HCG 84
Ursa Minor
Position (brightest member)
RA 16 44 23.0
DEC +77 51’ 31”
Size: 12'
Mag: 15.4-17.2(V)
Type: Galaxy Group
HCG 84 is a (surprise!) compact galaxy group in Ursa Minor, well located for easy field finding and well placed for at least some part of every single night, especially for northern latitudes. July evenings are the best times to observe this group as it reaches its highest point late in the evening.
HCG 84, the third HCG to feature in the OOTW series, consists of six components, ranging in magnitude from 15.4 to 17.2. Like all Hickson compact groups, the components are quite small. Thus, they benefit from high magnification and steady seeing conditions. The location of HCG 84, at declination of almost 78 degrees north, makes this group ideal for those who lack an equatorial platform as its movement in the eyepiece is quite slow. The six components are:
HCG 84A = CGCG 355-20A, mag 15.4, size 0.7’x0.4’
HCG 84B = PGC 58873, mag 16.5, size 0.6’x0.2’
HCG 84C = PGC 58884, mag 16.2, size 0.4’x0.2’
HCG 84D = PGC 58861, mag 17.2, size 0.3’x0.2’
HCG 84E = PGC 58881, mag 17.2, size 0.2’x0.1’
HCG 84F = PGC 58856, mag 17.2, size 0.2’x0.2’
(All data taken from Alvin Huey’s Hickson Group Observer’s Guide).
H84labels.jpg
I observed HCG 84 on two consecutive evenings, June 9th and June 10th 2013. As per my usual practice, whenever I observe a particular target for the first time, I avoid to read anything about its appearance beforehand, so as not to bias my observation.
On June 9th, transparency was so and so (SQM-L 20.95). I located the field quite easily, but I was unable to see anything beyond field stars in my 18” f4.5 dob.
On June 10th, from the same exact location, transparency improved considerably (SQM-L 21.15). Using my hooded vest, at 294X I initially could detect “something” with averted vision. Over time, two distinct fuzzy patches materialized, that I could hold steady all the time with averted vision, and could occasionally see them with direct vision. The southernmost one seemed more distinct and slightly elliptical, while the northernmost one seemed fainter and somewhat more elongated. The two faint patches appeared to be oriented nearly perpendicular to each other. Also, the northernmost fuzzy patch seemed to have a slight division in the middle. I could hold this “rift” feature steady only at times. After some more time observing, the “rift” in the northern fuzziness seemed to become steadily visible, giving this a “double galaxy” appearance. In addition, further to the south of the brighter patch, I could detect a very faint star that however much I tried I could not focus crisply enough. I tried to push my magnification to 588X, but the seeing was not steady enough and I could detect with certainty only the central fuzziness, the northernmost galaxy “pair” seemingly disappearing.
H84A1.jpg
After returning home and checking my drawing with DSS images and Huey’s labeled picture in the book, as well as with Megastar, I could identify the brightest patch as component “A”, CGCG 355-20A. The northernmost patch corresponded with component “B”, PGC 58873, and what’s furthermore, both photos show the presence of a superimposed star on the west edge of this galaxy – possibly the reason for the “double galaxy” image I perceived. To my surprise, Alvin does not indicate the “double galaxy” appearance in the drawing in his guide, nor is it mentioned in Reiner Vogel’s Hickson guide. Finally, the faint star that I could not focus crisply corresponds nicely with component “C”, PGC 58884. My only concern is that there is a faint star very near to the galaxy, so I suspect what I saw was the combined light from both PGC 58884 and the star. I did not detect anything that I placed on my sketch that could correspond with the other members.
So, HCG 84 can give us a nice set of challenges: What is the smallest telescope through which this group can be detected? It was hard in my 18” on a very good night, but I think that it might be detected with a 15” mirror or even less. What are the prerequisites for observing the “rift” created by the superimposed star in PGC 58873? Magnification, steadiness of seeing, or transparency? Finally, what is the minimum aperture and magnification needed for observing all six members of the group?
I know for sure that I will soon be returning to HCG 84. And don’t forget,
“Give it a go and let us know!”
Good luck and great viewing!
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