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Supernova

Throughout the 1800’s and into the early part of this century, galaxies had been within the range of telescopes, but no distinction could be made between them and nearby extended objects like M27.  One view that seemed quite reasonable at the time was that the Milky Way is the universe, and that those spiral patches of light were within the galaxy.  The competing view was that the spiral nebulae were themselves island universes far removed from our own.  As convincing as it was wrong, the argument that everything we could see with our telescopes was contained within the Milky Way was supported by assumptions about the supernova events that were occasionally observed in these ‘spiral nebulae’.  There is no way, it was thought, that these objects could be other distant galaxies like the Milky Way because it was impossible that a supernova could be so bright as to appear clearly at such distances.  This was one of the principle arguments used by the well-known astronomer Harlow Shapley in a famous 1920 debate with Herber Curtis.  Harlow Shapley was remarkably prescient on many of the cutting edge issues astronomical issues of the day, but he missed the boat on this one.

And why not?  Prior to 1945 the only explosive forces we were certain of were chemical, as in dynamite and flammable gases.  Sitting in a quiet café on a tree-lined street with the birds singing, who would reasonably assume as a likely possibility, a single instantaneous explosion with the energy of a billion suns?  But as we later discovered, without a doubt such explosions do occur.

Before much was known about their true nature, supernovae were classified on a phenomenological basis according to their spectra; Type Ia that display no helium absorption lines, Type Ib and Ic which have no hydrogen lines and Type II which have both.  We now know that these spectral  differences are an indication of quite different physical mechanisms.  Glossing over quite a lot of significant details, we can still say generally that a star will end up its lives in one of 2 ways.  If more than 8 times the mass of the sun it will become a Type II supernova leaving behind a neutron star, or if even more massive, the remains can collapse directly to a black hole. A Star with less than 8 times the mass of the sun becomes a white dwarf.  These white dwarfs may become Type I supernovae if they find a way to accrete more mass.  It is usually assumed that this sometimes occurs when they are coupled with another star in a binary system.

Type Ia thermonuclear supernovae are the result of a white dwarf star pulling (accreting) matter from the outer atmosphere of a companion star.[2]  The sequence of events leading to the supernova begins with a pair of ordinary stars that are tied together gravitationally in a binary system.  The more massive of the pair runs through its life cycle more quickly and expels its atmosphere in a planetary nebula (for example like NGC 1514 on page 12.)  After devolving into a carbon-oxygen white dwarf, it settles into an orbit with the other normal star (still on the Main Sequence.)  In its turn, the second star reaches the end of its life and begins to expel its atmosphere.  At this point the white dwarf will now pull this expelled atmosphere onto its surface.  When this happens, a peculiar effect of the quantum mechanical conditions is that as the dwarf gains mass, it shrinks in diameter. The dwarf’s carbon core has no fusion reactions taking place and is supported only by degenerate electron forces.  Under normal conditions electrons are free to find an energy state that corresponds to the temperature of the gas (The ‘equation of state’: Temperature = Pressure x Volume.)  Under gravitational pressure, a white dwarf’s electrons are forced to fill all the lower energy levels and the equation of state becomes soft.  The surface of the star tries to compress but is resisted by degenerate electrons trying to get themselves to a higher energy level.  The mass of the white dwarf can continue to build only until it reaches 1.44 solar masses (the Chandrasekhar limit) and at that point, a spontaneous and instantaneous carbon fusion reaction begins in the core[3].  The resulting supernova explosion has an energy output of 10 billion suns.  Type IA supernova are the brightest of the supernova events by a factor of about 6.

Type II, Ib, and Ic supernovas are quite different animals and occur when a star of 8 or more solar masses exhausts the fuel in its core and collapses.  In the early stages of evolution the star fuses a portion of it’s hydrogen into helium, then begin to fuse that helium into carbon and oxygen, then the carbon and oxygen into neon and magnesium and so on until it has a core comprised mainly of iron.  Even the fabulous temperatures and pressures at the core of a collapsing star are not sufficient to initiate the fusion of iron.  When sufficient iron accumulates in the core there comes a point at which radiation pressure from the remaining fusible elements no longer generate enough energy to support the atmosphere and a sudden catastrophic collapse occurs.  The immediate cause of the collapse is the draining of the core’s thermal energy by a process known as photodisintegration.  g + ⁵⁶Fe -->   13⁴He + 4N   Gamma ray photons combine with iron to form helium and neutrons in a fission reaction which absorbs heat.  Energy ceases to flow outward from the core and begins to flow inward.  Combined with the gravitational acceleration of the failing star the energy flow reversal causes a catastrophic core collapse taking place in only 1/10 of a second as electrons are forced into the nucleus.  That is a global collapse from a volume the size of the earth to one less than the size of the city of San Francisco.  This collapse, concomitant with the extinction of the supporting fusion reactions, pulls the rug out from under the trillions times trillions of tons of matter in the outer shells and they come smashing inwards towards the new neutron core at some 70,000 kilometers per second. 

The mechanism for the final collapse in the core is the failure of the quantum mechanical repulsive forces in the atomic nuclei. Unrestrained by the previously high temperatures and the concomitant radiation pressure from the fusion reactions, the huge gravitational forces blow the atoms of the core right through electron degeneracy support, electrons are forced into the nucleus, and a neutron star is created.  In this process a huge burst of neutrinos occurs carrying away with them some 10⁴⁶ joules of energy.  Ordinarily, neutrinos are elusive creatures capable of passing entirely through the earth without interacting with anything.  In this setting however, the neutrino flux is so high and the incoming atmosphere so dense that about 1% or 10⁴⁴ joules is reinvested.  This outflow of neutrinos collides with the incoming outer shells initiating nearly instantaneous runaway fusion reactions that are the supernova explosion.  The following explosion of kinetic energy and electromagnetic radiation, which is one of the more spectacular events in the universe, is actually only 1% of the total energy produced by the supernova.  The energy released by these fusion reactions is equivalent to the lifetime output of more than 20 billion suns and is bright enough to outshine an entire galaxy for a brief moment. It is at this short moment that all the elements heavier than iron are created and scattered throughout nearby space by the explosion.

The remnant of the explosion is a neutron star core with a mass of between 1.4 and 3 solar masses.  If the mass exceeds 3 solar masses, the neutron degenerate support is insufficient to stop further compression and the core collapses to a black hole – (a point with no size and the mass of the core) from which no light (or anything else) can escape because of the intense gravitational fields.  It is useful to think about the size of black holes in terms of the ‘event horizon’, the distance at which some light is able to escape the gravitational field.  Inside the event horizon, there are not going to be any direct observations made and speculation as to ‘size’ and makeup of the interior is going to come from mathematical models and the indirect evidence obtained by looking at how the surrounding material behaves.

There is some advanced notice of supernovae.  A barrages of neutrinos escape the event hours before any other electromagnetic radiation is able to make its way out.  This is because electromagnetic photons work their way out by being repeatedly absorbed and reemitted while the neutrinos pass through largely uninhibited.  Neutrinos from SN1987a were detected 3 hours before the light reached us.

For many the weeks after the initial explosion the spectrum of a type II supernova is dominated by 2 components of the outer shell, oxygen and calcium.  Later, as these layers expand and thin, light from the inner layers begins to dominate, particularly that of Nickel₅₆ which is created by fusion and then decays back to iron.

Back in the core at the time of the collapse, electrons combine charges with protons to form neutrons, particles with the mass of a proton but no electrical charge.  It is in the atmosphere that the heavier elements are formed.  If this collapse had taken place in the more sedate time frame of the star’s prior evolution, the universe would be quite different than it is and results would be far less spectacular.  Other than hydrogen, which is primordial, all the elements in the world around us were created in stars by the fusion of lighter elements into heavier. Every element heavier than iron absorbs energy when it is fused from lighter elements and in nature and only a supernova explosion can provide the excess energy required to manufacture them.

I am a part of the sea and stars      

And of the winds of the South and North

Of mountains and Moon and Mars

And the ages sent me forth.                         -Edward HS Terry

As a point of fact, the average age of the individual atoms making up your body right now is about 4 billion years.

Type II supernovae end up with a rotating neutron star at center.  In the case of M1 its neutron star has a mass equal to the sun and is compressed into a sphere 12 miles in diameter.  If you put a spoonful of this stuff into your morning coffee, you would be stirring in 500 million tons.  Only about 1,000 neutron stars have been observed to date, but current estimates are that there are at least 100 million in our galaxy

What is the likelihood that a nearby supernova has or could inflict damage on the living organisms on the earth?  Well, there have been several mass extinctions of life for which there is no cause yet nailed down.  Ionizing radiation from nearby supernovas is one of the prime suspects.  These extinctions have occurred on a timescale of hundreds of millions of years while the direct evidence of a supernova fades away on the timescale of tens of thousands of years, making the assignment of blame to a supernova event difficult.  We do know that the frequency that we can expect to have nearby supernovas is inadequate to explain more than one of the 5 major extinctions of the last 600 million years.  A prime suspect is the extinction that occurred at the Pliocene/Pleistocene boundary because at the time there was a large dense cloud moving by with many massive short lived stars embedded in it.  This cloud is now 450 light years away in the direction of Scorpius and Centaurus.

In the long term we are not completely safe even with the massive stars because they evolve much more quickly than lighter ones.  A star’s lifespan is proportional to 1/M^2.5  where M is its mass.  This means that if the mass of a star doubles, its life expectancy declines by 3x.  All the stars ever created with a solar mass of less than .8 solar masses are still in existence.  A 20x solar mass star will burn out its fuel in only 10 million years and become a supernova.  If a massive star were to be created in some nearby star forming region it would hopefully not be within 200 light years.   Currently, the closest such region is the Orion Nebula at 1,470 light years.

Photo –  Lick Observatory 3 meter telescope  

With a visual magnitude of –1.4, Sirius is the brightest star in the sky.  At a distance of 8.6 light years it is quite close.  This is a binary system, one component being a white dwarf.  Sirius B is in fact the closest white dwarf to the earth.  It is however, certain that there is too much separation between the 2 stars for Sirius B to be able pull sufficient material from Sirius A to reach the mass required to go supernova.  In any case, nothing will happen until Sirius A exhausts its core hydrogen and becomes a red giant in about 1 billion years.  

More interesting is the binary system HR 8210 at the right.  It is located  ~150 light years away and consists of IK Pegasi A, a magnitude 6.1, 1.7 solar mass star and IK Pegasi B, its white dwarf companion at a magnitude of 10.4[5].  Unlike Sirius B, Pegasus B is quite close to Pegasus A.  As if that weren’t enough, the larger star is in the late stages of its life.  At some point between now and a few hundred million years from now it will begin to pulse, ejecting its outer atmosphere as a nebula.     

 Image ©Mark A. Garlick.  "May not be used for any reason without permission"    http://www.space-art.co.uk/ 

K Pegasi B is already 1.17 solar masses, not so far below the 1.44 solar mass threshold where it will go supernova[6].  If the dynamics of accretion are just so and Peg B does manage to accumulate sufficient mass from IK Peg A, there will be some real excitement in this neighborhood of the galaxy.  If it were to happen now, the earth will probably come out ok.  Closer, and gamma radiation from the blast could destroy the ozone layer and the earth, in that eventuality, would be bombarded with ultraviolet radiation from the sun.  In 50-100 million years, we will without doubt be ok, because the HR8210 system is moving rapidly away from us and by then will be too distant to do damage.

Another (and much more common) case of this type of system occurs when the white dwarf is more massive than it’s companion.  Here the system becomes what is known as a cataclysmic variable.  This happens when the escaping atmosphere of the surviving star forms an accretion disk around the white dwarf and spectacular displays follow from this; novae, dwarf novae, superhumps. 

But this is not going to happen to IK Pegasi.  The A star is more massive than the white dwarf B star.  When Ik Peg A reaches the end of its life and begins to expel its atmosphere, the mass falling on the dwarf will be transferred further away from the common center of gravity of the system.  Angular momentum will be conserved in the system and the distance between the stars will necessarily shrink a bit.  Then as the orbit is compressed, more mass is transferred.  Which in turn causes the orbit to compress again.  A positive reinforcing cycle continues and quickly becomes unstable, so that the entire atmosphere of the A star can be dumped onto the B component in as little as a few years.  By the timescale of the universe this is breathtakingly fast.  And when the mass of the White Dwarf reaches 1.44 solar masses electron degeneracy can no longer support the star and, voila.

In terms of the initial explosion, two hundred light years separation is generally considered to be the distance we need to be comfortable.  The critical distance is both variable and debatable.  It is debatable because there are large uncertainties and no precedents to guide.  It varies with the type of supernova. Type I are more powerful than Type II.  But type I are known to be asymmetrical.  If the supernova is a Type I the impact here will depend upon whether or not we are shielded from the supernova by its larger companion.  HR 8210 has an orbital period of 21 days with the Earth being somewhere near the plane of the orbit, so that the white dwarf is eclipsed by the larger star on very short timescales.  At opposition, the cone defined by the shielding of the A star subtends a solid angle of about 35⁰. 

The density (flux) of the gamma radiation depends on the distance from the supernova simply because of geometry.  As the sphere that is the shock front of the explosion moves away from its source, the surface area grows by  4 P R² and the density of anything on that surface decreases by the square of the radius.  This is why 200 light years is calculated to be a comfortable distance in terms of gamma radiation.

 Recently, evidence has been found of a supernova that exploded some 41,000 years ago at approximately 250 light years from earth.  I mentioned above that elements created in star the goes supernova are “scattered throughout the nearby space”.  ‘Scattered’ is neutral term that doesn’t do justice to what appears to have happened when this star exploded 41,000 years ago.  There was an initial shock wave of gamma radiation and neutrinos that left no mark that we are able to find.  However, 7,000 years after the initial explosion, 34,000 years ago, material debris traveling at 10,000 kilometers a second (6,220 miles a second) would have arrived.  And in fact, mammoth tusks have been found which are peppered with small iron rich pellets.  The depths of the impacts are consistent with grains traveling at 10,000 km/sec and are located in sites dated at 34,000 years ago.

 Finally, 13,000 years ago, 34,000 years after the supernova, a 6 mile diameter, low density,  asteroid with a comet-like trajectory hit the earth.  Samples of sediment from around the world confirm that the materials in the asteroid were consistent with the elements and isotopes produced in a supernova explosion.[7]  The date of impact corresponds with the date of the Holocene Extinction; .a worldwide extinction of the large mammals.

 It appears that there is much more to worry about than the initial explosion and that even 250 light years is not a safe distance. 

Relativity and Supernovas

In 1905 Albert Einstein published the theory of special relativity.  ‘Special’ because is applied only to the case of unaccelerated inertial frames.[8]    It took him another 13 years to finish the work on accelerated inertial systems and his General Theory of Relativity was published in 1916.

As noted above, type IA supernovae are remarkably uniform events.  They produce light curves (a plot of the supernova's brightness as the days go by) that begin at around 90% of the maximum magnitude (intrinsic magnitude –19.8), rise to the maximum in about 15 days, and then fall off over the next 40 days to about 80% of the maximum.  In what is probably the most dramatic illustration of relativistic time dilation, the duration of these events are stretched out when they occur in galaxies that are receding from us at high velocities.  In other words, the faster the supernova is moving away from us, the slower we perceive its clock to be running, so that we might measure its maximum to occur after 20 days rather than 15.  When the effects of time dilation are corrected for in the measured light curves, the curves of all supernova events fall right on top of each other.

copyright 2006  Ben Davies

[1] Because Type I supernova all explode at exactly the same mass and that the quantum mechanical processes are the same, they should all be the same intrinsic brightness.  Because of this they make very good standard candles for determining distances, just as Cepheid variables do.  And because supernovae are so bright, they are useable out to great distances and have so far been calibrated at a redshift of .85, or 5 billion light years.  

[2]Or possibly the collision of two white dwarfs.  The exact mechanism is still up for grabs.

[3]  If the dwarf is spinning rapidly, the mass can exceed 1.44 solar.  Tidal locking of the orbits obviously inhibits this, but it can happen.

[5] Sort of.  Like all B and better stars, the output approximates a blackbody and it's peak frequency is at 820 Angstroms, a wavelength at which the atmosphere is opaque.  If you want to ‘see’ the full 10.4 magnitudes, you will need a satellite and an ultraviolet telescope such as the EUVE..   

[6]The supernova would have an absolute magnitude of –19.33 and because of its distance will appear as a –16 magnitude star.  That is 40 times brighter than the full moon.