Expert: Philip Stahl - 11/13/2008

Question
QUESTION: Dr. Stahl,
Please tell me what is involved in the formation of comet-like objects such as those found in a close-up view of the Helix Nebula that is named "Comet-Like Knots Around a Dying Star in the Helix Nebula" found on the Nebula Gallery section of the HubbleSite.
Thank you for your time.

ANSWER: Hello,

First of all, any protostar -type object (including the early Sun) will spin off detritus in the process of formation. This can include comets, asteroids as well as meteroids, depending on the initial composition of the stellar nebula.

In the case of the star at the heart of the Helix, it is believed (based on images obtained) that similar processes occurred and the "knots" to which you refer are simply a "halo" or shell of comets near the star. The seeming icy and dusty properties reinforce this view.

On the other hand, there may perhaps be some plasma processes occuring that are forming these entities, and they have little or nothing to do with the comets with which we are familiar. The most likely candidate might be shock waves (with high Mach numbers) being triggered in the interstellar plasma between the star and the apparent "ring".

Once in play, and if the temperature and pressure conditions are right - all sorts of plasma instabilities may come into play, including reverse-pinch, kink, and others (e.g. electrostatic, ion-acoustic)

More work will obviously have to be done before the theory behind the knots-"comets" is nailed down to everyone's satisfaction.




---------- FOLLOW-UP ----------

QUESTION: Thanks for the great answer.
Just to clarify, one "knot" would be a shell of many, many comets(?), which means there is a star inside the knot that we cannot see? Or, where is the star? And supposing this is the case, why would there be a "tail" on each knot?

Also, I am not sure what you mean by a "ring" in the second scenario.
Thanks so much,
Urania

Answer
Good questions all!

First, let me emphasize the "cometary" description, not "comets" literally. Recall I said that may be open to interpretation. After further checking it seems most astronomers who've studied this nebula don't believe these are literal comets but rather denote a cometary *appearance" with "knots" to boot. The "knots" then are knots of nebulosity - not of literal comets. Thus, as I first suspected, we are looking at the behavior of the plasma (electrically charged gas) in the neighboorhood of the spawning star (i.e. the star that gave way to the Helix).

The bright "balls" analogous to a comet's head, are actually photo-ionization fronts. These are regions where there is higher than normal ionization (removal of electrons from atoms) of the gas and re-absorption of the electrons, which then cause the (receiving) atoms to become excited to higher energy levels, after which the electrons are "de-excited" with photons given off. Thus the bright looking "head".

The tails flow from that and comprise regions where fewer photo-ionizations occur and which appear to "stream" backward from the source star. Most likely a "stellar wind" effect - caused by radiation pressure from the star.

Now about the "star" itself. It is actually NOT a star-star but the *remnant core* of a former star that has lost its outer envelope of gases - which became the nebula (see the linked NASA photo below of the full Helix). We call this object a "planetary nebula nucleus" and right now this object is extremely difficult to see given it is just gearing up to the white dwarf stage.

The "ring" can be seen below in this NASA image, and again, is the flourescing (expelled) plasma surrounding the radiating remnant core, which causes it to glow:

http://apod.nasa.gov/apod/ap030510.html