Cometary knot

On the near side of the Helix Nebula, the central dusty globule of each cometary knot appears dark against the background as it absorbs the [O III] 5007 Angstrom light emitted in the nebular envelope.

The latter case would imply that the conditions in the planetary nebula host would have, at a certain point, triggered the formation of molecular clumps in its nebular envelope.

In advection-dominated flows, μn0 is greater than αn02h, and most of the incoming photons reach the ionization front and ionize fresh gas.

[7] Several structures have been described as cometary knots or cometary globules that surround R Coronae Borealis, which is a peculiar star described as potentially the result of a white dwarf merger or final helium shell flash that periodically dims due to a build-up of carbon dust surrounding it, acting as a 'natural coronograph'.

[8] Three-dimensional modelling of NGC 6337, a planetary nebula with a close binary nucleus, suggests the presence of a "thick ring with radial filaments and knots."