North of Messier 16, at a distance of approximately 2000 parsecs, is the OB association Serpens OB2, containing over 100 OB stars. Around 5 million years old, the association appears to still contain star-forming regions, and the light from its stars is illuminating the HII region S 54.[125] Within this HII region is the open cluster NGC 6604, which is the same age as the surrounding OB association,[126] and the cluster is now thought to simply be the densest part of it.[127] The cluster appears to be producing a thermal chimney of ionized gas, caused by the interaction of the gas from the galactic disk with the galactic halo.[125]