uruviel2001

M57 is known for its extremely high dynamic range, with the interior several times brighter than the outer shell. This makes it hard to process in its own right, but those who take the time usually find a brilliant red outer glow of expanding gas. Not so in this case, to my surprise. The redness usually obtained with a Hydrogen Alpha filter also includes the ionized Nitrogen band (NII) which is missing from the stronger 3nm filters used here. Wikipedia tells us why: "All the interior parts of this nebula have a blue-green tinge that is caused by the doubly ionized oxygen emission lines at 495.7 and 500.7 nm. These observed so-called "forbidden lines" occur only in conditions of very low density containing a few atoms per cubic centimeter. In the outer region of the ring, part of the reddish hue is caused by hydrogen emission at 656.3 nm, forming part of the Balmer series of lines. Forbidden lines of ionized nitrogen or [N II] contributes to the reddishness at 654.8 and 658.3 nm"Fun surprise!