It is easy to forget that our 2-D images are actually representations of 3-D gas bodies, that are acting according to 4-D dynamics. In day to day life, we have many clues that we can rely upon including parallax views, perspective rules, lights and shadows, and actual physical interaction that we can use to assess the nature of objects in 3-D and 3+1 space. Unfortunately many of these clues are absent or confusing in our deep space objects. In this post, we analyze a 2-D image of the Great Orion Nebula and stellar nursery including its shape and orientation in 3-D space. Along the way, we will present an understanding of the three principle gas types in deep space photography and how these gas types actually show up in our photos so that we can apply the same principles to any image of a deep space object. %
The Great Orion Nebula
M42 – The Great Orion Nebula Narrowband Filter SHO Palette Version
Telescope: Televue 127is; AP 1100GTO Mount; Camera: QHYCCD QHY600M; Filters: Antlia Pro; Broadband Filters;
Frames: R,G,B: (22,24,32 x 100s, Bin 1, Gain 26); H,O,S: (26,28,30 x 200s, Bin 1, Gain 26); H,O,S:(38, 32,23 x 500s, Bin 1, Gain 26)
Total integration time = 19.8 hrs (Dec3,8,11 2024 & Jan.24,25,26 2025) Vancouver Island, BC, Canada
The Orion Nebula or M42 is very bright and generally not a difficult object to image, but for me, I have to content with very high trees to my south coupled with generally poor winter weather on Vancouver Island. This data was left unprocessed from last winter and I was wondering if I needed more integration time to get a decent image. So to finish my target planning for this winter, I thought I would process what I had. I used my own formulas for HDR composition and managed to get a good dynamic range from my short and longer exposure combination, HDRMT, and GHS stretching in Pixinsight.
The large yellow/orange/red region within a purple halo at centre is the Great Orion Nebula or M42 and within the brightest part of the nebula are young stars being formed in a cluster. To the naked eye only these bright young stars appear as the “single” middle star in Orion’s belt. In binoculars, that single star become a smudge, letting us know that it something more. The full stellar nursery actually takes up a larger portion of the night sky than the full moon. Just above the nebula a lone star appears to be blowing a spherical bubble and further up, another stellar nursery can be seen, known as the “Running Man” nebula or Sh2-279.
These nebula are distinguished from the surrounding turbulent, larger molecular cloud by the dull orange emissions and black regions that take up the rest of the image. In fact the cloud contains many more nebula off the image extends over hundreds of light years, while the Orion Nebula itself is 26 light years across.
Full resolution, downloadable images in Astrobin or Victoria Centre RASC Zenfolio sites.
That's pretty, but what is it?
Whenever I show anyone an image of a deep sky image, the first question I am often asked is “What is it?”. Once asked, I often feel I have to first gauge the person’s interests, time, background, etc. to formulate a response. Do they want to know what the subject matter is, or where and when I took the image? Do they want to know what the subject matter is made of, what it does, or how it got there? The full answer of everything will take too long and likely get me into trouble by stretching my knowledge base, so I generally respond back with name, rank and serial number of the object and try to enquire more precisely what it is they would like to know.
In this posting, I would like to take a fundamental approach to describing what the major components of our imaging subjects – from galaxies to nebula actually are. Then, with some basic thermodynamic and basic light interacting properties, we can get a sense of what our subjects look like in 3-D space and little of what the dynamics are too. I am taking our Orion Nebula as a test example, but a similar approach can be taken with the analysis of other nebula and galaxies too. In fact, the is the bulk of what I do when I interpret astrophotographs.
Step 1 in this process involves getting a sense of the material bodies in 3-D space. However, astronomy is one of those sciences where we must rely on remote sensing (via cameras and telescopes yielding 2-D images) to make this assessment. One can’t just walk around, touch, handle, smell, taste or listen to our target objects. One has to look at many images, of many similar objects, before one can get a sense of the 3-D structure. Even then, each object has its own features that may not be straight forward to unravel.
An additional technique is to compare our objects to more familiar ones down here on earth that you can examine in person and try and relate these to objects in space.
Three Gases and Stars
All of the broadband visible light we see in our images originated from the stars. Most stars are composed of hydrogen in a condensed (or supercritical condensed) form
Most of the volume (at least within all galaxies) is made up of three gas – all primarily made of hydrogen (and helium in second place) – but occupying different regions in 3-D and projecting their 2-D influence on our images. The gases are best characterized by the predominant form of hydrogen within them – ionic monatomic (H+ or proton), monatomic neutral (H – proton with a nuetron), or diatomic molecules (H2 or two covalent bonded atoms) – and the prevalent conditions that suit their ionic/atomic/molecular form.
I believe that the above chart is likely the most important in astronomy, and it exists (to various accuracy) in the astronomical/astrophysical data, yet I don’t believe that most professional astronomer or astrophysicists understand its implications. It is critical to the understanding of the dynamics of star formation, molecular cloud collapse, and even galactic structures – especially considering that 99% of a galaxies mass in contained with 1% of the galaxies volume! You will be sure to see this chart in various forms in many of my future postings. If you happen to be a chemical engineer, you might key in right away, but for the rest of us – I will gently reveal its implications to understanding astronomy.
I have highlighted the three main gas body types, dependent on their predominant form of hydrogen that make them up. The state conditions (temperature, pressure, density) of the gases within each type are not distinct – the gas at any point within the conditions can mix and transition. For example, warm neutral medium can cool off to become cold neutral medium if there is a physical mechanism (heat transfer, expansion, radiation) to do so. However, transition between colour groups (ie. types of gases) involves a chemical change to the form of hydrogen. This generally requires a third party particle (a slow electron, UV radiation, neutral Helium atoms, metallic dust, etc.) to accomplish this and it must occur under the right conditions that favour the transition (density, pressure, and temperature).
Seeing the layered cocktail
It may be counterintuitive at first to think of gas bodies as remaining separate, since usually we feel that when put them together, they spontaneously, and rapidly mix. However, just like layers cocktails, such as the one at left (Credit pealwithzeal.com), if the bartender is careful to avoid mechanical mixing and convection, by pouring in order of density, miscible bodies can remain distinct for some time. Diffusion is a very slow process, particularly on the time scales of space. Nonetheless, we can think of our images of nebula and even galaxies, as images of layered cocktails, only the different bodies are made of gas, rather than aqueous liquids.
There is just one problem with this. All three of the forms hydrogen that make up the gases in our images are both transparent and colourless. In one way, this is a good thing, because if the gases were opaque, we wouldn’t be able to see past the gas closest in our line of sight and wouldn’t even be able to see the stars. (Stars by the way, are a fourth form of hydrogen – a condensed form that due to its heat, emits broadband IR, UV, visible light, and other wavelengths of light too).
Indeed, both ionic media and neutral atomic media are transparent and colourless (at least in visible broadband terms). So is molecular hydrogen that exists invisibly within regions of galaxies where it does not contain dust. However, closer there is an relative abundance of dust within molecular clouds, left over from dead stars. This dust is concentrated in this gas body and not the others by mechanical and gravitational process that I will address in a different post. Fortunately the dust that does concentrate in the molecular regions is condensed, non-volatile material – and this means it is opaque and reflective, which means we can generally tell where the molecular gas bodies are by the fact that they are opaque and reflective.
The opacity of molecular bodies, depends upon the density of the dust (it can vary considerably – as seen in the table) and the thickness of the gas we are looking through. We are used to seeing such opacity in gas bodies when we look at clouds in the sky. Sky clouds are made opaque by condensed water droplets. That is why we refer to the these molecular gas bodies as molecular clouds. Stars that are behind the gas body get dimmed or even obliterated. Unfortunately, stars that are in front of the dusty body are unaffected and this can cause some confusion when trying to visually assess the opacity of the molecular gas body.
When proximal to the molecular cloud, dust can directly reflect the light from stars, and this directly reflected light maintains the colour of the stars – generally reddish, but sometimes vibrant blue. Indirect, diffuse reflections also occur that tends to turns the colour to grey to reddish brown. We see this reflected light as “reflection nebula” or even as the “dust lanes” in galaxies that communicate to us where the molecular clouds sit.
As a general rule, molecular clouds or H2 bodies don’t emit light at their cold temperatures. Fortunately, there are a few exceptions to this for the ionic, H+ regions. While hydrogen ions can’t emit light because they have no electrons, heavier ions, most notably oxygen (O++) ions can in a narrow band of greenish-blue wavelengths. These emissions from the ionic media is best captured through [OIII] filters, when normally assigned a blue colour, add semi-transparent blue colour to this gaseous region. This light is only given off when the oxygen concentration is low, and is ionized by very strong UV bombardment from the stars – that also ionizes neutral hydrogen atoms. In practice, this strong UV light emission is associated with young star clusters.
Within galactic disks, the ionized gas zone exists close to stars and the gas makeup can transition from ionic to neutral atomic and this is also dependent on the strength and intensity of the UV light. When UV light hits a neutral hydrogen atom, it can get excited and eject its electron – resulting in its transformation to an ion. If the UV light is not too intense, the now hydrogen ion can recapture its freed electron, return to neutral and start emitting photons of its own as it de-excites itself down to the ground state. A couple of these photons are within the visible range, the most intense of which we know as Hydrogen-alpha or Ha that gives the region of ionization red coloured brightness. Most of the time, this signal is very weak within neutral atomic gas bodies but intensifies strongly where UV light meets a molecular cloud.
This image of the Orion Nebula is the same narrowband image as above, only coloured using the red/green/blue filter information to more naturally colour the emissions and reflections.
Deep blue reflections can be seen off of nearby stars. Emissions and reflections of Ha and [SII] light from the molecular cloud give it shape. Meanwhile indications of turbulence and streamlines show how the other two gas bodies are moving against it. Closer to the nebula, the gas is streaming out of the nebula, superimposing it’s blue [OIII] colour. Again streamlines indicate flow out of the nebula. We can actually see the molecular cloud walls into the nebula, showing up as pink bright Ha light.
At these walls, molecular hydrogen is heated, disassociated, and expanded increasing the size of the nebula and ejecting ionic and neutral hydrogen out of the hole made in the cloud itself. The star cluster that formed within the molecular cloud is now mostly unobscured to us at the top of the Orion nebula.
The molecular cloud interface
In many circumstances, the opacity of dust does not let light penetrate far into a molecular cloud, except for perhaps the brightest of stars behind it. In these cases, most of the reflected light, and indeed emitted light, can be thought of as emitted by the near surface or interface of the cloud with other gases. How this surface reacts to what is going on outside of it or within in, gives us an ideal as to its overall geometry and what is shaping it. Ha emission is one such method of doing this, as it appears to emanate right from the surface of the cloud.
When UV light strikes the interface from either ionic or atomic bodies and the molecular cloud, it is prevented from entering deep into the cloud itself by the dust. The dust at the interface heats up and shifts the partition coefficient at the interface towards atomic. Either the heat or a direct hit by a UV photon disassociates the hydrogen molecule causing rapid expansion of the gas. This gas jets off the surface of the molecular cloud, pushing it back and increasing the pressure within the cloud and promoting the formation of additional stars. The atomic hydrogen leaving the molecular cloud goes through the ionization/deionization process and emit strong Ha signals near the surface of the cloud that faces the source of the UV radiation. UV light can also ionize and excite metals at the surface of the cloud, and ionized sulphur visible emissions [SII] can also leave the surface of the molecular cloud.
By examining the faces of molecular cloud we can orient the cloud interface to the directional source(s) of the UV light that causes it to shine. Conversely, darker regions that don’t shine in Ha or [SII] light are likely facing away from the UV source. Hard edges often form, giving an indication of the shape of the interface between the molecular cloud and other gases. Furthermore, molecular clouds can also reflect this narrowband line, yielding further clues in both reflections and shadows. Overall, in combination with star light, we can glean a 3-D perspective of the shape of the molecular cloud – at least in the vicinity of a strong UV source – such as the cluster in our Orion image.
Shadows and emissions can also give us huge clues as to the relative movement of ionized or neutral hydrogen against the molecular cloud. This is where the dynamic viscosity in our table comes in. As other bodies travel past the molecular cloud, shear forces and move and erode it. This erosion can result in tendrils of molecular cloud that don’t readily mix, but are elongated in the direction of relative flow. By looking at the shape of the direction of these tendrils we further our picture in terms of the dynamics involved.
Altogether, we we take all of these sources of light, reflections, opacity, and colour together, we can turn our flat, static 2-D image into a dynamic 3-D description of what is going on – especially when you couple it with a knowledge of physical systems. In my humble opinion, that is the art of astrophotographic interpretation. That, and the ability to stare a images for considerable periods of time.
The Orion Nebula - Never the same again
Star formation often occurs deep within a molecular cloud, obscured from our view. But when a new star ignites, its creates a lot of stellar winds (ionic gas) and UV radiation that, together with UV dissociated hydrogen creates an expanding chamber or bubble surrounding it. Within our Orion image, one such bubble can be see at the top of the Orion Nebula. I believe the star formation in this bubble was likely helped along by the pressure created in the main Orion Nebula and it is fortunate that we can see it close to the molecular cloud surface.
A single star likely created such a bubble deep within the molecular cloud. This star was shortly followed by others, creating a stellar nursery. This chamber eventually expanded to break the surface of the molecular cloud and now appears as a cavity or hole in the molecular cloud that we call the Orion Nebula. When we look into the Orion Nebula, we are looking into the cavity, at inner walls of the molecular cloud, brightly emitting Ha and [SII], with the ionic gas and [OIII] signal spilling out over the rim of the hole.
A second, similar breach of the molecular cloud has been made at the “Running Man” location. A bridge of turbulent molecular cloud seems to separate these nebula, and the surface seems to angle off at this bridge.
To me, the right hand side of the image resemble the face of an owl, with each of the Orion and Running Man Nebula forming by the eye-sockets of the owl. Looking into the nebula would be similar to looking into the empty sockets of the owls eyes. Based on the turbulents, apparent streamlines, it appears that the bridge and beak between the eyes corresponds to a raised area of molecular cloud.
Also, based on the position of the trapezium star cluster within Orion, it appears as if the stellar nursery cavity lies underneath the bridge and may connect up, somewhat like a bent pipe, with the Running Man being the other end of the same cavity.
Of course, I have no way of proving that the Running Man and Orion nebula form two ends of the same stellar nursery behind a bridge of molecular cloud, but now that I have formed this 3-D picture in my mind, I can’t look at this space the same. I definitely can’t look at it as a couple of 2-D independent regions.
I believe that thinking of these images as the three types of gas bodies in three dimensional space, with the surface of the molecular cloud supplying most of the light that we glean more about the nature of what we image.
