2019-08-01_Sky_and_Telescope

56 AUGUST 2019 • SKY & TELESCOPE

The Eagle and Its Nestlings

Object Type Mag(v) Size/Sep RA Dec.

Messier 16 Cluster / nebula 6.0 34 ′ × 27 ′ 18 h 18.8m –13° 50′

Trumpler 32 Open cluster 12.2 12 ′ 18 h 17.2m –13° 21′

Sh 2-48 Emission nebula — 10 ′ 18 h 22.4m –14° 36′

Sh 2-54 Emission nebula — 144 ′ × 78 ′ 18 h 19.7m –12° 04′

B95 Dark nebula — 30 ′ 18 h 25.6m –11° 45′

NGC 6631 Open cluster 11.7 7.0′ 18 h 27.2m –12° 02′

NGC 6517 Globular cluster 10.2 4.0′ 18 h 01.8m –8° 58′

Tau Ophiuchi Double star 5.3, 5.9 1. 5 ′′ 18 h 03.1m –8° 11′

NGC 6539 Globular cluster 9.3 7.9′ 18 h 04.8m –7° 35′

IC 1276 Globular cluster 10.3 8.0′ 18 h 10.7m –7° 12′

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 magnifi cation of the viewing instrument.

Right ascension and declination are for equinox 2000.0.

is NGC 6517, located 1.1° northeast

of Nu (ν) Ophiuchi and 5′ north-

northeast of a 10th-magnitude star. The

cluster is easy to overlook at low power

but emerges nicely through my 105-mm

refractor at 122×. It covers a scant 1½′

and grows brighter toward the center.

A 9′-long zigzag of six fi eld stars passes

west of the cluster. With my 10-inch

refl ector at 115×, NGC 6517 is 2½′

across and clasps a tiny bright nucleus.

The outer regions appear granular, while

the interior is coarsely mottled. NGC

6517 is elongated northeast-southwest

at 213×, and a faint star guards its

south-southwestern edge.

Moving 50′ north-northeast of NGC

6517 brings us to the lovely double star

Tau (τ) Ophiuchi. Tau is a visual binary

with a period of 257 years. Currently

the 5.9-magnitude companion is 1.5′′

west-northwest of its 5.3-magnitude

primary and coming 0.1′′ closer every

seven years. The pair is comfortably

split through my 4.1-inch refractor at

174 ×. To my eye, the brighter star glows

yellow-white, and its companion shines

yellow. As we’d expect for a double that

exhibits a visible shift in position over a

relatively short period of time, Tau is a

nearby pair only 170 light-years away.

At low power, Tau shares the fi eld

of view with the globular cluster NGC

6539 , which straddles the Ophiuchus-

Serpens border 44′ northeast of Tau.

Through my 105-mm scope at 87×, this

softly glowing ball of light is 5′ across

and patchy in brightness. Some faint

foreground stars dot the western side

of the cluster.

Nudging the telescope 1.5° east-

northeast takes me to the subtle lights

of IC 1276, nestled in the northern

corner of a 13′ triangle of 11th- and

12th-magnitude stars. This dim globu-

lar cluster presents a 2′ halo and a tiny

mottled center barely ½′ across. At

122 × I see a very faint star punctuat-

ing the cluster’s western side and an

occasional glint in the core. With my

10-inch scope at 213×, several threshold

stars form an east-west band across the

cluster’s face.

NGC 6539 has the same intrin-

sic brightness as NGC 6517, but it’s

closer to us and looks correspondingly

brighter. These clusters lie at distances

of 25,400 and 35,200 light-years,

respectively. IC 1276 is comparatively

nearby at 17,600 light-years, but it gives

off a fi fth as much light. This makes it

the faintest of the three globulars by

a small margin. The natural glory of

these clusters is diminished not only by

distance, but also by interstellar dust

between us and them. The dust grains

absorb and scatter the light emitted by

the globulars, dimming each by about

three magnitudes.

¢ Contributing Editor SUE FRENCH

penned this column for the August 2009

issue of Sky & Telescope.

pThe emission nebula Sharpless 2-54 spans more than 2º of sky. When scanning the nebula with

her binoculars, the author overlooked NGC 6604, the tiny knot of stars in the southwest sector of

the nebula. Can you see this open cluster in your telescope?

AUGUST 2019 OBSERVING
Deep-Sky Wonders

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NGC 6604