We use new Hubble Space Telescope and archived images to clarify the nature of the ubiquitous knots in the Helix Nebula , which are variously estimated to contain a significant to majority fraction of the material ejected by its central star .

We employ published far infrared spectrophotometry and existing 2.12 \mu m images to establish that the population distribution of the lowest ro-vibrational states of H _ { 2 } is close to the distribution of a gas in local thermodynamic equilibrium ( LTE ) at 988 \pm 119 K. In addition , we present calculations that show that the weakness of the H _ { 2 } 0-0 S ( 7 ) line is not a reason for making the unlikely-to-be true assumption that H _ { 2 } emission is caused by shock excitation .

We derive a total flux from the nebula in H _ { 2 } lines and compare this with the power available from the central star for producing this radiation .
We establish that neither soft X-rays nor 912–1100 Å radiation has enough energy to power the H _ { 2 } radiation , only the stellar extreme ultraviolet radiation shortward of 912 Å does .
Advection of material from the cold regions of the knots produces an extensive zone where both atomic and molecular hydrogen are found , allowing the H _ { 2 } to directly be heated by Lyman continuum radiation , thus providing a mechanism that will probably explain the excitation temperature and surface brightness of the 2.12 \mu m cusps and tails .

New images of the knot 378-801 in the H _ { 2 } 2.12 \mu m line reveal that the 2.12 \mu m cusp lies immediately inside the ionized atomic gas zone .
This property is shared by material in the “ tail ’ region .
The H _ { 2 } 2.12 \mu m emission of the cusp confirms previous assumptions , while the tail ’ s property firmly establishes that the “ tail ” structure is an ionization bounded radiation shadow behind the optically thick core of the knot .
The new 2.12 \mu m image together with archived Hubble images is used to establish a pattern of decreasing surface brightness and increasing size of the knots with increasing stellar distance .
Although the contrast against the background is greater in 2.12 \mu m than in the optical lines , the higher resolution and signal of optical images remains the most powerful technique for searching for knots .

A unique new image of a transitional region of the nebula ’ s inner disk in the HeII 4686 Å line fails to show any emission from knots that might have been found in the He ^ { + + } core of the nebula .
We also re-examined high signal-to-noise ratio ground-based telescope images of this same inner region and found no evidence of structures that could be related to knots .