NGC 7293 the Helix Nebula

The Helix Nebula was the first planetary nebula discovered to contain cometary knots. Its main ring contains knots of nebulosity, which have now been detected in several nearby planetary nebulae, especially those with a molecular envelope like the Ring nebula and the Dumbbell Nebula. These knots are radially symmetric (from the CS) and are described as “cometary”, each centered on a core of neutral molecular gas and containing bright local photoionization fronts or cusps towards the central star and tails away from it. All tails extend away from the PNN in a radial direction. Excluding the tails, each knot is approximately the size of the Solar system, while each of the cusp knots are optically thick due to Lyc photons from the CS. There are about 40,000 cometary knots in the Helix Nebula.