Object name: COLLINDER21 IC1731

Designation(s): COLLINDER21_IC1731,

Collinder 21, located in Triangulum, is a group of rather bright stars that make a good \"cluster\" for small telescopes. Most sources, however, say it is just an asterism rather than a true star cluster. That is, it is just some bright, unrelated stars that happen to appear close together due to our perspective. Viewed from a different angle the appearance of a cluster would vanish. But since it has a couple interesting galaxies I decided to put it on my to-do list. Being bright I took it on a very poor night with lots of clouds. This caused some nasty halos of various colors around the bright stars. Instead of a quick and easy object to process, it was quite difficult. While I initially balanced the star colors with eXcalibrator by the time I got done dealing with the halos I\'ll only claim the colors to be in the ballpark of correct. The bright star at the south end of the group is SAO 74965 a double star with a separation of only 0.32\". Thus, it appears as a single F5 star of 8.15 magnitude in my image according to TheSky. I saturated the star so can\'t measure it. I\'ll have to take TheSky\'s word for it. It places it at a distance of 562 light-years. The distance to the lower of the pair of stars on the right side of the group is 1553 light-years according to TheSky. TheSky places the red star at the top of the group at only 105 light-years. It doesn\'t have distance data on any others in the group. Still, this is sufficient to show it isn\'t a true cluster. After going though this I found this paper doing a complete analysis of the group, and two others, thinking they were clusters being torn apart but found they are all just asterisms. https://arxiv.org/abs/astro-ph/0407605

Only two galaxies in the image had redshift data. IC 1731 at the top is about 150 million light-years distant by redshift and 130 million by other methods. NSA 130512 at 640 million light-years by redshift is at the very top of my image with its upper part out of the frame. Being near the Zone of Avoidance there\'s little on the field. Everything NED had is noted in the annotated image -- not much. I was disappointed that the apparent rather large, distant galaxy cluster left of IC 1731 wasn\'t listed. It sure looks interesting. IC 1731 was discovered by Isaac Roberts on November 29, 1896.

Just NW of this image is the pair of galaxies, NGC 0672 and IC 1727 which I\'ve already taken. My field is just too small. The two images don\'t quite overlap so I couldn\'t make a mosaic. They are far more interesting than those in this image but there\'s no bright asterism in their image.

14\" LX200R @ f/10, L=4x10\' RGB=2x10\', STL-11000XM, Paramount ME