Sky and Telescope - October 2018 - 31

By disappearing when you look directly at it! The central
star that produced the nebula is bright enough (magnitude
10.6) to overwhelm the soft glow of the nebula when you
look straight at it, but averting your eyes brings the nebula
right back to surprising prominence. The
nebula isn't doing the blinking; you are
by moving your eyes away and back. It's
a great effect, probably best in an 8-inch
telescope, but you can play with magniﬁcation or an aperture mask to get it the
right brightness in just about any scope.
If you're having trouble ﬁnding the
Blinking Planetary, look for 16 Cygni, a
double star about ½° due west. 16 Cygni
is naked-eye visible (its components are
magnitudes 6.0 and 6.2, for a combined
brightness of 5.3) and easy to split (39″),
and it makes for a good signpost that
you're close to the Blinking Planetary. Since the planetary lies
directly east of it, just center 16 Cygni, wait three minutes,
and the Blinking Planetary will be centered.
As long as we're playing with interactive objects, there's
another great one to the south a ways: Epsilon (İ) Pegasi,
or Enif, the nose of Pegasus. This one is a wide double star
with enough difference in brightness between its components
(magnitudes 2.5 and 8.7) to play a neat trick on your eye. If
you wiggle the telescope in a direction perpendicular to the
binary split, you'll see the bright component move back and
forth just as you would expect, but the dim component lags

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behind looking for all the world like a glittering diamond
swinging back and forth at the end of a string attached to the
primary. For this reason, Enif is called the Pendulum Star.
What's happening here? It's a latency effect: Your eye takes a
moment longer to register a dim star than
it does a bright one, so when the stars are
in motion the dim one lags behind.
While you're in Enif's neighborhood,
you might as well move 4° to the northwest and have a look at M15, a tight,
bright globular cluster that's spectacular
in any scope. This is one of the most
tightly packed globulars in the sky, with
half its mass concentrated in the central 10 light-years of its 175 light-year
expanse. You can really see that when
you crank up the magniﬁcation and try
to split the core into individual stars.
There are just too many to do it!
Let's move back up toward the zenith again to pick up a
couple more neat objects we passed over on our way to Enif.
We've probably all heard of, and seen, Epsilon (İ) Lyrae, the
famous double-double star in Lyra. With two tight (2.1″ and
2.4″) pairs separated by a generous 210″, this is a favorite test
of telescope optics. But Lyra holds a second double-double
that I think is even prettier: Struve 2470 and Struve 2474.
These pairs are separated by an even more generous 10′, and
the individual pairs are separated by 16″ and 14″. Both of
them split east-west, and both have the brighter component
to the east. They look like a double and its reﬂection in still
water. This is a great object for star parties; they're much
easier to split than Epsilon, and more beautiful.

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JERRY OLTION (5)

NGC 6802

p EPHEMERAL CLUSTER
Soft and delicate NGC 6940 is
one of the most beautiful open
clusters in the sky. FOV = 1° 20′

u DO YOU SEE IT? The Coathanger looks
just like its namesake. In Newtonian scopes,
it's right-side up. Look for diminutive NGC
6802 just to the east of it. FOV = 3°
sk yandtelescope.com * OCTOBE R 2 018

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Sky and Telescope - October 2018

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