Yep those are bald face hornets. They are actually wasps in spite of their name. However they are the biggest and meanest around. I've been stung in the face by them before as well and for no good reason. I was just standing around and then bam, one lands on my face and stings me. Now I have a hatred for them, although they do eat tent caterpillars which eat my trees.
If I were you I'd get a pressure washer and blast that nest and then run like hell. Another trick is WD-40, I've discovered it kills wasps and hornets nearly instantly. WD-40 would probably hurt your tree though. Another technique I've used when it's cold and they are inactive is simply to spray expand-o-foam in the entrance of their nest. Not only does it block the opening of the nest nearly instantly it expands to fill the rest of the nest engulfing and suffocating all of the occupants.
That hurts just looking at it. I think I got lucky this summer early on. I had some to start building a nest in one of my peach trees and I did not even know it. I was spraying Permethrin and it killed them with the first spray. I only saw the nest after the spray had done it's job. Those buggers really make a spot.
If you use a pressure washer you better be really fast, them bad boys hit like a 22! I wait till right at dark and just use wasp and hornit spray and have never got stung. Make sure its the one that shoots the 20 ft. stream
Looks nasty. Good thing you were near the house and not miles away from home. I work for the school maintenance in knox county and we use wasp spray in a can. We can stand back about 10 to 15' and it will shoot strong and straight and always kills them at first contact.
sorry about the stings,ive had them and they are painful..mine itched like crazy when they started to heal..got a guy that will buy the real big paper nests for a hundred bucks apiece...I wait for a real cool night then slip a pair of doubled up garbage bags over the nest ,tying it tight at the top so no hornets can get out.then I saw the limb off and put it in the deep freeze for a week to kill them..no damage to the nest that way....