Cosmology Experiment C-4

Synthesis of Elements in Supernovae


The previous web pages, C-1, C-2, and C-3 explain that the big bang and stellar processes that synthesize the first 26 elements of the chemical periodic chart released the energy that we see as starlight and background radiation. But very little of the remaining elements were synthesized even in red giant stars.  Those remaining elements were synthesized in the spectacular brief final stage of some of the largest stars.

The smaller stars were never compressed enough to power the fusion of heavier atoms.  After they have consumed most of the Hydrogen available in their cores, they contract into small but still hot stars called white dwarfs.  In this final stage they slowly cool and fade out of view.

Carb NebulaMore massive red giant stars continue fusing atoms in their cores and in the surrounding shell-like layers until insufficient fuel remains in the core to prevent further collapse.  Then over a brief time as short as seconds, the surrounding layers, still containing large concentrations of atoms capable of fusing, are compressed, heated and undergo enormous amounts of fusion, at rates enormously beyond any previous in any red giant.  As a result of the rapid release of energy the layers of the star still further away from the core are blow free from the star with enormous force and great brightness.  Such very distant exploding stars were first observed on earth as new stars in what had previously been considered unchanging heavens and thus named nova.  In more recent years that name has been reserved for stars a million times brighter than typical stars.  However these exploding red giants become 108 brighter for a few weeks and so are now called supernova.  The light emitted is often briefly brighter than the rest of the galaxy.  The intensity decreases rapidly at first but lasts over many thousands of years.  For example the Crab Nebula (NASA photo to right) is the still visible remnants of a supernova observed by the Chinese in 1054 A.D.

During the explosion of a supernova, temperatures reach 4 x 109 K and density of 108 g/cm3.  At these extreme conditions, not only can further fusion occur, but nuclei can also dissolve into smaller nuclei and neutrons which in turn can provide as raw materials for rapid fusion of heavy atoms.  Vast amounts of electrons and protons are fused to form an abundant fluid of neutrons (Eqn. 1 below) which in turn can fuse in large numbers with existing elements to form extremely unstable super heavy isotopes with masses perhaps >270 (example, Eqn. 2 below).  These quickly undergo a large number of beta decays (example Eqn. 3 and 4) to form gradually more stable heavy elements at least as heavy as Uranium, 2892U. (example, Eqn. 68)  This rapid synthesis of elements is called the r-process.

0-1e + 11p ⇒ 10n                    (Eqn. 1)

185 10n + 5526Fe ⇒ 24026Fe:                    (Eqn. 2) typical r-process

24026Fe ⇒ 0-1β + 24027Co          (Eqn. 3)

24027Co ⇒ 0-1β + 24028Ni          (Eqn. 4)

. . .   many more successive beta decays

24091Pa ⇒ 0-1β + 24092U          (Eqn. 68)

Much of these new elements is ejected from the supernova and becomes available for later formation of second and third generation stars.  Our Sun is such a second or third generation star.  We know that because the Sun's spectra reveals that it contains the wide variety of chemical elements including those larger than iron.  Not all of the ejected material may form stars.  Some may be collected by gravity into planets and moons orbiting second or third generation stars.  We are composed of atoms synthesized in a red giant become supernova which has been collected back together for form our Sun, the planets and moons of our solar system, and in particular our planet Earth.

But while much of a large red giant is ejected in the supernova, the cores inside the immense fusion layers were further compressed by both the implosion and the surrounding explosion.  A star with mass from 1.4 to 30 times that of the Sun is compressed enough by the gravity and surrounding fusion to squash the plasma of neutrons, protons and electrons into s single ball of neutrons (Eqn. 1 above) called a neutron star. This tiny, massive body with the mass of a star but compressed to the size of a city has the density of 4 x 1014 g/cm3.  This tiny compact neutron star rotate rapidly, often faster than once per second.  With a magnetic field often not aligned with its axis of rotation (just at the earth's north magnetic pole is not exactly at the north pole of spin, a neutron star typically emits pulsing emissions and thus is also called a pulsar.  Such stars were first discovered in 1968 to pulse in the range of radio frequencies, but may also pulse visible light and even X-rays.  (Download and listen to the 43 minute audio tape made during the 1969 discovery of the first visible pulsar at the core of the Crab Nebula (photo above).  Doing so will be worth your effort!)

The cores of red giant stars more than 30 times larger than the Sun have sufficiently intense gravity due to their great density and mass so they continue to collapse so much that nothing, not even light radiation, can escape.  These now totally absorbing objects are known as black holes.  But details of that are another story told elsewhere.


  1. Some members of the general public apparently have a different notion of what is controversial and what is well established knowledge than do scientists.  For example few in the public consider the r-process controversial yet there has been no experiments to confirm what remains largely speculation by scientists.  At the other extreme, many people find the concept of a Big Bang controversial while scientists have found overwhelming evidence that it occurred.  Compare the differences in criteria used by the general population and by scientists to evaluate how information is determined to be correct.periodic chart

  2. Referring to the locations of elements on a periodic chart, determine which processes synthesized each element.

  3. Create a table of the conditions necessary to synthesize each kind of element.  Where in the universe does each synthesis occur and how long does it last?
  4. Write the equation representing the r-process for a 4020Ca atom to absorb enough neutrons to become the mass of a lead, 20782Pb, atom.  How many neutrons would need to be absorbed?  How many beta decays would be needed to for the intermediate nucleus to become lead?

Communicating technical information such as observations and findings is a skill used by scientists but useful for most others.  If you need course credit, use your observations in your journal to construct a formal report.



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created 1/17/2005
revised 1/31/2005
by D Trapp
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