The Cygnus Loop is a large spheroidal supernova remnant. What we are really looking at is material either blown out from the supernova star or gathered up by the shock fronts moving out from the explosion. If large enough, a star will undergo a supernova explosion when it is running out of fusing fuel and can no longer sustain its volume against gravity. While the core of a dying star implodes, the sudden release of energy causes a lot of the star to explode violently with both material and light.
This supernova was astronomically fairly close to us at just 1500 light years away, and occurred between 5000 to 8000 years ago. At the time, the starry explosion would have been the brightest object in the Milky Way galaxy sending material away at many times the speed of light. What’s left of the star itself is likely somewhere near the middle of the remnants that we see in the image, but now exists either as a neutron star or a black hole, with its exact current location unknown.
The supernova explosion itself would have been visible to the naked eye just a civilizations were beginning to appear on earth. Being closer and younger than many other supernova remnants in the sky, it is quite bright for the camera but it takes long exposures to bring out its features.
The supernova explosion itself would have been visible to the naked eye just a civilizations were beginning to appear on earth. Such brightness does not last long, however, with brightness peaking and starting to fade generally within hours or days.
The light show that was the Cygnus Loop supernova has long faded, but the material thrown out of the explosion is still visible and can be captured through the long exposure magic of our cameras.
The whole Cygnus loop takes up about 50x the area of sky than our moon. Even with the short focal length of the telescope used for this image, capturing the whole thing required a mosaic of two full frame images.
Cygnus Loop (Supernova Remnant (SNR)) in SHOAskar FRA500; iOptron HEM27; ASI6200MM, – Antlia Pro 3.5nm Narrowband Filters
2×1 Mosaic: H,O,S : (73,68,55 x 780s Bin 1, Gain 100) Total integration time = 42.5 hrs (July 9 to 17, 2024) Maple Bay, BC
For full resolution, downloadable image, link to Victoria RASC Zenfolio or Astrobin.
A supernova is actually an implosion of a massive star at its inner core to a neutron star or possibly a black hole, releasing a huge amount of energy thermal and electromagnetic. The outer shells of the imploding star are imparted with enormous temperature, pressure, kinetic energy and outward momentum that they simultaneously explodes as the core implodes – suddenly accelerating a sphere of material away from the star core. (Part of this energy, and there is a lot to go around is used to nuclear fuse this material into elements heavier than the exothermic iron atom, but this is for another posting).
So much momentum and energy is released so suddenly into the exploding material that it is accelerated to speeds faster than pressure or temperature can disperse it. It forms a spherical wall of material, known as a shock wave, that sweeps up and incorporates any other particles it meets in its path (either molecular cloud or ISM). The speed that pressure or density waves move through a material medium is known as its sonic velocity because it is these waves that transmit sound. The actual speed depends upon the properties including pressure, temperature, and density of the material it is relatively travelling through and we normally call this speed Mach 1. Material travelling faster than Mach 1 creates an almost square wave function of high density and pressure moving through undisturbed material in front of it, and a strong vacuum behind it. We call this a square wave of material a shock wave because – in terms of thermodynamic properites such as pressure, temperature, and density there is no warning that it is coming and anything the wave encounters must be truly shocked at its arrival.
Since ejected material is moving faster than pressure can disperse it (Mach >1), it forms a uniform wall or sphere of material moving – like an inflating ballon -outwards. The speed that pressure/density waves can move through any medium is known as sonic velocity and is represented in a dimensionless number or Mach number of 1.0. When the particles are ejected faster than Mach 1, they cannot disperse and create a shock front that sweeps up any new particles it encounters in its path. This lack of dispersion gives the shock front an apparent integrity with all of the particles within the shock front moving at the same velocity.
Additional heating is achieved due to the enormous friction and momentum transfer through the particles caught up within the shock front and temperatures can reach over a million degrees in these shock fronts. At such temperatures, the atoms within the shock front start emitting light – ultimately allowing our cameras to capture their location.
Eventually, due to both geometry and the continuous bulldozing of material in front of it, the shock front will run out of steam and its velocity drop down below Mach 1. At this time, material at the edge of the shock front will begin to disperse. Alas, temperature will eventually cool too, and even though this takes much longer, the shock front will fade from our astrophotographic capture.
In the picture at left (I believe was taken from the original Trinity atomic testing), a dark shadow can be seen around the explosive fireball. The supersonic shock front is travelling into stationary air increasing its density and refractive index that we see as a shadow. The study of supersonic flows of fluids is a large field of study in itself, but is more often combined with flow past solid bodies. In any event, we sometimes use that fact that we often make chokes (or small orifices) to limit the fluid flow rates (particularly gases) to the subsonic rates that pressure differentials can generate. It is obviously, very useful in the study of ballistics or supersonic air travel.
You may be wondering why the Cygnus Loop doesn’t appear as a shadow – instead it actually emits light that is captured by the camera. The reason is that all that friction between material at the shock front heats the atoms and molecules to a very high temperature – 2.9 Million degrees C. Many of the atoms are of ions, and most are in quantum stimulated energy states. As their electrons transition, they emit the same spectral lines that we are used to imaging in other nebula. Particularly strong are the red hydrogen alpha line and the blue/green line of ionized oxygen..
You may also be wondering why the Cygnus Loop also doesn’t appear spherical. The answer to this required a considerable amount of staring on my part and trying to visualize the perspective. At right I have taken an older image of mine, and drawn where I see the outer limit of the shock wave effects. Overall, the shock wave appears spherical, except for a large growth at the bottom. As for the highly visible material, it looks as if two “ends” of a sphere have been blown off, leaving a donut shaped strip in the middle. Ok, so it is kind of spherical but there must be something else going on here.
Another thing to note, is that why are there tendrils of material representing multiple shock front planes, and not one shock front as in our atomic detonation? Finally, a last question involves the colour of the tendrils that define something about the composition and properties of the tendrils themselves. In our simple model of a single mass of material sweeping outward, why would the tendrils be different in composition and mass, appearing as differences in color and brightness.
The answer has a little to do something to do with both the nature of the supernova itself and supersonic travels of particles. The supernova cannot be expected to be spherically symmetric along all axes. Even if the outward velocity of particles were exactly the same, there will be variations in particle mass and momentum (remember the point about fusion going on). The energy imparted may also vary someone, as well as the timing of the explosion. The supernova explosion potentially may occur as a series of explosions too with location/timing somewhat variable. In the case of Wolf Rayet stars or other types of supernovae, the explosions can be periodic. As these particles travel outwardly variations in velocity and momentum will be exaggerated due to geometry, leaving us with an asymmetric supernova remnant.
We are not sure what causes hook of material at the bottom of the image, but a small neutron star has been found near the centre of the bubble formed by the hook. Could this be the results of a second supernova, potentially triggered by the first? Or perhaps during the initial supernova the initial star broke apart?
Of greater importance is any inhomogeneity or anisotropy in the material surrounding a supernova. In terms of momentum transfer, the shock front will lose momentum faster in areas where there is more material to bulldoze up to supersonic speeds. Here the relative mass of the shock front to the density of the surrounding material cause variation in the travel distance of the various wave fronts / shock fronts. This can easily result in non-spherical supernovae remnants, and make the true epicentre of the original explosion difficult to pinpoint.
Of more of importance is likely the density and nature of the ISM medium the these supersonic particles. Although in reality what we have is a particle leaving the star and entering a rarefied sea of relatively stationary other particles. Nonetheless, I think is useful to think of the supersonics particles like a supersonic jet travelling through air. You will note the jet plane at right, that represents our particle moving supersonically, generates a conical shock wave in the medium of air it is travelling through. As it turns out, the shape of the shock wave (the angle formed to the leading edges of the plane itself) is dependent on the speed of the plane, but also the density and other properties (including sonic velocity) of the material it is travelling through. The density and composition of the ISM that our supersonic particles travels through will not at all be homogeneous, so it is not a stretch of the imagination that the tendrils form as sort of conics in the wake of many supersonic particles.
Unlike our jet plane which is provided with thrust as it travels, our supersonic particles only have the momentum they were imparted with at the supernova explosion. Although rarefied, the particles will experience increasing drag as it gathers more and more ISM into its shock wave. Eventually, some of the supersonic particles will slow down and stop, leaving their shock fronts to slowly dissipate under subsonic conditions.
Supersonic shock waves are indeed waves. They will also reflection partially off of areas of higher density, and indeed can even reflect off of themselves – causing constructive and destructive interference. Individual particles or groups of particles within the shock wave, will naturally move ahead into the assumed stationary medium, causing shock wave cones that move away from any leading particles. The geometry of these cones depend upon the Mach number or speed of travel of the lead particles. Such cones are likely to overlap and interfere as well to create the tendrill like structure to the brightness.
This all adds to the intricate patterning we see in these supernova remnant images. If you look closely, you will see, outside of the tendrils a vestige of particles that essentially form the initial supersonic shock front and fainter Ha emissions that have left the interstellar tendrils behind. This front is of various intensity and distance out from the centre, depending largely upon the density (and level of drag) they have experienced on their travels. The background of the Cygnus Loop indicates that this supernova actually took place within a molecular cloud – evident in the very weak background emissions further out still. You may not that within the centre of the SNR donut and within its two caps, these emissions are largely absent with the material now supersonically swept to form the tendrils.
It is perhaps natural to assume that when we have different coloured tendrils of material at different locations within a supernova remnant that these colours represent tendrils of different composition. For example, initially one might think that the red tendrils indicate places where H2 concentrations are highest and blue tendrils where oxygen is dominant. While this may be true (and component separation process should be of extreme value to astronomers), it is not necessarily true. The dominant color of the emissions are likely temperature dependent and or even density dependent (in the case of forbidden emissions). Any way, the resultant images are spectacular in both color and brightness.
One pattern that has emerged for me as an astrophotographer though, is that whenever I see sharp multiple tendril like fronts, it likely indicates either a supernova remnant or Wolf Rayet star and supersonic. Particularly the [OIII] or oxygen signal, which usually yeilds diffuse, or at best singular sharp fronts (such as bow fronts), only yeilds shaper multi-tendril patterns when supersonic relative velocities are involved. Subsonic flow give much softer edges to oxygen emitting regions, and I consider this very diagnostic.
At this point, on this website we have encountered two dimensional numbers that control the appearance of nebula to us. In our description of galactic structure we had introduced the concept of Reynolds number – that determines weather our nebular flows are laminar or turbulent. The Reynolds number is the ratio of inertial (mass based) forces to viscous drag. In this posting, we have introduced a second, number: the Mach number another fluid mechanical measure that determines whether the velocity of particles are travelling supersonic relative to the medium or not. There are a few other dimensionless numbers and concepts that we will likely introduce in the future that are important in understanding how deep space objects appear to us – so stay tuned.
Finally, one does not have to capture the whole of the Cygnus Loop to use it as a target. There are multiple areas of the Loop that create beautiful “star scapes”. The ones I show here have been cropped from the initial mosaic…
