Messier 57 is a planetary nebula with all the consequences. It was formed about 7,000 years ago, when a dying red giant star expelled its shell of ionized gas to form the nebula. And this star then became a white dwarf – a hot, dense stellar remnant roughly the size of the Earth and surface temperature about 100,000K. The nebula is expanding at a velocity 20 to 30 km/s. M57 Ring nebula is located about 2,300 light years away in constellation Lyra, and its apparent magnitude is 8.8mag.
M57 was discovered by the French astronomer Antoine Darquier de Pellepoix in January 1779. He wrote then:
This nebula, to my knowledge, has not yet been noticed by any astronomer. One can only see it with a very good telescope, it is not resembling any of those already known; it has the apparent dimension of Jupiter, is perfectly round and sharply limited; its dull glow resembles the dark part of the Moon before the first and after the last quarter. Meanwhile, the center appears a bit less pale than the remaining part of its surface.
The same month later Charles Messier discovered it independently and then placed in his catalogue. The outer halo of the nebula was discovered in 1935 by J.C Duncan. Deep images have revealed, that this shell extends to more than 3.5 arc minutes. This shell most probably contains stellar wind matter, that has been emitted before the nebula was born. Nebula cannot be observed with naked eye, it can be spotted with binoculars, but due to small apparent dimensions larger power is required to resolve it as nebula. Central white dwarf has apparent magnitude 14.7mag and is difficult target for visual observations.
I collected subframes to this image of M57 Ring nebula over a few short nights in May and June 2018. Outer shell contains 3.5 hours of subframes made through Ha narrowband filter. And there is additionally about 2 hours of exposures made with LRGB filters. The halo is really faint, I tried to reveal it a little but also to keep the proportions somehow.
Image technical data: Date: May-June 2018 Location: Nieborowice, Poland Telescope: Meade ACF 10" Corrector: AP CCDT67 Camera: QHY163M, gain 100 Mount: SW EQ6 Guiding: SW 80/400 + ASI290MM Exposure: Ha 42x5 minutes, LRGB 50:24:18:20x1 minute Conditions: seeing average-good, transparency good