Sky and Telescope - October 2015 - 55

Sue French

Sue French welcomes your comments at scfrench@nycap.rr.com.

Ruprecht 173

The Stars of Cygnus
Object
HD 197292

Type

Dec.

41.7m

+35° 33′

Open cluster

40.0′

X Cygni

Variable star

20 h 43.4m

+35° 35′

Ruprecht 175

Open cluster

15.0′

20h 45.4m

+35° 31′

9.0′

20h

43.4m

+37° 01′

20h

41.7m

+36° 37′

Open cluster

Dolidze 47

Open cluster

5.0′

DoDz 11

Open cluster

13.1′

20h 51.0m

+35° 50′

Remnant cluster

16.2′

21h 10.8m

+37° 34′

Dolidze 45
HD 197419

RA
20h

Ruprecht 173

Ruprecht 174
X

Size

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.

BERNHARD HUBL

table is from the 2013 catalog of Nina V. Kharchenko and
colleagues.
A half degree southwest of Ruprecht 174, we ﬁnd
Dolidze 47, named for Georgian astronomer Madona
V. Dolidze. Unlike many candidate open clusters, the
Dolidze groups weren't proposed due to their visual
appearance. In the course of her spectral studies of stars
at Abastumani Astrophysical Observatory, Dolidze noticed
groups of hot young stars with similar apparent brightness and other groups with a seemingly cluster-like distribution of spectral type and brightness (main-sequence).
Despite this promising-sounding method of discovery,
21h 10m

21h 00m

20h 50m

20h 40m

+40°

σ
61
+38°

CYGNUS

Do 45

Ru 174
Do 47

λ
DoDz 11

+36°

X
Ru 173

Star magnitudes

north of the circlet's western edge emit a golden glow. A
wide W of ﬁve stars sits southwest of the circlet, and 40
to 50 fainter stars are loosely scattered about, forming a
group with indeﬁnite borders that's only slightly more
prominent than the starry background. The catalog in
Star Clusters (Brent A. Archinal and Steven J. Hynes)
gives this cluster a diameter of 40′, while Sky Catalogue
2000.0 (Alan Hirshfeld and Roger W. Sinnott) lists 50′.
Sharing the ﬁeld of view with Ruprecht 173, Ruprecht 175 is a very granular haze overlaid by several stars
magnitude 9 and fainter. Boosting the magniﬁcation to
91×, Ruprecht 175 displays about 20 moderately bright
to faint stars over a dim, patchy haze roughly 8′ across.
This corresponds fairly well to the 9′ size given in Sky
Catalogue 2000.0, but according to Star Clusters, Ruprecht 175 spans 15′.
A 1998 paper in the Astronomical Journal further
complicates the cluster situation. Canadian astronomer
David G. Turner proposes that Ruprecht 175, X Cygni,
and the southernmost part of Ruprecht 173 may form a
physically related group. He gives the sizes of Ruprecht
175 and Ruprecht 173 as 9′ and 50′ × 30′, respectively.
Ruprecht 174 also inhabits this area of the sky, 57′
west-northwest of Lambda (λ) Cygni. My 130-mm scope at
48× shows a grainy patch of mist just south of a 10th-magnitude star, with a few extremely faint stars involved. At
91× I see 15 faint to very faint stars over unresolved haze
about 6′ across. With my 10-inch reﬂector at 187×, I count
37 moderately faint to very faint stars gathered into a 7½′
× 4½′ oval, tipped northeast. A more densely concentrated
region stretches northeast from the center. A relatively
bright, trapezoidal group of several stars, including the
10th-magnitude one, rests atop the cluster.
The position for Ruprecht 174 given in this article's

3
4
5
6
7
8
9

Ru 175

+34°

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Sky and Telescope - October 2015

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