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n last month’s column, we began a
survey of colors visible in the summer
night skies. That column focused on
star colors. We continue now with the
hues of summer’s various nebulae and
star clusters — with a bonus at the end.
Extra colors in the brightest
planetaries. An arc of bright planetary
nebulae is visible from Lyra to Aquarius
on late-summer and early-autumn
evenings. These glowing grave wreaths
of stars are the Ring Nebula (M57) in
Lyra, the Dumbbell Nebula (M27) in
Vulpecula, the Saturn Nebula (NGC
7009) in Aquarius, and the Helix
Nebula (NGC 7293) also in Aquarius.
The Saturn Nebula displays the most
intense characteristic color of these neb-
ulae: blue (or greenish-blue if the lenses
of your eyes have become too yellowed by
decades of exposure to the Sun’s ultra-
violet radiation). The Helix Nebula has
great total brightness yet is spread out so
much that it has low surface brightness,
which makes it hard to see its color.
But what about the Ring Nebula
and Dumbbell Nebula? In small- and
medium-size telescopes the Ring may
not show color, appearing just like a
mystical phosphorescent smoke ring
fl oating in space. But long ago I was
delighted to glimpse red, yellow, and
blue in the Ring for the fi rst time
— with one of the fi rst commercial
13-inch Dobsonians. Soon after, I did so
with a 10-inch telescope. (By the way,
you may need a night of good seeing,
i.e., excellent observing conditions with
little to no atmospheric disturbance, to
detect these colors in M57.)
You may have to work to draw color
out of the gauzy white haze of M27, too.Celestial
Palette
We continue with our
exploration of colors in
the heavens.If you start with the nebula’s apple-core
shape in small scopes and then move
up to larger scopes, you begin fi lling in
(un-eating!) the apple. At what apple-
ture — I mean aperture! — do you start
detecting colors?
Our Sun will puff out into a plan-
etary nebula as it dies. But more mas-
sive stars produce supernova remnants
(SNRs). The most famous SNR of
summer is the Veil Nebula in Cygnus,
whose strands are visually red, white,
and blue — in large amateur telescopes.
How much aperture (maybe 16 inches?)
do you need to detect its colors?
Diffuse nebulae and both kinds of
star cluster. Photographs show the red
of emission nebulae and blue of refl ec-
tion nebulae in parts of some bright
summer nebulae like the Trifi d Nebula
(M20) and the Lagoon Nebula (M8).
Can you start detecting these colors in
fairly small telescopes (see page 22)?
And what about open star clusters?
The Double Cluster in Perseus is still
pretty low at the time of our all-sky
map on pages 42–43, but one mem-
ber of this dynamic duo of clusters is
famous for housing a smattering of
warm-hued stars. My favorite open
cluster of all is the Wild Duck Cluster
(M11) near the meridian in the south
on our map. I see the basic fan shape
of this cluster as an avalanche of stars
tumbling from the 8th-magnitude
brightest sun, which shines near theapex of the fan. This star’s color has
been called “saffron” by the skilled and
poetic observer Steve O’Meara.
O’Meara also looks for color in glob-
ular clusters, noting that M2 in Aquar-
ius has “a yellow outer core that has a
diffuse, pale-blue halo.” I certainly see
this. Do you agree with O’Meara that
the Great Globular Cluster in Hercules
(M13) has a yellow core with a slightly
greenish halo?
The palette of atmospheric optics.
For a change of pace from the wonder-
ful subtle colors of objects in night
skies, switch to day and the intense
colors found in many phenomena of
atmospheric optics. The yellow (between
red and blue) in some parhelia (better
known as “sun dogs” or “mock suns”)
can be especially vivid. Look in cirrus
clouds for the rainbowlike circum-
zenithal arc way above the Sun when the
Sun is 32° or less above the horizon, and
the circumhorizontal arc way below the
Sun when the Sun is 58° or more above
the horizon. When thin clouds partially
veil the Sun, look for a blue-green and
red “cloud corona” around the Sun’s
refl ection in a car’s windshield or a
humble puddle of rainwater. For more
on these and numerous other phenom-
ena, check out Les Cowley’s superb
website atoptics.co.uk.¢FRED SCHAAF welcomes your letters
and comments at [email protected].Under the Stars by Fred Schaafskyandtelescope.com • AUGUST 2019 45
A circumhorizontal arc shimmers
over rooftops and treetops.