|Designation:||M57, NGC 6720.|
|Apparent size:||1.4 x 1.1 arc minutes.|
|Diameter:||0.6 light year.|
|Distance:||1,400 light years.|
|Exposure:||36 x 179.3 sec = 107 minutes.|
|Field of View:||28.4 x 28.4 arcmin; up is 122 degrees E of N.|
One of the prettiest planetary nebulae, M57 is visible during July and August low in the northern sky here in SW Sydney, at a maximum altitude of about 22½º – just above the Sydney light dome.
A planetary nebula occurs when ageing stars similar to the Sun reach the end of their time on the main sequence, swell up to become red giants and then puff off their outer layers to produce expanding nebulae.
The process leaves the star’s core behind as a small hot white dwarf star. It no longer generates nuclear reactions and just slowly fades away to eventually become a black dwarf. My understanding is that this final process takes many trillions of years and the Universe is not old enough yet for any black dwarfs to exist!
The nebula itself glows due to the energetic ultraviolet radiation from the white dwarf but eventually dissipates into the inter-stellar medium over the course of about 10,000 years. The fluorescent colours are dependent on the chemical composition of the nebula. The reddish outer region consists of ionised hydrogen and nitrogen and the inner region consists of doubly ionised oxygen.
The central white dwarf star is 15th magnitude and has been reduced to the size of a terrestrial planet – but with a surface temperature of over 100,000º K!
M57 looks a ghostly grey/white through the eyepiece but I remember the surprise and awe when I first clapped eyes on it many years ago, when using my Meade LX-90. What an object!
|SkyWatcher Esprit 120 mm apochromatic 3-element refractor.|
|840 mm focal length @ f/7 with field flattener & 2x Powermate|
|Baader L-Booster UHC-S light pollution filter 2458276.|
|SkyWatcher EQ6-R Pro mount.|
|Camera: ZWO ASI 071 MC Pro (CMOS 28.4mm 16 Mpx)|
|ZWO ASI120 guide camera, using PHD2 software.|
See annotated image at Astrometry.net