4He + 4He ↔ 8Be+ γ
8Be + 4He ↔ 12C + γ
The net energy release of the whole process is 7.275 MeV.
Supernovae (Type 11)
In sun-like stars after the helium core has burnt to carbon and oxygen the core collapses to a carbon-oxygen white dwarf as already discussed in the web-site page dealing with the Sun.
Stars with larger masses than the Sun can continue the fusion process beyond the process of carbon and oxygen synthesis as described above. Stars with much larger masses than the Sun burn helium to carbon and oxygen and without becoming red giants proceed to burn the carbon and oxygen to more massive nuclei. Exactly how far a star goes in this process depends upon its mass but stars of over 8 solar masses form Red or Blue Supergiants and then proceed to explode as supernovae.
Massive stars burn extremely rapidly especially in the later stages just before the supernova explosions. A large number of different reactions occur which give rise to a large number of different nuclei leading up to the final production of nickel-56 which undergoes radioactive decay by two stages to iron-56.
The way the nucleosynthetic processes operate in stars is that the fuel burns to produce the ash which becomes the fuel for the next process.
Alpha Particle Pathways
Hydrogen is the fuel for the first burning - the ash is Helium-4 pp Series and CNO
Helium -4 is the fuel for the second burning - the ash is Carbon-12 Triple Alpha via 8Be
Carbon-12 is the fuel for the third burning - the ash is Oxygen-16 12C + 4He → 16O + γ
Oxygen-16 is the fuel for the fourth burning -the ash is Neon-20 16O + 4He → 20Ne + γ
Neon -20 is the fuel for the fifth burning - the ash is Magnesium-24 20Ne + 4He → 24Mg + γ
Magnesium-24 is the fuel for the sixth burning - the ash is Silicon-28 24Mg + 4He → 28Si + γ
Silicon-28 is the fuel for the seventh burning - the ash is Sulphur-32 28Si + 4He → 32S + γ
Sulphur-32 is the fuel for the eighth burning - the ash is Argon-36 32S + 4He → 36Ar + γ
Argon-36 is the fuel for the ninth burning - the ash is Calcium-40 36Ar + 4He → 40Ca + γ
Calcium-40 is the ash for the tenth burning - the ash is Titanium-44 40Ca + 4He → 44Ti + γ
Titanium-44 is the fuel for the eleventh burning - the ash is Chromium-48 44Ti + 4He → 48Cr + γ
Chromium -48 is the fuel for the twelfth burning- the ash is Iron-52 48Cr + 4He → 52Fe + γ
Iron-52 is the fuel for the thirteenth and final burning - the ash is Nickel-56 52Fe + 4He → 56Ni + γ
Nickel-56 is highly radioactive and decays via Cobalt-56 to Iron-56 a stable isotope
The series shown above is an over simplification of what occurs in the interiors of very massive stars since many other reactions also take place. The successive addition of Helium-4 is only one of the typical pathways. Three other important types of burning occur - They are carbon - carbon, carbon - oxygen and oxygen - oxygen burning. A few examples are given below
12C + 12C → 23Na + proton + γ
12C + 12C → 24Mg + γ
12C + 12C → 23Na + proton + γ
12C + 12C → 20Ne + 44He + γ
16O + 16O → 32S + γ
16O + 16O → 31P + proton + γ
Besides burning reactions, some reactions occur in which energy is taken in (to the chemists among you these reactions are called endothermic or endergonic. Some of these result in the production of helium-4 nuclei (at the high speeds involved otherwise called Alpha particles)
24He minus energy
16O+ 16O → 24Mg + 24He minus energy
Other reactions occur in which neutrons are expelled with the loss of energy
12C + 12C → 23Mg + neutron minus energy (-2.5993MeV)
Temperatures and Time of Burning Processes in Degrees Kelvin
|Fuel||Ashes||Temperature of Burning OK x million||Approximate Time of Burning|
|1H (Hydrogen)||4He 14N from CNO||10 million up to about 25 million depending on pathway||millions of years|
|4He (Helium)||12C 16O 22Ne||500 million||500,000 years|
|12C (Carbon)||20Ne 24Mg 16O 23Na 25Mg 26Mg||800 million||600 years|
|20Ne (Neon)||16O 24Mg 28Si||1,000 million||1 year|
|16O (Oxygen)||28Si 32S||2,000 million||6 months|
|28Si (Silicon)||56Ni||3,000 million||1 day|
|56Ni undergoes radioactive decay to 56Fe||56Fe enormous cooling to 1H 4He and neutrons and neutrinos||Huge temperatures are reached in the cloud ejected from the supernova explosion||Core collapse can be measured in milliseconds|
|56Fe||Iron core undergoes gravitational collapse.|
(at nucleosynthetic temperatures it is not important whether the temperature is stated in Kelvin or Celsius) The data is taken from David Arnett's book on Nucleosynthesis and Supernova
The dramatic nature of the burning processes in massive stars becomes strikingly obvious in the table above. Massive stars just below the size needed to produce a supernova burn up to the point where the temperatures needed for the very last stages of burning can not be reached.
It must be pointed out that nuclei are charged and so repel each other - they need kinetic energy (thermal) to overcome this Coulomb barrier. This explains why such stupendous temperatures are needed as the charge on the nucleus increases. - hydrogen has only one positive charge, helium two, carbon six, oxygen eight and silicon fourteen
It should come as no surprise that the most abundant isotopes are hydrogen, helium and those produced in hot stars up to and including iron-56
The 25 most abundant nuclei in the Solar System are given in the table below
|Rank||Name||Symbol||Origin||Z (number of protons)||Number of Neutrons||A (Mass of Nuclei)|
|2||Helium-4||4He||Big Bang and pp1,2,3 and 4 and CNO||2||2||4|
|4||Carbon-12||12C||Helium Triple Alpha process||6||6||12|
|5||Neon-20||20Ne||Carbon / Oxygen and He||10||10||20|
|14||Iron-54||54Fe||e Process and Silicon||26||28||54|
|16||Calcium-40||40Ca||Silicon and Oxygen||20||20||40|
|18||Nickel-58||58Ni||e Process and Silicon||28||30||58|
|20||Helium-3||3He||Big Bang and pp Processes 1,2,3 and 4||2||1||3|
|21||Silicon-29||29Si||Carbon and Neon||14||15||29|
|25||Hydrogen-2 or Deuterium||2H||Big Bang||1||1||2|
The table is derived from Anders and Grevesse reproduced by David Arnett in his excellent Book Nucleosynthesis and Supernovae
It should come as no surprise that MOST of the early elements in the chemical periodic table are represented by at least one isotopic form which is relatively common. Thus carbon, nitrogen, oxygen, fluorine, neon, sodium, magnesium, aluminium, silicon, phosphorus, sulphur, chlorine, argon, potassium, calcium, scandium, titanium, vanadium, chromium, manganese, iron, cobalt and nickel are all represented by at least one commonly occurring isotope.
However the three elements lithium, beryllium and boron are not particularly common - they are not produced in the interiors of stars. Small amounts if lithium were produced shortly after the Big Bang and all three of these earlier elements are produced by cosmic ray activity and to some extent on the surfaces of stars..
To explain the elements of the rest of the periodic table we must continue our story and introduce two further types of reaction which do occur in stars. These are known as the s (slow ) process and the r (rapid ) process.
Slow Neutron Capture
A large fraction of nuclei heavier than Fe-56 are produced by slow neutron captures (s process). The main astrophysical site of the s process has been identified to be the deep layers of stars including sun-like stars at the end of their evolution when they leave the main sequence branch and become helium burning red giants. The s-process products are brought to the stellar surface by recurrent episodes of deep mixing and they are carried into the interstellar medium by strong stellar winds.
The process occurs in two stages
1) the capture of a neutron by a nucleus. For example a stable nucleus of iron 58Fe reacts with a neutron to form a radioactive isotope of iron namely 59Fe
2)the conversion of a neutron in the unstable isotope 59Fe to form a stable isotope of cobalt 59Co
Summarising 58Fe + n → 59Fe → 59Co + e- + v
iron-58 + neutron → iron-59 → cobalt -59 + beta particle + neutrino
The beta particle is a highly energetic electron. It is caused by one of the neutrons in the iron-59 changing to a proton thus forming cobalt-59
This s process can proceed right up the periodic table as far as the last stable isotope namely bismuth-209
As a result of the s process stable nuclei above iron are formed in red giant stars and are carried out into interstellar space by the stellar wind.
Rapid Neutron Capture
In supernovas, the time scale in the cloud of escaping debris during which neutron capture followed by beta decay occurs is of the order of a few hours as compared to the s process which lasts for thousands of years. Large amounts of nuclei beyond iron-58 are formed just as with the s process.
There is a distinction between the isotopes that can be produced in the two processes. While some can be obtained by either, there are some which can only be produced by the s-process and some that can only be produced by the r-process. Elements beyond bismuth-209 can only be produced by the r process. They are radioactive isotopes with long half lives such as thorium and uranium.
One look at the periodic table will show that certain elements are made in more abundant quantities than others. Thus copper and zinc are relatively common and gold and platinum are rare.
The Layers of the Onion
During its final stages a supernova star assumes an onion shape the innermost region is the iron core (shown as green in the diagram). It is surrounded by the silicon/sulphur core(cross-hatchedred) surrounded by the oxygen/neon/magnesium layers (yellow) which are in turn enveloped by the carbon/oxygen layer (blue) which is surrounded by the helium burning core (green). Finally hydrogen (grey) forms the outermost burning layer. All the layers are burning ferociously until the last moment when the inner core undergoes gravitational collapse within seconds to a neutron star and a massive shock wave causes all the outer layers to explode into interstellar space at tremendous velocities. Many scientists believe that the shock wave causes the very low pressure gas and dust clouds in the surrounding interstellar cloud to undergo rapid condensation and initiate the formation of new stars. Whether that is true or not, it is a fact that clear evidence of the existence of medium long lived radio-isotopes of aluminium, iodine and plutonium have been found in very ancient rocks and in meteorites. Although they have long since decayed to more stable nuclei their very presence in the early years of the existence of the Solar System indicates that much of the solar material was derived from a fairly recent supernova explosion.
It must be emphasized that not all the material in a given star system such as our own Solar System is derived from a supernova. There are considerable amounts of carbon, oxygen, nitrogen and the other elements of the periodic table which even includes some 'heavier' than iron that are blown out in the stellar winds of massive starts many of which are not quite massive enough to reach the supernova stage.
Mention should be made in this page of the web-site of Fred Hoyle, F.Fowler and the wife and husband team E.M. and G.R.Burbridge who as early as 1957 worked out the way in which chemical elements are produced in stars and the brilliant discovery by Fred Hoyle of the way in which helium-4 produces carbon-12 via beryllium-8. It is to be regretted that Fred Hoyle never got the Nobel Prize.
So we close this page of the web-site with some understanding of the stars, those magic furnaces that produce the wonderful variety of chemical elements that make up the world we live in.
In the next chapter we shall deal with the importance of isotopes in the study of biology, geology and astronomy.