The Caldwell Objects

qualifier—the word "light" — has subsequently
been dropped in the decades that have passed
since he drew attention to the nebula's curious
color. Without the qualifier, one expects to see a
remarkably blue color that really isn't there. In
my notes I describe NGC 7662 as having a
"slightly bluish glow"; Christian Luginbuhl and
Brian Skiff say it has a "faint bluish light"; and, in
his Astronomical Objects for Southern Telescopes,
Ernst J. Hartung calls it a "vivid pale blue disk"
(all emphases mine). But how much of this is
suggestion?
A planetary nebula shines when its diffuse
gases fluoresce after being energized by strong
ultraviolet radiation from its hot central star.
That star's extremely high surface temperature
can be anywhere from 30,000 to several hundred
thousand degrees Kelvin. (NGC 7662's central
star has a surface temperature of 75,000° Kelvin.)
The visible-light spectrum of a planetary nebula
is dominated by two spectral lines of doubly
ionized oxygen, whose wavelengths are 4959 and
5007 angstroms, respectively; these lines are in a
portion of the visible spectrum that some refer to
as "green" and others "blue-green." Thus we
should not be surprised, perhaps, that
astronomer David A. Allen used the words "rich
green" to describe his view of NGC 7662 through
the 60-inch reflector atop Mount Wilson — the
numerous "blues" reported in the previous
paragraph notwithstanding. Your eye, the nebula's
altitude, the clarity of the air, and your telescope's
aperture, magnification, and design — all
these factors and more may affect the color you
see when looking at a planetary nebula. Some
would argue that no star, let alone a deep-sky
object, should show any color, since the nightsensitive
rods are colorblind. But it is conceivable
that bright, condensed objects like NGC 7662 do
emit enough light to stimulate the eye's colorsensitive
cone cells.
The Caldwell Objects
22
NGC 7662 is one of the many planetaries imaged
by the Hubble Space Telescope, whose repaired
optics are expanding our envelope of knowledge
about these enigmatic objects. Bruce Balick
(University of Washington) and his colleagues
recently combined HST images and groundbased
spectra of NGC 7662 to study the nebula's
internal motions. They found that the
northeastern region of the planetary's bright
interior ellipse is blueshifted while the
southwestern part is redshifted, and they
concluded that a good three-dimensional
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