The Ring and the Dumbbell − two favourite planetary nebulae

2014 May 24

Callum Potter

The summer months are not the best for challenging deep sky observing, so it’s a good time to return to a few old favourites. The variety of morphologies of planetary nebulae make them particularly interesting to observe, and to compare and contrast. Two of the brightest are well placed in the summer months, and bright enough to observe even in less than ideal conditions. These are, of course, the Dumbbell Nebula, M27, and the Ring Nebula, M57.
M27 was the first planetary nebula to be discovered, by Charles Messier in 1764, though William Herschel coined its common name. Although not the brightest planetary, it is one of the brightest – and it is perhaps more obvious due to a higher surface brightness. It shines at around magnitude 7.4, with an extent of something like 8 by 6 arcminutes.
Distances to planetary nebulae are difficult to quantify. An accurate measurement has been made of the central star using the Hubble Space Telescope and trigometric parallax, which suggests a distance to M27 of around 1,360 light years. When the Gaia astrometric satellite becomes operational a more precise distance measure should be possible. Age can then be estimated from the apparent size and rate of expansion, but these can give a wide variation in results. For M27 they suggest an age of around 10,000 years.
M57 was the second ever planetary nebula to be discovered, in 1779, by French astronomer Antoine Darquier de Pellepoix observing from Toulouse. Messier independently found it a short time after Darquier, and added it to his catalogue. It is somewhat fainter than M27 coming in at around mag 8.8, and smaller in extent – about 1.4 by 1.0 arcmin. Measurements of the central star place the nebula around 2,300 light years away, suggesting an age of about 7,000 years.
Simple observing of these two objects might initially suggest completely different morphologies of the nebulae. But in fact these are two good examples of the difference that perspective makes. With M27 we are viewing the nebula side on (as it were), and with M57 the view is down the pole.
Both these objects are quite easy for the visual observer, and may be noticed with binoculars or a small telescope. Both are also popular targets for imagers, and interesting results can be had with DSLR, LRGB and narrowband filter imaging techniques.
For visual observers with larger telescopes, the central star of a planetary nebula is a challenge. The star is a white dwarf, with incredibly high temperature (around 85,000K for the central star of M27) which is causing the nebula to shine. M27’s central star is mag 13.5. M57’s is rather more challenging at mag 15.7 and a large aperture instrument is needed to view it. Of course for imagers, the central stars are fairly easy captures.
It’s interesting to note that M27’s white dwarf has a companion – a 17th magnitude yellow star. Some professional astronomers are of the view that planetary nebulae are only formed where the central star is part of a binary or multiple star system – though Prof Albert Zijlstra, speaking at the Deep Sky Section meeting this year, was not convinced.
Another interesting star in M27 is the so-called ‘Goldilocks’ variable – this was discovered by Leos Ondra, who noticed the star was present and then missing on the cover photos of two magazines – Astronomy of 1990 May and Deep Sky in autumn 1990. Which shows that careful inspection of any images can reveal hidden messages. The star is a long period Mira-type variable – perhaps not so remarkable, except for the nature of its discovery.
Callum Potter,  Director, Deep Sky Section