Armagh Observatory: School Student Essays
Supernovae
Luke Haughian, Brownlow College, Portadown, 11 Oct 2002
Introduction
One of the most energetic explosive events in space is known as a supernova. These occur at the end of a star's lifetime, when its nuclear fuel is exhausted and it is no longer supported by the release of nuclear energy.The story of the supernova begins with a stars long struggle to survive against collapse by gravity. In order to not contract due to gravity the star has to produce enormous amounts of internal pressure, generated from nuclear fusion. Nuclear fusion is the energy-producing process, which takes place continuously in the sun and stars. Hydrogen is converted to Helium in the core of the sun (temperature 10-15 million degrees Celsius), providing enough energy to sustain life on earth.
This process of hydrostatic equilibrium acts almost like a diesel engine. As gravity tries to crush the star, it acts like a descending piston, forcing energy into the gas which raises the temperature of the star, thermonuclear fusion reactions occur when the intense heat and gravitational force in a stars nucleus force hydrogen atoms together. The atoms merge, or fuse together, creating helium atoms and releasing large amounts of energy in the form of electromagnetic radiation and heat. The difference between the two being energy:
E = mc2 (General theory of relativity, where c is the speed of light)
Within most stars this extra burning of hydrogen causes the 'descending piston' to turn and work in the opposite direction, so as the gas expands the star cools (or becomes smaller). However when the fuel runs out, the star will die, often in a grand explosion)
Types of supernovae
There are two main types:Type I supernovae: The light curves exhibit a sharp maximum (reaching about 10 billion solar luminosity) and die off gradually. They are formed from a white dwarf which accumulates mass to exceed the maximum mass the electron degeneracy pressure can support (1.4 solar masses).
The mass (1.4 M¤ solar masses) is called Chandrasekhar Limit:
M = 5.87 / u2 M¤
u is the mean number of nucleons per electron. For iron this would be 56/26 (as it has the highest binding fraction) so that M = 1.26M¤
A more modern value however, is 1.4M¤ .
Type II supernovae: Have a broader peak at maximum (emitting about 1 billion solar luminosity's) and die away more rapidly. They are formed from the collapse of the iron core of a dying star. Also, it will dim irregularly after the explosion.
Basically, If gravity wins you get a Type Il supernova, if fusion wins you get a Type I.
The energy released by the two types of supernova is the same.
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The deaths of stars
Many supernovae are remembered for the amazing art, which they produce when they die. But do they produce it when they die? In actual fact stars create their most artistic display as they die. This is true for low mass stars like the sun, they transform themselves into a white dwarf star by getting rid of their gaseous outer shell. This expended gas forms the impressive display called a planetary nebula
Low Mass stars
If the star is round about the same size of the sun or below the Chandrasekhar limit it will have a different effect than those bigger than the sun. These stars can never get hot enough to allow carbon to begin to fuse. The thermonuclear process at the stars core is complete. The outward pressure produced by the reactions can no longer balance the inward gravitational attraction between atoms. Resulting in the core collapsing under its own weight. As it does so, the star implodes, transforming gravitational energy into kinetic energy, or energy of motion and becomes a red giant. The core of the star collapses in on itself, but as it does so, it transfers kinetic energy to the stars atmosphere. This sends the atmosphere expanding outward from the stars core or the high temperature central regions drive the outer half of the star away in a brisk stellar wind, lasting a few thousand years. The particles of the stars atmosphere begin moving rapidly away from the star, tearing apart the stars atmosphere, the star starts to peel off like an onion. When the process is complete, the remaining core remnant is uncovered and heats the distant gases and causes them to glow thus creating a planetary nebula. Finally the core will cool into a white dwarf star and then into a black dwarf.
Massive stars
For stars which are bigger than the sun they start to fuse helium and carbon together due to the lose of hydrogen in the core. After the helium is gone, their mass is still enough to fuse carbon into heavier elements oxygen, neon, sulfur etc. Once the core has been changed into iron it can no longer burn, the star collapses under its own gravity and begins to heat up. Inside the core protons and neutrons are so tightly packed that they merge together to form neutrons, this then causes the iron core to shrink to a neutron core within the space of a second. The outer layers of the star now fall inward upon the core causing it to be crushed further, naturally the core heats up (to billions of degrees) and eventually explodes (a supernova). The remains of the star can either form a neutron star or a black hole (depending upon the mass of the star).Even a few days after the supernova has occurred, it is still too hot and dense to allow any insight into what happens after it occurs. Once it starts to clear astronomers are able to look much more closely at the supernova. One of the things, which they found was that most of the light, doesn't actually come from the heat of the combustion from the supernova itself. When you see a supernova, you aren't seeing the energy of the explosion itself. The energy of the explosion is dissipated almost immediately. The substance that powers the supernova so that it can temporarily outshine the hundred billion other stars in its galaxy is nickel. It is just like pocket change, but it is radioactive, and it transforms into radioactive cobalt in a just a few days! This happens because the nickel, called nickel-56, has extra neutrons. These neutrons are unstable and can decay. In the case of nickel-56, the decay occurs in the form of electron capture, so that a neutron absorbs an electron to become a proton, simultaneously releasing a neutrino. With an extra proton, what was nickel-56 is now cobalt-56, but it is still radioactive. By the time the burning wave has flickered out about 40% of the solar mass has been converted into cobalt -56. The glowing cobalt radiates away its own essence until it is turned into iron by the same process over a few months. These continuous decays give off copious photons. Initially, these photons are trapped in the center of the expanding blanket of gas above them. As the supernova expands and cools, more and more radiation is able to leak out. After about 20 days from the explosion, the light output from a supernova reaches a peak (called maximum light) and starts to decline. Brighter supernovae are brighter because they have more nickel. As a result, they are also hotter and are more opaque. It takes longer for a photon to find its way out of a hotter supernova. Able to trap in radiation for a longer period of time, their brightness declines more slowly.
Nuclear synthesis states that elements above iron in the periodic table cannot be formed in the normal nuclear fusion processes in stars. Up to iron, fusion yields energy and thus can proceed. But since iron is at the peak of the binding energy curve, fusion of elements above iron dramatically absorbs energy. So to produce heavier elements, enormous amounts of energy are needed. Current opinion is that they must be formed in supernovae. In the supernova explosion, a large flux of energetic neutrons is produced and nuclei bombarded by these neutrons build up mass one unit at a time to produce the heavy nuclei. With large neutron excesses, these nuclei would simply disintegrate into smaller nuclei again were it not for the large flux of neutrinos which make possible the conversion of neutrons to protons via the weak interaction in the nuclei. The layers containing the heavy elements are be blown off by the supernova explosion, and provide the raw material of heavy elements in the distant hydrogen clouds which condense to form new stars.
Supernovae on white dwarfs
The explanation of the explosion of a white dwarf star with a runaway fusion reaction in its core is almost impossible to find, as there are no certainties. But however there is a starting point, mathematical combustion theory shows that there are only two types of explosions. DEFLAGRATION (subsonic waves which act like fast moving fires) and DETONATION (a supersonic wave that happens when combustion produces an ultra fast shock which compresses and ignites the material ahead of it). Waves, which start in one density regime, can travel to another, lab tests have shown that it is not certain that a detonation wave will stay like that but can change into a deflagration wave (and vics versa). With this in mind today's scientists believe that a type I supernova starts with a deflagration wave in the center of the star. However as it moves outward (approx. 0.7 stellar radius) until it can no longer be supported, as it is not dense enough, where it changes into a detonation wave. This (in theory) should explain most real world observations but many more simulations to refine it. Most rely on simplified two dimensional calculations, full simulations are still a decade away.
Cosmology from type l supernovae
Astronomers use type l supernovae to measure things like the scale of the universe and how it is expanding. If all type l supernovae have the same brightness, then the dimmer one appears, the further away it must be. By using a precise brightness-distance relation astronomers are able to estimate the expansion of the universe and also the geometry of the universe:
One consequence of general relativity is that the curvature of space depends on the ratio of rho to rho(crit). This ratio is called:
Omega = rho/rho(crit)
(rho is equal to the density of matter in the universe and rho(crit) is the critical value of the density of the universe, which is used to see if the universe is open or closed.)
For Omega less than 1, the Universe has negatively curved or hyperbolic geometry. For Omega = 1, the Universe has Euclidean or flat geometry. For Omega greater than 1, the Universe has positively curved or spherical geometry.
High redshift supernova are still the most saught after piece of information in space, both the "High-Z Supernova Search Team" and the "Supernova Cosmology Project" have launched second generation searches which will capture hundreds of distant supernova over the coming years. Red shift is the name given to the movement of absorption lines toward the red end of the spectrum (the opposite being blue), the changes between them being called the red shift and blue shift. Red shift is also a term used within the evolution of galaxies. It came about from the pioneering work of Edward Hubble in 1929. He calculated that:
v =c(Dl/ l0) = cz = H0r
l0 = laboratory wavelength, H0 = Hubble's constant
l= measured wavelength, z = red shift
r = distance, c = speed of the light.
The water vapor from the earth's atmosphere prevents the ground-based astronomers from achieving accurate spectra and brightness data from type l supernova. This includes supernova who are at the cosmological redshifts beyond z = 1. Knowing this, a group at "Lawrence Berkeley National Laboratory" have proposed to send a probe (S.N.A.P - a two meter long telescope) into orbit to monitor 20 square degrees of space at a time in rolling searches. They hope that it will discover supernovae one-tenth as bright as the best ground based searches. The detectors upon the probe will be optimized for type l's in the cosmological interesting redshift between z = 0.5 and z = 1.7.
How can some be overluminous and others underluminious? These are some of the questions, which are facing the astronomers today. Current theory can accommodate subliminous supernova if the deflagration to detonation transition comes late enough but the question toward overluminious supernova is the problem of figuring out how the explosion could create so much nickel 56 to power such a bright supernova. The only aid, which the astronomers have, is that immediately following the explosion; the supernova enters a phase of free expansion. It means that the structure of the relative position of the elements remains fixed after the initial explosion. Since the velocity of an element is proportional to distance, astronomers can work out where an element is physically by measuring its velocity. Fortunately it is easy to measure the velocity of elements in a supernova with a spectrograph, which splits the light into the colors of the rainbow.
Conclusion
There are many stars in our galaxy, which will go supernova, perhaps not in our lifetime but millions of years from now. That is why astronomers are working so hard today to unravel the mysteries behind their cause and the effect they have upon space and other stars in the vicinity. They may be quite a while off coming to this conclusion but with the advances in computer technology, the telescope and the understanding of space, it will only be a matter of time.
Description of Ni-56 decay corrected, CSJ 03/03/06