I chose this image of Abell 93 as the subject matter of this inaugural planetary nebula posting because of the stunning beauty of its simplicity. But the majority of planetary nebula are far from being simple. Complexity is part of their evolution and even their creation. Most of us can related to spheres, and planetary nebula are complicated enough, so let’s start out simply shall we? As we shall see, the simple spherical Abell 93 PN is the rarity and not the rule.
![RASC Victoria Centre: Dave Payne &emdash; Abell 39 Planetary Nebula in LRGB + NB "[OIII]"](https://rascvic.zenfolio.com/img/s/v-10/p1937354469.jpg)
Abell 39 Planetary Nebula in LRGB + NB “[OIII]” in Hercules
Planewave CDK12.5 – ASI6200MM; A-P 1100 AE GTO, Antlia Pro BB and 3nm NB filters
L: (28 x 150s exposures, Bin 2×2, Gain 100); R,G,B: (20, 20, 20 x 180s exposures, Bin 2×2, Gain 100); O: (25 x 540s exposures, Bin 2×2, Gain 200)
Total Integration Time = 7.9 hours, June 23,24 2023, Vancouver Island, Canada
Planetary Nebula are the small, bright nebula that are left over when a sun-like main sequence star dies. The remains of the original star consists of an outer shell of gas surrounding the now highly compacted and hot products of nuclear fusion. Both are visible in the centre of this image – the star typically shows up small and near the centre of the gaseous shell – in this case slightly to left of centre of the spherical shell.
The star is very small – comparable in size to the earth, yet contains most of the mass of the original star so is very dense. The small, dense state of the star is what led to these nebula being called “planetary nebula”, although it really has nothing to do with planets. The star itself is characterised as a white dwarf star, and while fusion has now ceased in the star, it still extremely hot, and its ultraviolet emissions are what charge the surrounding gas to emit its own light.
The nebulous shells are sometimes visible in RGB, but are most often imaged through the same narrowband filters we use for star forming regions. Abell 39 shows up exclusively in [OIII] light (the blue/green “forbidden” emission line given off by the excited O+2 ion). This colour often shows up in planetary nebulae (PNs), although hydrogen alpha and even [SII] often make appearances – they just happen to be absent in Abell 39. In order to give off their particular emission colour – two things have to be present. UV light of sufficient energy and intensity must be present to excite and ionize the elements – both emitted by the star and not spent by adsorption along the way from the white dwarf. Secondly, the elements not only have to be present, but at the correct concentrations and pressures to emit. This second condition is a bit of a goldilocks issue, where if too rarefied there will be insufficient emissions to record on our cameras, but if too concentrated – collisions with other atoms will stop the [forbidden] emissions from occurring.
Whatever the cause, the gaseous shell around the white dwarf in Abell 39 appears partially transparent and empty of other material. A close look of Abell 39 reveal other stars and even a galaxy that can be seen right through it. The degree of transparency of a PN also depends upon how much of the emitting gas we are looking through. That is why the edges of the sphere appear more intense than the middle, where we are looking perpendicularly through the face and back of the sphere. Peaking at the closeup may also reveal to you a distinct halo that exists around the outer edge of the sphere. Such halos are common around PNs, so be sure to look them in PN images.
At first, my expectations was that all planetary nebula should be spherical in nature. However, when I understood the processes involved in their creation, I now believe the opposite is true. It should be and is rare, that a planetary nebula is spherical (only <20% are) and extremely rate that one should be as beutifully spherical as Abell 39. Abell 39, as with most PNs are quite small and physically about 1 to 2 light years across. Abell 39 is ~3800 light years away in the constellation of Hercules.
For full resolution, downloadable versions please visit either the Victoria RASC Zenfolio site or my gallery on Astrobin.
From main sequence star to white dwarf and planetary nebula
Main sequence stars, like the sun, are balls of primarily liquid metallic hydrogen undergoing nuclear fusion at the core. When hot enough (say 15 or so million Kelvin) and under enough pressure (2.7E3 GPa or about 265 billion atmospheres) at the core, two protons (ie hydrogen nuclea) fuse to form, first deuterium, and then Helium atoms. (Note this process also swallows up a couple of electrons and produces some neutrinos or anti produces some antineutrinos or some combination). The main point is that this process gives off a tremendous amount of heat, and provides the heat that we enjoy every day shining down on our planetary planet (context is everything).
The conditions that allow fusion, however, only (initially) take place at the core of a star. What we see from earth is the outer surface of sun, with a still hot, but much more modest 5800 Kelvin and 265 bar or atmospheres at the surface – giving it a yellow hue of broadband light, since the liquid metallic hydrogen provides for near black body emissions. The lower pressures at the sun’s surface transform hydrogen from a liquid metal within the sun into a gaseous plasma form that forms the sun’s atmosphere. In any event, outside of the core of the star, the lower pressures and temperatures do not support fusion. Similar descriptions can be made of many “main sequence” stars just as it applies to our sun.
As an aside, you may be interested to know how, even at such high core temperatures and pressure, how can two protons be brought together, overcoming Coulomb’s forces. The answer lies in a quantum probability trick, called quantum tunneling. This is an interesting rabbit hole to explore, if you are so inclined, but I need to return to the main point.
Somehow, the heat generated at the fusing core must get out to the surface, in order to be radiated out to the solar system and grow our plant life amongst other things, and this part of the story is critical to understanding how stars evovle and PNs form. In fact, the standard explanation star evolution (including the core heating up while the surface cools) makes no sense without understanding that this is a heat transfer problem. It is likely as much a surprise to astrophysicists as to climatologists that the main mechanism of heat transfer is via convection and not radiation. Radiative heat transfer only dominates at the sun’s (star’s) surface. Thanks to Maxwell’s equation, in a star the convective movement of hot core material to the surface of the sun and cold material back to core is driven primarily by the electromagnetic forces driven by (differential) spins of liquid metallic hydrogen. (Note that I mean metallic in its real sense – like sodium, mercury, and iron, and not in the astronomical sense of heavier than Helium – that I refer to as dust.) At we have seen, the core can be thought of as having spherical shells of hydrogen between the core and the outside surface. Magnetic fields push hot material from within outwards, where it cools and then returns back.
Heat is transferred to the surface of a sun-like star via electromagnetic convection. We can even see the magnetic line on the surface of the sun in this Nasa image (Credit: NASA Solar Physics) of the sun, taken through a Ha solar filter. This mechanism, while still allowing for a temperature and pressure difference between the hotter core and the cooler surface, can maintain a pseudo-steady state condition to occur within a star for billions of years.
Aside from a pressure and temperature gradient between the surface and the core, the star will eventually build up a compositional gradient between the two regions as hydrogen fuel in the core is depleted through conversion to helium. Helium build-up/hydrogen depletion increases the density of the core and this has a knock-on effect by increasing the pressure at the core. More importantly, helium is decidedly non-metallic and non-magnetic and this has a profound effect on circulation of material and heat transfer. The magneto that has been driving core/shell loses effectiveness causing the core to heat up at the same time as the shell/surface region cools down. It is this separation between the core and the shell that will ultimately develop into the two components that make up the nebular gas of a plantary nebula and the white dwarf star.
Within the star, as hydrogen is converted into helium, temperature slowly rises as the core’s ability to shed heat effectively is reduced. This process is very slow, but eventually, there becomes a shortage of hydrogen to burn at the core. Despite what you might intuitively think, this causes the star to heat up due to convection curtailment and the star expands due to this temperature increase. The colour of the star, however, seems to contradict this warming by exhibiting light radiation associted with a cooler colour (reds and orange as opposed to yellow and white). The apparent contradiction is explained by reduce convective heat transfer from the core, coupled with an increased cooling of the surface associated with the larger area of an expanding star. The contradiction is further dispelled by the star becoming brighter, due to the increase in photon emissions albeit at a lower energy per photon.
The increased temperature within the star allows hydrogen to begin fusing to Helium away from the core in an expanding shell away from the spent helium remaining within the core itself. The point where the star expands, becomes redder, hydrogen fusion moves away from the spent core and into progressively larger shell is the point where we consider the star to have left the main sequence of star behavior and evolved into a red subgiant or giant. (I love the term subgiant – I think it is the same thing as “big”). This is the first stage that a star that is somewhere between a third to 8 times the size of the sun that a star takes to eventually become a white dwarf and planetary nebula. Below, is a diagram (credit Spring 2018 ASTR), that you can follow my description.
We aren’t talking about subtle changes here, the star can expand to 10 to 100 times is initial size as a red giant – for the sun this could mean that it expands out to the earth’s orbit. (Please don’t loose sleep over this, because I have faith that the astronomers have done their calculations correctly and this won’t happen for another 5 billion years or so). In any event, it is unlikely that much of the star has ceased to be solely liquid metallic hydrogen at this stage, and is more likely to be a boiling soup of liquid and dense plasma. he now red giant is likely somewhat unstable as in its effort to achieve heat transfer, the core and shells likely burp up large ejections of plasma as solar winds. The red giant stage is thought to persists for about a billion years (or 10% of its total life) in this mildly unstable state, but for larger stars, this process is much quicker.
During the red giant phase, a star’s inert helium core will continue to warm and compress due to its increasing heat transfer issues and being surrounded by a shell of fusion. The hydrogen fusing shell continue to send much denser helium down to the core adding to the pressure at the core as well. The temperatures at the core can reach an astounding 100 million Kelvin and I don’t really have a pressure value, but pretty darned high. At some point, the increased core pressure and temperature starts helium itself to fuse to primarily oxygen and carbon (via the triple-alpha (particle?) process apparently).
Helium fusing to oxygen and carbon is really fast and the initial ignition is termed a helium flash and takes just seconds to hours to occur). It is likened to a billions of nuclear bombs going off in the core – and yet there is little evidence of this occurance outside of the star other than a slight brightening, slight contracting, and a shift to a hotter colour (red to yellow, white, or even blue) again. This is the start of yet another phase in the death march of the star – it begins to be either a helium burner, or more likely a helium burner near the core and a hydrogen burner at some radius from the core. The helium burning tends to occur in fits and starts, and it is generally stars in this phase of life that we note as having variable brightness and therefore variable stars. For the sun, this phase will last only about 100 million years or 1% of its lifespan. At this stage the star is also knows as the unimaginative “helium core fusion star”, but we are more likely to call it a variable star due to its continual variation in brightness, colour, and size as helium at the core fuses in fits and starts as it is supplied by the shell of hydrogen fusion. Helium fuses to carbon/oxygen much more rapidly than hydrogen to helium.
Helium fusion occurs rapidly and before too long the helium (now fuel) at the core gets exhausted. and also begins to burn within a shell of increasing outward radius, lagging the hydrogen fusing shell further out. Of course, this deserves another phase name, and apparently the best we can do is the double shell fusing red giant on the AGB (asymptotic giant branch). The star swells again, now dwarfing the red giant size and taking on the even more hyperbolic name of “red super giant” – bigger, redder, unstabler, and (at least at the outside, an even cooler than before).
Of course, the stars heat transfer issues haven’t gone away. Instead they have only gotten worse due to both phase, and property changes of the star structure. The core gets hotter while the outer surface gets cooler as the star expands. The heavier elements gathering at the core come under ever increasing pressure. Mass ejections go into overdrive in an effort to shed heat and the star can loose from 50% to 80% of its original mass as stellar winds at this stage. The helium tends to ignite in pulses, adding to the chaotic ejections of material. The pulsating mass ejections of often take the heavier elements along with them for the ride. It is this ejected material, consisting of hydrogen helium, carbon, oxygen, sulfur, etc. that will become the nebulosity we see in a planetary nebula.
One last super ejection of material leaves the hot dense core of the star exposed and we call this penultimate star phase a white dwarf. The super hot, high pressure core is laid bare for us to see. Still no relief to the heat transfer issue, as now the sole method of star cooling is simple broadband emissions. The star will eventually become a black dwarf when it cools down, but don’t hold your breath – it will take many times the current age of the universe to occur so we haven’t identified any black dwarfs yet.
Other than black body radiation, there are no acitve processes really going in a white dwarf. Along with its huge heat content, there is a lot of gravity in such a small volume and the core remains under enormous pressure. The planet sized volume that it does have is maintained by the Pauli exclusion principle and the Heisenberg uncertainty principle ganging up against gravity. The electrons within the atoms in the white dwarf cannot be squeezed together any further. This is called the electron degeneracy pressure because it can only be overcome by getting rid of electrons by having the combine with protons to become neutrons – but that is for another posting.
In the meantime, the white dwarf, now gravitationally separate from the material it ejected, and lights up the ejections with its ultraviolet photons and lets them glow. Eventually the gases will expand or be blown away from the white dwarf and the planetary nebula will fade (10,000 to 50,000 years).
PN Shapes and their Bigger Role
If you read the previous section of this post, then congratulations and I thank you for your interest and perseverance. With all this star pulsing, chaotic ejections, it is astounding to me that such a spherical beauty such as Abell 39 could ever be produced. My only guess is that for the star that produced Abell 39, the last super ejection must have been dominant a largely spherical to produce Abell 39 – but this is only a guess.
Stars, may look spherical from afar, but even a stars spin causes it to be oblate You would think any lack of spherical symmetry of the original star would be exaggerated during it may phases and pulses of material ejection. By staying tuned to these postings, I will be showing families of planetary nebulae and speculating as to what may be ocurring to cause their unique character. Lots of this will be somewhat more whimsical than this posting as a lot of it will involve more guesswork than deduction.
But there is an even bigger picture here – more important to the galaxy and universe than even the death of a star. Stars, via their fusion reactions (and neutron bombardments) are responsible for all the elements in the period table pretty much above Lithium. Even more importantly, these elements or dust elements as we often refer to them, are a necessary for new star creation.
The stars are not only dust manufacturers, but distributors as well. Some stars become planetary nebula in the spiral arms of a galaxy, while other evolve in the gaps between spiral arms. Some star will deposit their dust in hydrogen arms that have been waiting for this final ingredient to create stars in regions that have been hitherto unproductive. Astronomers often refer to the carbon and oxygen as fusion “ash”, but like the fabled Phoenix, with this ash new star will arise.
As stated in the previous section, the described star evolution into a planetary nebula on really applies to stars that are up to eight or so times the size of our suns. Stars larger than this undergo an explosion event called a supernova (we sure like the work super, don’t we) that I will describe in a future posting. We often image the remnants of supernova explosions too, but unlike a planetary nebula, supernova remnants (SNRs) glow under their own heat. They can be distinguished in images by their typically larger size, but even more readily by their woven-tendril shaped shock fronts associated with supersonic propagation. Unlike a supernova, that is more analogous to a supersonic detonation, the ejection of material in the formation of a PN is more analogous to a deflagration explosion, where the products travel at subsonic speeds.
In the meantime, I will end this posting with a second planetary nebula that I shot a few years ago, that I think better records the mass star ejections that make up the complete history of its progenerating star.
The Blue Snowball Planetary Nebula (NGC 7662)
Planewave CDK12.5 – AIS6200MM; A-P 1100 GTO AE; Antlia 3.5nm NB & BB filters
H, O filters: (2 x 21 x 360s exposures, Bin 1×1, Gain 100; & 2 x 21 x 120s exposures, Bin 1×1, Gain 0 & 2 x 41 x 20s exposures, Bin 1×1, Gain 0)
S filter: (22 x 240s exposures, Bin 1×1, Gain 100)
R,G,B filters (3 x 15 x 120s exposures, Bin 1×1, Gain 0)
Total Integration Time = 7.62 hours, October 2022, Vancouver Island, Canada.
