly or
Appendix C 463