101 SKY OBJECTS
WWW.ASTRONOMY.COM 119 The Blinking
Planetary
Here’s an object that’s a lot of fun to show to others
at summer and fall star parties. Through 8-inch
and smaller telescopes, the Blinking Planetary
(NGC 6826) in Cygnus appears to blink when
viewed with direct, and then averted, vision.
To find this nebula, point your telescope 1.4°
east of the magnitude 4.5 star Theta (θ) Cygni.
This planetary nebula measures 25" across and
glows at magnitude 8.8. It lies some 2,000 light-
years away.
The first observers to record this effect were
American astronomers James Mullaney and
Wallace McCall at Allegheny Observatory in
Pittsburgh. They wrote in August 1963 about the effect, which they saw while using the obser-
vatory’s 13-inch Fitz-Clark refractor.
More recently, astronomers studying images of the Blinking Planetary have identified two
slightly fainter spots on either side of the nebula. Christened Fast Low-Ionization Emission
Regions (FLIERs), they are clouds of gas moving away from the central star much faster than
the planetary’s shell. They’re a bit fainter because high-energy ultraviolet radiation from the
star, which ionizes the gas in the expanding shell, doesn’t have the same effect on the gas in
the FLIERs. The higher density of these regions probably accounts for this difference.
With a medium-sized scope and using direct vision, you’ll spot the 11th-magnitude central
star easily, but the nebula won’t be visible. Use averted vision and look just a bit to the side of
the star, and the nebula pops into view. Not only that, its apparent brightness under averted
vision swamps the star’s light. So, by looking back and forth with averted and then direct
vision, you can make this object blink. It takes some practice, but the result is quite fun.
The best way to see this effect is from a dark site with a 6-inch scope and a magnifica-
tion of about 100x. Do note that if you use too large a scope, the brightness of the nebula will
overwhelm the star and you won’t see the blinking effect. — M.B.1 1 The Coma
galaxy cluster
As you might guess, the Coma galaxy cluster
resides within the boundaries of the constellation
Coma Berenices. If you have access to a large ama-
teur telescope, you can search for it 2.7° due west of
magnitude 4.2 Beta (β) Comae Berenices.
The Coma Cluster is huge, with over 1,000 galax-
ies within a 4°-wide field. The richest area, however,
is the central 0.5°, a region that covers as much sky
as the Full Moon.
This cluster’s designation is Abell 1656, taken
from American astronomer George O. Abell’s
catalogs. His original list contained 2,712 clusters
in mostly the northern sky. An extension (after his
death) covering the far southern sky brought the
total number to 4,073 galaxy clusters.
Because this cluster lies some 330 million light-
years away, not every galaxy in it is going to be a
standout. What’s more, this cluster is dominated
by elliptical galaxies that show few details. So, if
you can view through a 12-inch or larger telescope
from a dark site, at least try to identify the brightest
members.
Four galaxies glow brighter than 12th magnitude:
NGC 4884 (magnitude 11.5), NGC 4889 (11.5),
NGC 4793 (11.6), and NGC 4874 (11.7). Five sur-
pass 13th magnitude: NGC 4789 (magnitude 12.1),
NGC 4839 (12.1), NGC 4921 (12.2), NGC 4911 (12.8),
and NGC 4827 (12.9). If you start with a good finder
chart of the region, you should be able to identify
several dozen more by using these nine galaxies as
starting points.
The Coma Cluster has an invisible claim to fame.
In 1933, Swiss astronomer Fritz Zwicky was studying
the motions of galaxies within Abell 1656. He con-
cluded that the galaxies he could see were revolving
around the center of the cluster 400 times faster
than they should be, based on the visible mass
of the cluster. He called the unseen mass dark
matter. — M.B.JOHN CHUMACKRONALD BRECHER
TIM LOCKHART10 NGC 6781
Planetary nebula NGC 6781 lies in the constellation Aquila the Eagle. But while this star figure
ranks 22nd in size among the 88 constellations, you won’t find any Messier objects or emission
nebulae in it. In fact, although it sits squarely within the Milky Way, it contains few even reason-
ably bright star clusters.
NGC 6781 doesn’t make up for all that, but it is a fine target for amateurs with medium-sized
telescopes. This interesting planetary lies some 3,500 light-years away and is about 2 light-years
in diameter. Like all such objects, however, the bubble continues to expand and will do so until
it’s too far from the central white dwarf for the star’s radiation to ionize it. At that point, it simply
will fade from view.
To find NGC 6781, aim your telescope 3.8° north-northwest of magnitude 3.4 Delta (δ) Aquilae.
What you’ll see is an almost perfect bubble of gas cast off by a star that once generated energy
like our Sun, but which has long since stopped fusing hydrogen into helium within its core.
Through a 6-inch telescope at 100x, magnitude 11.4 NGC 6781 stands out well against the
rich, star-filled background of Aquila. Through larger instruments, you’ll see that the disk
doesn’t have a sharp edge and is slightly oval-shaped. If the seeing (atmospheric steadiness)
at your observing site is
good, you’ll notice that
the central region of the
nebula is darker than its
surroundings.
If you’re able to
observe this planetary
through a 14-inch or
larger scope, you’ll see
lots of structure in its
thick ring. At high powers,
you’ll spot small, dark
blotches across its face.
The planetary’s central
star, now a slightly blu-
ish white dwarf, glows
weakly at magnitude 16.2,
so don’t spend too much
time searching for it. — M.B.ADAM BLOCK/MOUNT LEMMON SKYCENTER/UNIVERSITY OF ARIZONA