Object of the Week October 14, 2018 - Collinder 21 with background galaxies (IC 1731)

[Uwe Glahn]

Collinder 21 (OCL 371, C 0147+270)
Type: Open Cluster (Asterism)
Constellation: Triangulum
RA: 01h 50m 12s
Dec: +27° 04.8’
Size: ~7’

IC 1731
Type: Galaxy
RA: 01h 50m 13
Dec: +27° 11.8’
Magnitude: 13.4vmag

Moon is just rising, time for an easy but beautiful pair of an Open Cluster and some background Galaxies, particularly IC 1731.

In the southern part of Triangulum, 5° SE of Messier 33, we found a showy ring of several 8mag-11mag stars. "Per Collinder", a Swedish Astronomer catalogued this group as an Open Cluster "Collinder 21". Modern research [A&A 428, 67-77 (2004)] shows that Cr 21 is not an Open Cluster because of very different velocity components of the stars. Today we better should name the star group "Asterism" which makes it not without interest for us visual observer.

Directly north of Cr 21 we found a brighter galaxy - IC 1731. It was discovered by the Welsh Astronomer Isaac Roberts in 1896. With his silvered 20-inch Newton, he photographed the galaxy and published his results in 1897 [1897AN....143..341S], also with the reference to the star group. With its 13.4vmag the galaxy is a target for almost 6-inch to 8-inch telescopes.

For owner of very large telescopes there are some other reachable galaxies in the field, the easiest one is MCG +04-05-016 with given 14.6bmag at NW edge of the Asterism. 2MASS J01501150+2708581 with its 18mag is somewhat fainter but in reach of many telescopes of this forum.

Beside the beauty of the pair, the different distances makes it interesting to observe. First the stars within Cr 21 itself differ between 200 Lj and 4800 Lj, but IC 1731 is around 130 MLj away.

DSS blue 15'x15'
Cr21_15b.jpg

sketch: 8", 114x, NELM 6m5+
Cr21.jpg

sketch: 27", 172x, NELM 6m5+
Cr21_27.jpg

Give it a go and let us know.

[Steve Gottlieb]

Nice juxtaposition of different object types!

This observation was made back in 1999 using a 17.5-inch.

IC 1731: Very faint, fairly small, low surface brightness glow elongated 3:2 WNW-ESE. IC 1731 is located 34' SE of NGC 672.

Cr 21, immediately to the south of IC 1731, is a distinctive 5' group of 10 brighter mag 8.5-12 stars, including STF 172, a mag 10 pair at 18" separation. The brighter stars form a rough semicircle open to the east with five fainter stars bringing the total to ~15.