The best source of information about the Collinder list is here: http://www.cloudynights.com/item.php?item_id=2544
Open clusters are formed when clouds of cold, dense interstellar gas condense under the influence of gravity to form a cluster of hot, young stars.
Once formed, these gradually drift apart, so any that open clusters that we see, are relatively young on a cosmic scale.
The stars are often of large mass and high luminosity (Classes O and B) and shine brightly in UV causing any remnant gas to either reflect the radiation (reflection nebula) or emit their own, characteristically in the red HII band (emission nebula).
Some OCs are of outstanding beauty. Examples include the Pleiades, the Hyades and the double cluster in Perseus. A personal favourite of mine is NGC 1981 in Orion (Collinder 73).
Open clusters are relatively easy for an amateur to observe as they are of stellar brightness rather than the “faint fuzzies” of distant galaxies and planetary nebulae.
If there’s a lot of light pollution about or its a bright summer night, I tend to concentrate on OCs and double stars.
Per Collinder (1890 – 1975) was a Swedish post-grad astronomy student whose 1931 doctoral thesis (On Structural Properties of Open Galactic Clusters and Their Spatial Distribution) included a list of 471 OCs – these included previously documented OCs as well as some he identified for the first time. In addition to listing OCs, he classified them according to their appearance.
The list was drawn up in the 1930s, so there are a few errors – one of the most famous members of the list, Collinder 399 (the Coathanger), is actually an asterism rather than a cluster but it still makes me smile every time I see it!
Like the Caldwell list, the Collinder list covers both Earth hemispheres, so I’m not going to be able to complete the list from 55 North.