Sky & Telescope - September 2016 - 56

OBSERVING

Deep-Sky Wonders
16

Around the Summer Triangle

Sh 2-84
Roslund 3

Mag(v)

Size/Sep

RA

Dec.

Double star

6.3, 6.8

1.5″

19h 48.7m

+11° 49′

Emission nebula

-

5′

19h 49.1m

+18° 23′

5′

19h

58.7m

+20° 31′

2′

19h

48.9m

+21° 13′

19h

46.0m

+25° 20′

Open cluster

NGC 6827

Open cluster

Sh 2-88

-
-

Emission nebula

-

17.5′ × 9.0′

Sh 2-88A

Bright nebula

-

1.2′ × 1′

19h 46.6m

+25° 12′

Sh 2-88B

Bright nebula

-

1.2′ × 1′

19h 46.8m

+25° 13′

17 Cygni

Multiple star

5.1, 9.3, 9.4

26″, 108″

19h 46.4m

+33° 44′

3.0″

19h

45.6m

+33° 36′

84′ × 42′

19h

53.6m

+47° 17′

Struve 2576

Double star

Vultus Irrisorie

Asterism

8.5, 8.6
4.2

Angular sizes and separations are from recent catalogs. Visually, an object's size is often
smaller than the cataloged value and varies according to the aperture and magnification of the
viewing instrument. Right ascension and declination are for equinox 2000.0.

The Sh 2-88 complex is about 8,000 light-years distant.
Sh 2-88A's source of excitation is the visible star within.
Sh 2-88B houses a youthful star cluster that can be seen
on some infrared images. One of its stars is the suspected
power source for the observable part of the nebula.
Sweeping northward into Cygnus, we'll find a set of
multiple stars bejeweled with colorful suns, all sharing
the field of view through my 105-mm refractor at 68×. 17
Cygni is a bright, yellow-white star that's attended by two
gold, 9th-magnitude companions. One rests 26″ eastnortheast of its primary and is a suspected variable star.
The other sits southeast at four times that spacing, but it's
not physically related to the other two stars. Struve 2576
(Σ2576 or STF 2576) is a visual binary with an orbital
period of 232 years. It shows two nearly matched gold
stars, currently 3.0″ apart, with the slightly dimmer star
south-southeast of the primary. Struve 2576's stars have
the same proper motion (apparent motion on the celestial
sphere) and distance (69 light-years) as 17 Cygni; therefore,
they may have a common origin.
Let's end our sky tour on a happy note with Vultus
Irrisorie, the Smiley Face, a binocular asterism fashioned by Alabama amateur Roseann Johnston on July
21, 2001. Although it dwells just beyond the Summer
Triangle's northern border, this winsome face is simply
too delightful to pass by. If you place Delta (δ) and Omicron1 (ο1) Cygni in your field of view, the asterism will be
between the two and a bit farther away from the Swan's
body. Five stars form a north-south, mouth-stretching
grin, while two stars closer to Delta are the eyes. The
stars range from magnitude 5.6 to 7.6. Through most
binoculars, the face will span about one quarter of the
56

September 2016 sky & telescope

θ
+50°

CYGNUS

ο2

+48°

ο1

Vultus
Irrisorie

2

+46°

δ

3
4
5
6
7
8

+44°

20h 10m

20h 00m

19h 50m

19h 40m

field. Many telescopes can't attain a low enough power
to show an object as large as Vultus Irrisorie. Using an
eyepiece with a 50°° apparent field, you'll need a magnification of 25× or less to frame the group well.
Sadly, Roseann is no longer with us, but we can still
see her smiling down from her perch in the sky, saying,
"Have a nice night!" ✦

Sh 2-88

Sh 2-88B
Sh 2-88A

Sh 2-87

If you have trouble spotting the emission nebula Sh 2-88, try
adding an O III filter to the eyepiece. The above image shows a
1° 56′ × 1° 28′ field of view as recorded in thirty 40-minute exposures through a 530-mm f/5 astrograph with an H-alpha filter.

MIKE SCHUSTER

Pi (π) Aquilae

Type

Star magnitudes

Object

26



Sky & Telescope - September 2016

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