NGC 40 is a peculiar PN characterized by a low nebular ionization due to a low temperature central star ("only" 35000 K). This temperature causes higher flux of HI than other typical species (as in higly excited nebulas). Structurarly, it's made of an expanding elliptic central structure overlapped to a secondary structure, perhaps of an originary bipolar strucure. These separated structures' expansion speeds give hints for a possible double-emission-phase.
This nebula is not much faint, and from a good observation site we can observe nice details. Unluckly OIII filters don't help the observation so far, because of the nebula's low ionization.
The structure observed in a telescope is a pair of semi-arch surrounding a bright central star. With a large diameter telescope we can also spot lights-and-shades in the inner part, and also try to see external lobes, hardly visible and fainter. With long exposure imaging its colour is red, typically associated to hydrogen-emission nebulas.
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