Pickering's Triangle is a small part of the Veil Nebula supernova remnant in constellation Cygnus.
Veil Nebula is a cloud of ionized gas and dust, leftovers from an exploded star. The star went off some 5000-8000 years ago at distance of about 1470 light years. This, relatively faint target, is difficult to image due to the large angular diameter, about three degrees, and a dense star field.