The Necklace Nebula (at left) consists of a bright equatorial ring, measuring 12 trillion miles across, dotted with dense, bright knots of gas. The knots glow brightly due to absorption of ultraviolet light from the central stars. The knots are made up of glowing hydrogen and oxygen gas. Each knot also dons a small tail pointing away from the central star.
The Helix Nebula (at right) was the first planetary nebula discovered to contain knots. Its main ring contains knots of nebulosity, which have now been detected in many nearby planetaries. These knots are highly radially symmetric and are described as "cometary", each containing bright cusps and fainter tails. All extend away from the PNN in a radial direction. There are more than 20,000 cometary knots estimated to be in the Helix Nebula.