Currently viewing the tag: "Supernovae"

NASA’s Chandra X-ray Observatory (along with optical data provided by the ESO and infra-red data supplied by the Spitzer space telescope) has produced a truly amazing image of the star cluster NGC 1929 located within the nebula N44.

N44 and NGC 1929 Credit:X-ray: NASA/CXC/U.Mich./S.Oey, IR: NASA/JPL, Optical: ESO/WFI/2.2-m

The nebula and its star cluster are located in the Milky Way’s largest satellite galaxy – the Large Magellanic Cloud (LMC) –  at a distance of 160,000 light years from Earth - 940.6 quadrillion miles.

The star cluster is composed of primarily newborn stars that have only recently been forged from the surrounding material.A great number of these are many times the mass of the sun and produce a precipitous amount of hard radiation and vicious solar winds, before burning out in (on the time scales of the universe) sort order as supernovae generating incredible outpourings of energy.

These shockwaves along with the continual bombardment from radiation and particle stream gouges out massive ‘bubbles’ in the surrounding nebula. The x-ray data provided by Chandra (shown in blue) shows the regions of the nebula that are at the highest temperatures – the areas under the heaviest onslaught of radiation or reeling from one or more shockwaves . The cooler gas and dust as detected by Spitzer is displayed in red with the yellow regions show where the radiation is actually causing the surrounding material to glow in the visible range (this data was collected by the ESO’s Max-Planck telescope).

Astronomers have been having a problem with N44 and other similar ‘superbubbles’ in the LMC for sometime now – they are producing too many x-rays.

Before anyone panics, this is not a medical problem (we aren’t all going to suffer radiation poisoning thanks to a few over-active nebulae in another galaxy), it only refers to the measurements pointing to such nebulae producing more x-rays than could be explained using current knowledge – our knowledge of such objects must be incomplete.

A previous study had suggested that the shockwaves of supernovae impacting the bubble’s walls along with the evaporation of hot material from the sides of the bubble could perhaps explain this anomaly. This set of observations at least doesn’t find any supporting evidence for these ideas though it has been the first time that the observations have been sensitive to distinguish between these and other possibilities so progress is being made.

This post was co-written by PeterC & HannahH

SS 433 is a very interesting system with an equally enthralling history behind it. It is located well within our own galaxy at a distance of around 18,000 light years (roughly three times the distance to the Crab Nebula) in the direction of the constellation Aquila – The Eagle. It is visible using a large (8 inch or larger) amateur telescope, though as it is a 14th magnitude object (in visible wavelengths) you would need to improve your eyesight  by a factor of 10,000 to have a chance of seeing it yourself from an urban back garden. It sits just outside the galactic plane as the image below shows rather nicely.

RGB calibration of SS 433 and galactic plane. R is at 60 microns G is at 25 microns B is at 12 microns Data: IRAS Calibration: Hannah Hutchins

SS 433 is a binary star system; that is two stars orbiting around a point of common mass known as a barycentre.[1] No two binary systems are exactly the same, tiny differences in the mass ratio and age of both stars as well as the precise orbital dynamics; produce a vast range of possible systems. In the case of SS 433 the system has undergone extreme stresses in its lifetime, approximately ten thousand years ago the larger mass star in the system ran out of the nuclear fuel required for the fusion reactions in its core and detonated as a supernova. This created the supernova remnant nebula W50 in which the system is located – and as can be expected near the centre.
The resulting collapsar remnant (the dense object left by the supernova rather than the gas and dust thrown out by the explosion to form a nebula) is believed to be either a neutron star or a stellar mass black hole.

The secondary, lower mass star, has been identified as a spectral class A7Ib star. For those of you unfamiliar with stellar spectral classification, such a designation identifies the star as a relatively low luminosity blue-white supergiant with a surface temperature of around 8200 kelvin. Research into the mass of the secondary star have provided estimates of anywhere between 3 and 30 $M_{\odot}$, though a middle value of around 13$M_{\odot}$ is the standard mass of a member of said spectral class so who knows![2]
As the two orbit the barycentre, they periodically occlude one another from our point of view – the fancy way of saying one passes over and blocks out some of the other’s light. This can be represented in general terms like so;

The graph along the bottom of the image shows apparent luminosity, which falls to its minimum when the dimmer star passes in front of the brighter.

SS 433 is a considerably more complicated and interesting system than this primitive illustration. It falls into the class of X-ray binary stars; these systems are unusually bright at X-ray wavelengths. This can only be explained by the system containing exceedingly hot material as X-rays are only emitted (in any large quantities) at such high temperatures. Given that the system contains a collapsar remnant (more likely a black hole than neutron star as we shall see later), and a large supergiant star we can account for the production of said X-rays via frictional heating.

The likely situation involves the original system being in the form of a detached binary; essentially each of the stars evolves unaffected by its partner. This situation would have remained unchanged even after the higher mass star went supernova, as evidence suggests, that in most cases at least, supernova detonations do not drastically affect the nature of binary star systems. However after the lower mass star aged it expanded, as it did so it filled its Roche Lobe – the area of space where its gravity exceeds the gravity of its partner.

After it did so material began to be sucked off its surface like water flowing from a drain pipe. This is pulled by the companion star into a flat disk around its equator forming an accretion disk. A simplistic way of describing this would be a CD-ROM. The gap in the middle contains the neutron star or black hole (and thus in reality there would not be a gap), material enters the disk at the outer edge and then spirals inwards on decaying orbits until it is sucked onto the central object’s equator from the inner edge.

This explains where the material comes from though does not explain how the material heats up enough to produce the X-rays. For that, we must look into the conservation of energy.
Material on the edge of the disk has to ‘fall down’ the gravitation potential well that the mass of the central object creates.
This is formed by the concentration of several $M_{\odot}$ into a small region of space, as such high concentrations of matter significantly distorts space-time forming a deep ‘well’ (indeed for a black hole, which has an infinite density, the walls of such wells are infinitely steep). A representation of such a potential well is below.

Credit: AllenMcC.

As material moves closer to the central object, it must lose the corresponding amount of gravitational energy. As energy cannot be created or destroyed (as every pupil in a secondary school science class will tell you) this must be converted to another form, in this case it is converted to kinetic energy, meaning that material accelerates as it approaches the inner edge (more properly explained as the conservation of angular momentum).

As the material is spiralling towards its doom, there are many internal collisions between the constituent molecules of the disk. Such collisions generate the heat that is required to generate the X-rays. However, it is only the inner regions of the disk that are at the millions of kelvin required to produce the detected level of X-rays. The intensity of thermal energy within the disk increases moving towards the inner collapsar; this is directly related to the number and ferocity of the collisions in this region of the disk.

As you move through the disk, particle concentration increases as equal numbers of particles are compacted into smaller regions of space. This combined with the increased velocity of each particle, creates a larger number of violent collisions, that through friction, generate the extreme temperatures required.
There is another area of the accretion disk where temperatures reach such ludicrous levels. All the material flowing from the secondary star enters the disk via a small region; this drastically increases friction within that area producing a hot spot.
A large amount of the X-ray emission of the whole object comes from this tiny region at the edge of the accretion disk.

The SS 433 system could (in a highly simplistic view of course) look something like the diagram below.

I should also note the colour of the secondary star is a little too blue as that would be more representative of a B class star compared to the true blue-white colour of A class stars.

The ESO has also produced a diagram of the system, that is slightly more ‘professional’ than my attempt:

Credit: ESO

I had previously mentioned that the dense body in SS 433 could be either a neutron star or black hole; though evidence suggests that it is indeed a black hole. I shall now have a brief look at this evidence.
Neutron stars occupy a tight mass band between an upper and lower limit. They can’t have less than around 1.4$M_{\odot}$ as that would be a mass below the Chandrasekhar limit and thus the star would have formed a White dwarf, as it had too little mass to overcome the resistance provided by electron degeneracy. However, they can’t exceed any more than (though the exact value is much less certain) 3$M_{\odot}$ as this would be greater than the Tolman–Oppenheimer–Volkoff limit and so neutron degeneracy would be overcome and they would collapse into a black hole. So it follows that if SS 433′s dense object is a neutron star it should weigh in less than 3 $M_{\odot}$ any heavier and it is more than likely that it’s a black hole. Up until recent times we had, no real way of determining what is the case though recent research may hold the answer.
Optical and Near I-R spectroscopy has identified an inner ring of material around the system that contains around 40$M_{\odot}$. Of this 16$M_{\odot}$ can be linked directly to the collapsar and its accretion disk. So either it’s a black hole with a reasonable mass and accretion disk or a neutron star with more than four times its own mass in orbit around it, I think we can see which is more likely.

Update: We can confirm that the mass of the collapsar is 16$M_{\odot}$ and so it is indeed a black hole

The Discovery of SS 433

SS 433 has been documented extensively since its discovery, with well over a hundred papers documenting all aspects of the system on record.

A study in 1978 identified an X-ray source – A1909+04 – within the confines of the supernova remnant W50. It was within the margin of error for the location of a radio source that had been detected within the nebula. The location of both sources was not exact as at that time radio and X-ray astronomy were still unable to precisely locate objects. As such, the X-ray and radio source could not be directly paired with a visible source. Eventually a spectroscopic study identified a visible ‘star’ with what, at the time appeared to be a standard spectrum of an X-ray source.

That star had been previously catalogued by Bruce Stephenson and Nicholas Sanduleak as number 433 in their list of H-Alpha emission sources – hence the name SS 433.

Detailed, longer term spectroscopic observation of SS 433 revealed that this was no ordinary object. The spectra showed that along with the normal spectral lines of hydrogen and helium there were other lines at seemingly random wavelengths. To make matters even more maddening these lines changed in wavelength between the observations; an exceedingly rare phenomenon. The explanation for these moving lines comes from the Doppler shifts experienced by rapidly moving material that is constantly changing direction.
The seemingly random lines are actually composed of two sets of lines. Each set moves in opposite directions before crossing over. After each set reaches a fixed, extreme wavelength, they reverse direction and move the other way, the total time for one complete cycle (with two crossings) is 164 days. This defies common sense though the explanation is deceptively simple.

These seemingly random lines are actually Doppler shifted hydrogen lines. One set is blueshifted and the other redshifted, which can be explained if we are seeing the jets of the Microquasar with one pointed towards us and the other away. The lines move as the black hole moves around in its orbit within the system and so the orientations of the jets alters relative to us over a period of time. We see the un-shifted spectrum as the central star isn’t moving with any great speed compared to us so the Doppler effect is minimal. Taken from the Doppler shifts of the jets we can conclude that jet material is moving at very high speeds, a good fraction the speed of light.

The Jets of SS 433
As mentioned SS 433 is a microquasar. As you can see they are quite similar to AGN in that they have an accretion disk and a pair of jets, but instead of a super massive black hole powering it, we have stellar mass black hole (though neutron stars are also possible power sources). Due to their similarity, they are very useful as a ‘close-by’ lab to see how the dynamics of AGN in far distant galaxies work, since microquasars follow the same principles.

The material that flows into the disk that isn’t sucked into the grips of the black hole is siphoned out of the system altogether as bipolar jets. As the gas spins round it, and the radiation produced as a result can escape through the gap between the black hole and the disk, since there’s less material in here for it to get caught up and swept round once again.

The jets themselves are forced out at 20 degrees near the black hole’s axis of rotation, beaming out at about 80 degrees away from our planet. The jets are moving at velocities of up to 0.26 c, all the while completing one rotation on its axis once every 164 days. To give you an idea as to just how fast 0.26 c is, the jet will cover one kilometre in just 13 microseconds!

The jets also fluctuate in brightness as the radiation they contain slams into material getting in the way, sending electrons flying out of their atoms and creating yet more ionizing radiation.

Another interesting effect is time dilation. As the jets are moving at 26% percent the speed of light, it’s sufficient enough for normally negligible effects to become problematic if you don’t take them into account in your calculations. If you take the redshift of the system at face value, it’s travelling away from us at 12,000 km a second when really this isn’t the case; since if it was it’s just exceeded (by quite a few times) the escape velocity of the Milky Way– 450 km/s – and as such would be rocketing out of the galaxy rather than inhabiting its stable orbit of the galactic core.

The faster the photons (that is the guise light takes when it behaves as a stream of particles) moving in the jet vibrate the more squashed their wavelengths of light they emit will be, conversely the slower the atoms vibrate the more stretched out wavelengths of radiation will be. Now in the SS 433 system you’d imagine that the system, according to its redshift, would be travelling at a velocity that matches other objects orbiting in the Milky Way and not 12,000 km per second, and in fact it is, but time dilation skews this:

Let’s imagine Hannah and Peter each had an atomic clock. Peter stays behind on earth with his, and Hannah leaves earth with hers and reaches a relativistic velocity. Now if Peter could see her clock he’d see time was moving significantly slower than his own as well as Hannah herself behaving more slowly that usual. Though from her point of view, Hannah experiences the ticking of time just as it always has been and would see Peter and his clock behaving in an accelerated way.

In our jet the atoms are vibrating quickly away just as they should, but from our point of view they are moving much slower; just as Peter could see Hannah’s clock ticking by slower than his own. Because the light is stretched out more than it should be the redshift is also higher than it is, causing us to measure, the velocity as much faster than what it is in reality.

Observations of SS 433 and W50

The surrounding nebula (W50) as mentioned before, was produced by the supernova detonation of the primary star 10,000 years ago from our point of view (28,000 years ago taking light lag into account). The nebula has long since faded from visible wavelengths but the dust it released is still visible in the infrared. NASA’s Spitzer Space Telescope has taken several observations of the area at various wavelengths. This first image shows some of the detail within the nebula, with SS 433 being the bright source in the centre. This short wavelength allows us to gaze at the dust within the nebula with a reasonably high resolution, though nowhere near the detail we can observe structures in the optical with instruments like Hubble.

Credit: NASA, Spitzer Calibration: Peter Clark

This second image is at a much longer wavelength (70 microns is equivalent to 700,000 ångströms which is also roughly the same as the thickness of a human hair) which is able to penetrate all but the densest clumps of dust surrounding the system, with a similar shape of clumps being visible in both images. Unfortunately, at this wavelength the fine structure of the nebula has been lost from view.
Note that the scale of the two images is not the same with the second image being more zoomed onto SS 433.

Credit: NASA, Spitzer Calibration: Peter Clark

A colour coded VLA radio map reds and yellows show the highest levels of radio detection with greens and blues showing less powerful areas of emission; SS 433′s radio jets are also clearly visible.

Credit: Blundell & Bowler NRAOAUINSF

A greyscale version of the above image with the calculated path of particles emitted at varying speeds at ten-day intervals. Beads of the same colour have been ejected at the same speed. The main point of this image is to show that the emission from the central object is not constant and varies over short time scales.

Credit: Blundell & Bowler, NRAO, AUI, NSF

This is a compilation of several VLA radio images taken over a consecutive period of time. It shows clearly the uneven concentration of material being emitted by the object, even over a short space of time.
Quasars and radio galaxies are predicted to show similar variation but over much longer time scales, much longer than a human lifetime and so such variation has yet to be observed. Therefore, in a way SS 433 allows us to peer into the heart of Quasars and learn about their structure in a way that isn’t currently possible for the full scale versions.

This two panelled image shows X-ray and radio observations of SS 433 and W50 in two different ways.
The first panel shows in detail the radio emission of the region with both jets visible though the eastern jet is much more pronounced. The overlaid contours map the regions of X-ray emission that more clearly define the jets.
The second panel is the reverse, with radio contours overlaid on to the X-ray data. The jets are strikingly visible with the image also highlighting just how powerful an X-ray source SS 433 is. The images also show the distortion of W50 by the jets of material emitted by SS 433 further showing their power.

Credit: Goodall, P.T.; Alouani-Bibi, F.; Blundell, K.M., Mon.Not. Roy. Ast. Soc. 414 (4) pp. 2838-2859.

The Sky at Einstein’s Feet by Professor William Keel (Chapter 2 contains lots of information on SS 433).

This truly stunning image of the Eastern Veil SNR was released at the very end of 2010 by the Issac Newton Group of Telescopes.

NGC 6992 Credit: A. Oscoz, D. López, P. Rodríguez-Gil and L. Chinarro

The nebula is located approximately 1470 light years from Earth and was produced by a detonating star that died between 5000 and 8000 years ago.
The nebula is the visible portion of the much larger Cygnus Loop and is divided into several arcs, with the image above showing part of the eastern section. Since it’s formation the remnant has expanded to a size that makes it appear to have a diameter around 6 times that of the full moon, or 36 times it’s area when viewed in the night sky. This translates to roughly 50 light years in physical diameter.

The loop is one of the brightest features in the X-ray skyscape as viewed from Earth. The nebula contains large quantities of hydrogen, sulphur and doubly ionised oxygen (OIII) each of which have been picked up in the filters used by the Newton Telescopes. They are displayed in the image as red, blue and green respectively.

The classification name given to this section is NGC 6992 of the nebula, and the Eastern section is also happens the brightest region of the loop.
The nebula was first observed by William Hershel in September 1784.
As the nebula is part of the Cygnus loop it can be viewed in the constellation Cygnus and is most spectacular when viewed through an OIII filter.

The Western Veil Nebula Credit: Nick Howes

There are various classes of stars from blue supergiants to black dwarfs. This guide is a quick run through of each of the main types.

For this guide to make sense some simple aspects of stellar formation must be understood: -

All stars form when a stellar nebula of dust and gas undergoes gravitational collapse. This collapse causes the nebula’s core to increase in density and temperature. It forms a hot dense ball of gas at is centre called a protostar. This protostar begins to shine as it heats up; it is in a sense a baby star. If the nebula has enough mass to continue the gravitational collapse then temperature in the protostar’s core reaches a critical point;  at 10 million kelvin (about 10 million 273 degrees Celsius) nuclear fusion begins in the protostar’s core. Once this occurs the protostar has now become a fully-fledged star.

The Birth of a Protostar Credit: NASA/JPL-Caltech/R. Hurt (SSC)

Larger nebulae produce larger, hotter stars. Oddly the hotter the star the shorter its lifespan. The smallest, coolest stars can exist happily for as much as 1000 times longer than a medium sized and temperature star like the Sun which in turn can last for as long as 10 billion years! The most massive stars burn for only around 10 million years.

This relationship between mass, temperature and lifespan can be explained with a single fact: – A more massive star has a higher temperature in its core so it fuses hydrogen faster than a lower mass star to an extent that it depletes its store of hydrogen faster, even through it had a lot more to begin with – So the relative rate of stellar fuel depletion increases with increasing mass.

Now that we have touched on some of the basics of stars, lets have a look at some of the varieties that litter the universe.

Brown Dwarfs – Technically not true stars. A brown dwarf forms in the same way as a normal star as described above but it forms from a nebula with less than 8% of a solar mass ( 1 Solar Mass = the mass of our sun) and so temperatures at the core of the nebulas protostar never reach high enough temperatures to initiate nuclear fusion.

Red Dwarfs – The smallest and coolest type of main sequence star. They form from small stellar nebulas and contain around half the mass of the sun. They unlike brown dwarfs do have the temperature in their cores to initiate and maintain nuclear fusion. They are the dimmest and the longest lived type of stars burning for many billions of years. The closest star to Earth (not including the Sun of course) – Alpha Proxima Centauri – is a Red dwarf. These are the most common type of star, indeed 21 of the 31 nearest stars to the Sun are red dwarfs. Despite their abundance they are exceptionally dim and cannot be viewed with the naked eye.

Yellow Dwarfs - These are medium size and temperature stars. An example of a Yellow dwarf is our sun. The term yellow dwarf is actually a misnomer as they can range anywhere from white to yellow.

Red Giants – When a medium sized main sequence dwarf (0.5 – 10) fuses the last of its hydrogen, its core begins to collapse. This collapse causes the star’s core to heat up and it begins to fuse helium, this causes the star to expand. Its heat is now spread out over a much larger surface area and this results in cooler surface temperatures. Once lower mass red giants fuse the last of their helium into carbon, nuclear fusion stops and the star’s outer layers are released into space forming a plaentary nebula with a white dwarf at its centre.

Red Supergiants – When the most massive stars deplete their supply of hydrogen they, like smaller stars, begin to fuse helium into carbon. However unlike smaller stars this process does not stop with the fusion of helium; the star continues to fuse elements to form heavier and heavier ones. This causes the progressive expansion of stars into supergiants dwarfing even red giants, a supergiant can be as much as 7AU in diameter! (1 AU = 1 Astronomical Unit = the average distance between the sun and Earth) As they continue to fuse elements they take on a layered structure with the lighter elements closer to the surface. The process by which stars release energy via nuclear fusion (stellar nucleosynthesis) can continue until iron forms the majority of a star’s core. Iron does not release energy when it is fused, instead  it absorbs energy. This causes the nuclear fusion reactions in the core to stop, this in turn causes the star to rapidly contract under the force of its own gravity. This then heats the star back up again, very rapidly, the result of this process depends on the stars original mass. Most red supergiants heat up to the extent that the explosive force of the contraction ‘beats’ the compressing force of the star’s gravity and it explodes in a massive detonation - a Type II supernova. The most massive of stars are unable to overcome the force of their gravity and continue to collapse until the entire mass (many times the mass of the sun) is concentrated into a tiny area (compared to it mass for example a typical stellar black hole containing around 30 solar masses would have an event horizon (the ‘hole’) of around 30 km), this gives it an incredibly strong gravitational field from which not even light can escape.

The Crab Nebula - The Aftermath of a Supernova Credit: NASA

Neutron Stars – After a supernova dissipates only the star’s core remains, it about the size of Earth and is composed mostly of neutrons (giving the star its name). Some neutron stars emit beams of electromagnetic radiation as they rotate rapidly (several times a second) giving them their nick name,  – pulsars.  Other neutron stars have incredibly strong magnetic fields giving them the name magnetars.

White dwarfs – When a red giant star finally stops releasing energy through nuclear fusion, it slowly loses its outer layers into space. This leaves its core as the last remnant of the star – a white dwarf. A white dwarf is around the size of Earth and glows due to its latent heat, however as all nuclear fusion has stopped in its core and it is no longer producing heat it slowly cools and fades.

Black dwarfs – This is the final stage in the life of an average mass star. After a white dwarf has cooled and is no longer emitting light all that remains is a small, cold dense ball of matter that emits no light – this is a black dwarf. White dwarfs cool so slowly that there should be no black dwarfs currently in existence - the Universe is not old enough for a white dwarf to have cooled sufficiently.

I hope this guide has helped you to understand the basics of stellar categorisation however please bear in mind this is only a basic guide and does not include all subtypes or all the fact about the general classes however it does provide the basics. Once again I hope this has been useful.

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White dwarfs are often thought of as the quiet remains of stars that quietly cool over the aeons not troubling anyone except in the most extreme circumstances (See my post on Type Ia Supernovae for more details), though recent research indicates that many may be timebombs slowly ticking down to destruction.

An Artist's Impression of a Type Ia Supernova Credit: David A. Aguilar (CfA)

White dwarfs have a generally accepted upper mass limit of about 1.4 (termed the Chandrasekhar limit), any such star that exceeds this upper value will be unable to prevent itself undergoing gravitational collapse this creates a runaway nuclear fusion reaction within the star which promptly tears itself apart as a Type Ia Supernova. Two main theories have been put forward to create the conditions required for such an event to occur.

1. Two sub Chandrasekhar limit White dwarfs within a binary star system merge forming a single super Chandrasekhar limit White dwarf that immediately goes supernova.
2. A single White dwarf can slowly accretes mass from a non white dwarf binary partner which eventually pushes the White dwarf over the critical mass initiating the supernova.
The preferred explanation for most such events is the slow accretion scenario as stellar merger events are known to be extremely rare.  Though this scenario comes with its own set of problems, specifically proving beyond reasonable doubt that it is indeed the true picture of what is occurring.
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Observationally there should be certain signs that the explosion has been produced in this way.  The area around the supernova should contain traces of hydrogen and helium from material that the White dwarf hadn’t managed to gobble up and from its disturbed partner that would have undergone mass loss as a result of the supernova. As of yet no such feature as ever been detected, to make matters more interesting still the partner of a white dwarf that has gone supernova has never been detected even long after the fires of the explosion have faded.
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A team of researchers working on the problem think they have found an explanation for this troubling lack of supporting observations.
As a White dwarf accretes material from its binary partner it gains two ‘things’. The most obvious is of course the mass of the material being accreted, the second is perhaps less well known – angular momentum. Angular momentum is the product of an object’s angular velocity (the speed at which an object is rotating) and its moment of inertia (its resistance to changes in the rate of rotation), and line its counterpart must be conserved if no external forces are acting.
As the material being accreted is rotating around the White dwarf at considerable speeds it has a correspondingly large amount of angular momentum. As this must be conserved when the material eventually ‘falls’ onto the White dwarf this momentum is transferred to the dwarf which responds by increasing its rotation rate – like an ice skater spinning faster as they pull in their arms.
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Faster rotation rates can help to support the dwarf against the crushing force of gravity, as the rotational motion acts to hold the core up preventing the core from reaching the critical density required to initiate the catastrophic fusion reaction which would trigger the supernova.
Thus, White dwarfs can exceed the Chandrashekar limit provided they are rotating fast enough.
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Now we arrive at the most interesting point of this research.
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As White dwarfs slowly ‘spin down’ (their rotation speed slowly decreases over time) meaning that for a White dwarf that has accreted enough matter to exceed the Chandrashekar limit, but are rotating fast enough to stabilise themselves are literal time bombs ready to detonate.
Depending on their mass, initial rotation speeds and rate of rotational slow down, a super-Chandrashekar limit White dwarf may remain stable for up to a billion years after it ceases accreating matter from its companion. This allows the companion to evolve in its own right potentially into another hard to detect White dwarf (explaining why no companion stars have yet been identified) and is more than ample for any surrounding gas and dust from the accretion to dissipate explaining why the expected hydrogen and helium outer shell has been detected, they may not be there to detect!
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That means that there may me millions, if not billions of White dwarfs out there that are slowly heading towards their destruction. Perhaps as you are reading this, somewhere out in the depths of the universe, the time has come for one such White dwarf which has just been torn apart in a final blaze of light and radiation. Perhaps one day the light from that explosion will reach Earth and be observed by astronomers completing a story that has spanned eons.
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