DSO of the Month
Messier 57
AKA: Ring Nebula; NGC 6822
Due south at 00.18 (BST) on 16 July
Position: 18 hr 53.6 min 33 deg 01 min 45 sec
Image of M57 by HAS Member William Wood. Used with permission.
Planetary nebulae are rare and those which can be easily seen in a small telescope are even rarer. I discussed planetary nebulae in some detail in the DSO of the Month for February 2020 (Caldwell 39) and explained that they are rare because they are short-lived in astronomical terms. However the summer months allow us to observe two easy planetary nebulae, namely M57 and M27: I will discuss M57 this month and M27 next month.
The Ring Nebula has one big advantage, it is easily found between the two stars which form the bottom line of the parallelogram of bright stars that form the main core of Lyra, namely Sheliak (Beta Lyrae) and Sulafat (Gamma Lyrae), being slightly nearer Sheliak. However it is rather faint at magnitude 8.8 and small (estimates of its size vary), so it cannot be seen with the naked eye even in dark-sky areas or with 10x50 binoculars. It can be seen in a 125mm telescope, but needs a high magnification of about 100 times. Its appearance in a small telescope is somewhat underwhelming, looking like as a small light grey smoke ring. Very large telescopes and photographic images will show its yellow colour and the very dim central star.
Like all planetary nebulae, the Ring Nebula is a dying star with a flattened bubble of expanding gas at a 30 degree angle to our line of sight which is excited by the radiation from the white dwarf at its centre. The ring shape comes from the greater concentration of material at the equator of the bubble. The original star was several times more massive than the Sun and it exploded 4,000 years ago forming a red giant, but then faster moving gas slammed into the outer layers of the red giant, making it glow. The gas in the nebula is now fading away as it expands and it will become invisible in about 10,000 years’ time, a blink of an eye in astronomical timescales. It is 2,000 light years away from Earth which is why it is so small and dim.
The Ring Nebula was first observed by Charles Messier in January 1779 and rediscovered by another French astronomer Antoine Darquier de Pellepoix soon afterwards. Both Messier and William Herschel believed that the nebula was composed of stars that were too faint to be resolved and it was only with the development of astronomical spectroscopy in the mid-nineteenth century that it became clear that the Ring Nebula (and other planetary nebulae) were composed of excited gas.