observing hickson groups


                                            Adam Block and Vic Eden/Mount Lemmon SkyCenter/University of Arizona


Introduction Observer's Guide Highlights Observing Reports


The Hickson Catalog comprises 100 compact groups of galaxies and was compiled by the professional astronomer Paul Hickson from Vancouver, Canada. To qualify for the catalog, the individual groups needed a) to be compact, b) to have at least four members in a certain magnitude range, and c) to be isolated, excluding groups belonging obviously to the larger dense galaxy clusters (1). The astrophysical background of Hickson's survey on the red plates of the Palomar Sky Survey (POSS) was to monitor the groups for members with discordant red shift, to study the galaxy interactions resulting from dark matter, and to examine galaxy evolution (2). In the abstract of a review on compact groups of galaxies (2), Paul Hickson writes:

"Compact group of galaxies have posed a number of challenging questions. Intensive observational and theoretical studies are now providing answers to many of these, and at the same time, are revealing unexpected new clues about the nature and role of these systems. Most compact groups contain a high fraction of galaxies having morphological or kinematical peculiarities, nuclear radio and infrared emission, and starburst or active galactic nuclei (AGN) activity. They contain large quantities of diffuse gas and are dynamically dominated by dark matter. They most likely form as subsystems within looser associations and evolve by gravitational processes. Strong galaxy interactions result and merging is expected to lead to the ultimate demise of the group. Compact groups are surprisingly numerous, and may play a significant role in galaxy evolution. "



For the observing amateur, Hickson Compact Groups (HCGs) are fascinating targets. There are a few groups that are nice associations of bright galaxies, as for instance Hickson 44 in Leo or Hickson 68 in Canes Venatici. These two groups are easy targets already for small telescopes and there are many pictures of them available on the internet. But these two groups are clearly exemptions. Most Hickson Groups include only fainter galaxies and are much more compact. Hence, their successful observation profits from aperture (and dark skies, of course) and many of the remaining catalog members are interesting targets mostly for observers with larger telescopes of 12" or even more aperture. These groups include famous examples  as "The Box" in Coma (Hickson 61), Stephan's Quintet in Pegasus (Hickson 92), Copeland's Septet in Leo (Hickson 57), or challenging Seyfert's Sextet in Serpens (Hickson 79). Other challenging examples without own names include Hickson 55 in Draco or Hickson 56 in Ursa major.

Most of my observations were made with my selfmade 14" and 22" Dobsonian telescopes. While many details could be observed or at least suspected already with the 14", the 22" was always the better choice.  

The first impression of a typical Hickson Group in a low power eyepiece is a distinct "something", clearly non-stellar, often only fleeting. Only with higher powers, the individual members become distinct, and I generally use exit pupils of 1.2 to 2 mm (resulting in 250 to 440x in my 22" f/4) for getting the details. Some of the groups, Hickson 79 (Seyfert's Sextet) being one of the most prominent members of them, remain even at this magnification partly unresolved.  

To me, observing a Hickson Group is like solving a puzzle. Usually some of the brighter group members are easily visible, sometimes even with direct vision. The challenging part is resolving the other, fainter members or splitting the group into individual galaxies. For the most difficult groups, you may easily spend from half an hour to indefinitely long for dissecting the group even with larger telescopes.

Hubble Space Telescope images


The Observing Atlas

I started observing Hickson groups with the material provided by Jim Shields and Steve Gottlieb on their website Adventures in Deep Space  (at least to me, this is the most valuable source of information and stimulation for observing the Deep Sky. Thanks, Jim and Steve!  ). Jim Shield's collection of "32 Interesting Hickson Groups" is an exciting start into observing these compact groups of galaxies. My compilation of the Hickson groups presented here resulted from a certain emptiness that approached me, when I had observed 30 out of these 32 groups in early Spring 2007, with the remaining two groups being clearly targets for late summer only .

What to do? With my new 22" Dob on duty for almost a year at that time, I decided to prepare DSS images and finder charts also for the 68 remaining Hickson groups that are not included in Jim's list, just to see if they were equally rewarding targets as the other 32 groups. If your are venturing for the first time into the "Hickson business", the best start would be to pick those groupgs that are also on Jim's list.

I was initially not sure whether to include my own observing notes into this atlas, as these are, of course, very subjective results of my own efforts. Furthermore, there is always an uncertainty between distinctions as  "sometimes suspected", "intermittently, but definitely seen with averted vision after staring at a single empty point in the sky for 30 minutes under a piece of black cloth", and a discouraging "clearly not seen". Due to this "subjectiveness", I used to be reluctant in publishing own observing reports and have judged my observation successes generally in a quite conservative manner. Nevertheless, I found Steve Gottlieb's notes always a big help when observing the groups included in Jim Shield's list. I therefore decided to include over time my own observing notes into this atlas. My observations were generally made under quite good, but not excellent sky (usually mag 6.5 skies) at about 1200 m elevation in the Black Forest in Southern Germany . Most of the groups could be observed under these conditions. Some of the fainter groups were, however, close to the detection limit. With few of them (HCG 20, 27, 28, und 39), the observation was therefore not certain and one (HCG 50) was negative.

Download the Hickson Observing Atlas

 (pdf file 14 MByte, last update: 2009)


The finder charts have a field of view of 3° and are sufficient to locate the field in combination with Sky Atlas 2000 or Uranometria. If you print  the Atlas double-sided, you can quickly switch between DSS image and data on one side and the finder chart on the other side for easy use at the telescope. 

To the right are two sample pages of the observing atlas.

I hope that this atlas is some help for others and avoids duplicate efforts in putting together data, downloading POSS images, and preparing finder charts.  


A selection of the most interesting Hickson Groups


This is a personal selection of thirteen of the most interesting Hickson Groups, starting from bright and extended groups accessible already with an 8" telescope to tightly packed groups of tiny galaxies being a challenge even with apertures as large as 20".


Observing Report of all 100 Hickson groups are here


Hickson 44 in the head of Leo is probably the most photographed Hickson group and makes a nice quartet of bright galaxies. In large telescopes, the dust lane of a may be discerned as a sharp edge. The faintest group member, d, is a barred spiral with a relatively low surface brightness. 

Hickson 68 at the NE border of Canes Venatici is similarly bright as the preceding group, but more compact. With five members, this is the most attractive of the brighter Hickson groups. 30 arcminutes ENE is spiral NGC 5371, which is not part of the group.

In Hickson 16 in Cetus, the four members are already somewhat fainter and the appearance becomes more typical for a Hickson Group.

Hickson 92 in Pegasus is famous Stephan's Quintet, which can be found SE of NGC 7331. The challenge in this groups is splitting b and d and seeing the sixth (non-member) galaxy offset from the group.

Hickson 61 in Coma is named "The Box". Though the four galaxies have visual magnitudes between 12.2 and 13.3, the surface brightness in particular of b is quite low.

Hickson 40 in Hydra welcomes you to the more compact groups that require higher magnification.

With Hickson 51 in Leo we enter to the more challenging groups. Here 6 out of 7 members were seen with 22".

Hickson 57 in Leo is also named "Copeland's Septet". Though it is a dense group, it is less compact than "Seyfert's Sextet" further below and therefore easier to split into individual galaxies.

Hickson 79 aka "Seyfert's Sextet" is the prototype of the extremely tightly packed Hickson Groups with five galaxies squeezed into a horseshoe of only 2 arcminutes. Resolving this group requires highest magnification on galaxies reaching only mag 15 to 16, a challenge even in large telescopes.

Hickson 56 in Ursa Maior is next to bright galaxies NGC 3718 and 3729. Four galaxies are lined up in a very compact chain of a mere 90 arcseconds and their complete resolution is a challenge. Even the detached primary member a is a difficult target with low surface brightness.

Hickson 55 is an extremely compact chain of very faint and tiny somethings in Draco.  Even with large aperture this group requires a lot of patience to pin down the precise locations of at least some of the individual members.

Hickson 54 is another very tight chain of tiny galaxies in Leo. This group stretches over only 40 arcseconds with members of only 5 arcseconds diameter and appears even more difficult than preceding Hickson 55.

Hickson 50 in Ursa Maior is the most difficult entry in the Hickson Catalog. Five almost non-existent galaxies form a circle of 25 arcseconds next to a "bright" mag 17 star. The brightest galaxies have visual mag 18 and detecting the group at all is a major achievement.



1. Hickson P (1982) Ap J 255: 382

2. Hickson P (1997) Annu Rev Astron Astrophys 35: 357



Paul Hickson’s webpage with more information on the background of the Hickson Catalog and links to publications

Jim Shield's and Steve Gottlieb's website with a selection of "32 Interesting Hickson Groups"

Ray Cash's website on observing Hickson Groups with a compilation of images and observational data from several observers

Robert Mc Gown's and Miles Pauls's Galaxy Groups observing project.

Alvin Huey's Hickson Observing Guidebook

Uwe Glahn's webpages on observing the Hickson Groups


DSS images copyright notice

The Digitized Sky Survey was produced at the Space Telescope Science Institute under U.S. Government grant NAG W-2166. The images of these surveys are based on photographic data obtained using the Oschin Schmidt Telescope on Palomar Mountain and the UK Schmidt Telescope. The plates were processed into the present compressed digital form with the permission of these institutions.


Introduction Observer's Guide Highlights Observing Reports