The Caldwell Objects

56
Astronomical Objects for Southern Telescopes he said
it was faint in a 12-inch. Now let's throw in the
monkey wrench: when I observed NGC 246 for
this book in December 1996 and December 1997,1
had no problem seeing the central star in a 4inch.
Before addressing these problems, we have
yet one more puzzle to consider. Herschel wrote
that the nebulosity is faint. "I question his
description of it as faint," Houston argued, "since
I can see it with the 4-inch Clark." But Hartung
called the nebula "rather faint" in his 12-inch.
Wilson calls it bright in the 13.1-inch, while
Christian Luginbuhl and Brian Skiff state that
"[t]his planetary nebula is faint, but can easily be
found at low power in [a 6-inch]."
Let's tackle the last mystery first. Photographs
tend to make NGC 246 appear deceivingly
small, but it isn't. With a diameter of nearly
4.6', NGC 246 is 60 percent larger than the
famous Owl Nebula (M97), a planetary in Ursa
Major. It also is a full magnitude fainter than the
Owl. And anyone who has searched for the Owl
knows how difficult the task is under anything
but dark skies. The same is true for NGC 246,
only more so. But the popular literature seems to
have perpetuated a myth. Houston writes that
"[m]odern estimates placed the . . . planetary at
magnitude 8.5." Roger Clark makes this same
claim in his Visual Astronomy of the Deep-Sky} and
the Webb Society's Handbooklists NGC 246's
magnitude as 8.0. Actually, NGC 246's
photographic magnitude is 8.0, but its visual
magnitude is 10.9. So the nebula appears much
brighter (relative to comparison stars) in
photographs than it does to the eye. Of course,
one's telescope size, choice of magnification, and
observing site also enter into this equation. But,
truth be told, NGC 246's brightness is simply a
demon to estimate, for the faint nebula has
relatively bright stars superimposed on it.
224
The central-star puzzle is tougher to explain,
though I suspect it is largely the result of an
optical illusion. Of course, some planetaries'
central stars do vary in brightness, and variations
would certainly explain why Hartung thought
NGC 246's central star faint in a 12½-inch
reflector in the 1960s, and Morales and Wilson
failed to see it in the mid-1980s, while I saw it so
readily in the mid- 1990s. Indeed, pho-tovisual
and photometric observations of the central star
have revealed brightness variations over the
decades. In 1930 the central star's photographic
magnitude was as bright as 9, while it dipped to
11.2 in 1969. But this difference in magnitude is
certainly not great enough to explain the abovecited
discrepancies. After all, an 11th-magnitude
star would practically blaze in a 12½-inch
telescope.
I believe there is more to this mystery than
meets the eye. Note that Morales saw three similarly
bright stars, and a fainter, fourth star,
superimposed on the nebula. His description of
these stars matches photographs perfectly.
Morales just did not realize that the secondbrightest
star of the four is the central star. The
same goes for Wilson. You see, the eastern rim of
the nebula is exceedingly faint and fully onequarter
of the shell is all but invisible, even in
photographs. Visually, then, the central star looks
off-center against the nebula. As for Herschel's
observation, I believe he did not see the eastern
part of the nebula. If so, his trapezium could be
formed by the stars labeled a, b, c, and g on the
photograph on page 223.
Astronomers have determined that NGC
246's 200,000° Kelvin central star has one of the
most highly excited spectra known. Most
prominent in this spectrum are lines of thriceionized
carbon (C IV), five-times-ionized oxygen
(Ο VI), and probably some singly ionized helium
(He II). Nitrogen is absent, so this star is quite
evolved and probably has undergone
Deep-Sky Companions: The Caldwell Objects