September 2026 - Picture of the Month
Helix nebula (NGC 7293) in Aquarius
The Helix Nebula in southern part of Aquarius is very well known to and observed by the amateur astronomy community. It's one of the closest planetary nebulae with its central white dwarf star, WD 2226−210, about 199.9±3 parsecs (651.6±3 light-years) distant. As a result it's also large for a visual planetary with an apparent size of around 12 arc-minutes.
However observers might find its apparent magnitude of about 7.3 is deceptive because its large size results in a very low surface brightness. This makes it a challenge to see much of NGC 7293's outer structure, but Patrick has some suggestions that might help. It's an good imaging target.
Planetary nebulae form when an intermediate-mass star reaches the end of its life, swells into its asymptotic giant branch (AGB) phase, and ejects much of its gaseous envelope in the process leaving a white dwarf star at the centre of a glowing shell of expanding gas.
For NGC 7293 this process seems to have started with a 1.5 solar mass star about 12,000 years ago with the material ejected forming what is now the outer ring of the nebula, a portion of which we see in most of this image. About 5,500 years later the current inner ring started to expand into this earlier material photoionised by the 13.5 magnitude central DAO-type white dwarf star, now about 0.7 solar masses and with an effective temperature well in excess of 100,000K. 1,2 Whilst this star is out of shot in this fabulous image, we're looking at the consequences this expansion.
This image from JWST’s NIRCam (Near-Infrared Camera) is full of cometary knots, named for their extended structure, that formed as the faster moving hot gas of that inner ring ploughs into the gas and dust of the earlier outer ring. There are several layers of these knots that become less dense as they're progressively eaten away by the solar winds in this young expanding nebula.
The knots are dense regions that maintain temperatures low enough for the formation of the complex molecules from the elements present in the envelope of that long dead star. The interest for astronomers and planetary scientists is that these at the materials that will eventually find their way into the formation of other solar systems.
I'm surprised by the number of galaxies that can be seen in larger versions of this image, they are very detailed and worth downloading. I'm not sure whether all the galaxies on display could be captured in a single image as this would suggest the nebula is extremely tenuous. There are galaxies that appear superimposed on some of the densest parts of the nebula. I wonder whether this might it be an artifact of the image processing?
There's also a wonderful image combining one of the whole nebula by ESO's VISTA with this image for those that need to find their bearings.
James Whinfrey - Website Administrator.
References
- O'Dell, C. R., McCullough, P. R., and Meixner, M., “Unraveling the Helix Nebula: Its Structure and Knots”, The Astronomical Journal, vol. 128, no. 5, IOP, pp. 2339–2356, 2004. doi:10.1086/424621.
- Iskandarli, L., Farihi, J., Lothringer, J. D., Parsons, S. G., De Marco, O., and Rauch, T., “Novel constraints on companions to the Helix nebula central star”, Monthly Notices of the Royal Astronomical Society, vol. 534, no. 4, OUP, pp. 3498–3505, 2024. doi:10.1093/mnras/stae2286.
