Visions of the Cosmos

Astrophysics

STARS

The Magic Furnace

All Life is One  Life

All Men are Brothers - All Women are Sisters

The Atoms of our Planet were Fabricated in the Blazing Heat of Gigantic Stars

The Big-Bang

About 14 billion years ago the Universe was born in an event we call the Big-Bang.   We shall return to this event and its aftermath in a later page of the web-site.  For the moment however we shall briefly touch upon the early times shortly after the event.  The debris from the Big-Bang was fairy simple.  In the very early years the only important constituents of the Universe appeared to be a vast amount of radiation in the form of gamma rays, electrons, and a few atomic nuclei.  The most important and also the simplest nucleus was the hydrogen nucleus, a simple proton.  During the few minutes after the production of protons the temperature and pressure conditions were high enough to cause some of the protons to react in what we now call a thermonuclear reaction to form a nucleus called deuterium (or hydrogen-2) consisting of one proton and a new particle called a neutron.  Further reactions occurred to give fairly large quantities of a nucleus called helium 4 consisting of 2 protons ands 2 neutrons.  Small amounts of two other nuclei were formed namely helium-3 and lithium- 7.

At this stage atoms did not exist.  It was only after about 300,000 years that the temperature dropped low enough for electrons to become associated with what we now call atomic nuclei to form atoms.   As time passed matter condensed into volumes of high gas concentration and the first stars were born.    It was these early stars, some of them very massive indeed, that eventually produced all the elements of the periodic table.  It is this process that is the main subject of this page of the web-site.

Although the detailed chemistry of the chemical elements is not the subject under discussion in this web site it is desirable to get some idea of the Periodic Table which is shown below.

The Periodic Table

  01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18
Period 1 01 H                                 02 He
Period 2 03 Li 04 Be                     05  B 06 C 07 N 08 0 09 F 10 Ne
Period 3 11 Na 12 Mg                     13 Al 14 Si 15 P 16 S 17 Cl 18 Ar
Period 4 19 K 13 Ca 21 Sc 22 Ti 23 V 24 Cr 25 Mn 26 Fe 27 Co 28 Ni 29 Cu 30 Zn 31 Ga 32 Ge 33 As 34 Se 35 Br 36 Kr
Period 5 37 Rb 38 Sr 39 Y 40 Zr 41 Nb 42 Mo 43 Tc 44 Ru 45 Rh 46 Pd 47 Ag 48 Cd 49 In 50 Sn 51 Sb 52 Te 53 I 54 Xe
Period 6 55 Cs 56 Ba 71 Lu 72 Hf 73 Ta 74 W 75 Re 76 Os 77 Ir 78 Pt 79 Au 80 Hg 81 Tl 82 Pb 83 Bi 84 Po 85 At 86 Rn
Period 7 87 Fr 88 Ra 103Lr 104Rf 105Db 106Sg 107Bh 108Hs 109Mt 110Ds 111 112 113 114 115 116 117 118

Due to the limitations of screen width the elements 57-70 and the elements 88-102 are shown in the supplementary table below

Elements 57-70 are called the Lanthanides or Rare Earths   Elements 89-102 are called the actinides

Period  6 57 La 58 Ce 59 Pr 60 Nd 61 Pm 62 Sm 63 Eu 64 Gd 65 Tb 66 Dy 67 Ho 68 Er 69 Tm 70 Yb
Period  7 89 Ac 90 Th 91 Pa 92 U 93 Np 94 Pu 95 Am 96 Cm 97 Bk 98 Cf 99 Fm 100 Es 101 Md 102 No

Elements shown in green are the main elements found in living organisms .  A few others such as fluorine, copper, zinc and iodine are used in trace amounts by many organisms.  A particularly striking example is the use of copper instead of iron in oxygen transport in some animals such as the cephalopods  (the squid and the octopus)

Elements shown in red have no completely stable isotopes.  Note that despite their relative low mass compared to the other radioactive elements there are no stable isotopes of Technetium and Promethium.  Isotopes of both elements have been artificially produced and isotopes of technetium have been detected in certain types of stars.

Isotopes

The subject of isotopes is very important and plays a major role in understanding nucleosynthesis.   The nucleus of an atom, except of course for hydrogen, always consists of a number of protons and neutrons tightly bound together by strong nuclear forces.  As a result nuclei can only undergo thermonuclear fusion at extremely high temperatures.  The chemical properties of an element depend on the electronic structure  and the number of electrons is equal to the number of protons in the nucleus.  Thus the chemical properties of oxygen-16, oxygen-17 and oxygen-18 are very nearly the same but not entirely.  This is a fact which now plays a part in biological sciences and even in the hunt for life on Mars and the moons of the giant planets such as Europa.

There  are two basic types of isotope

1) Stable.  These are said to be non- radioactive.

2) Radioactive.  Many nuclei transform into other nuclei with the liberation of energy.  This takes the form of the expulsion of fast high energy helium-4 which are called alpha rays, high energy electrons called beta rays, high energy 'light' called gamma rays and (a fact which is for obvious reasons often overlooked) the expulsion of positrons - the positrons immediately react with electrons in the environment to form gamma rays (these are of an exactly known energy).

Radioactivity plays an important role in many branches of science.  It is measured in half life periods.   Some nuclei have very long half lives measured in millions of years such as isotopes of thorium and uranium.  Other nuclei are so unstable and have half life periods measured in millionths or even trillionths of a second.   An example is Beryllium-8 which is mentioned shortly when the triple alpha reaction is described.

Thermonuclear Reactions and Nucleosynthesis

Scattered in the vastness of space and separated from one another by inconceivably vast distances there are places where the interstellar gas and dust has gathered together under the force of gravity to produce those immense thermonuclear furnaces we call the stars.  We have already discussed how the Sun produces its prodigious outpouring of energy by the conversion of hydrogen nuclei (protons) to helium.  The time has now come to discuss in detail those processes which make massive stars burn to form other nuclei at immensely greater temperatures to synthesize all the chemical elements of the periodic table.  The process whereby elements are produced is called NUCLEOSYNTHESIS and the reactions that bring about the transformations are called THERMONUCLEAR REACTIONS.

Ordinary chemistry is concerned with the production of new substances by the rearrangement of electrons between atoms.  An example is the formation of water from hydrogen and oxygen.

When hydrogen burns in oxygen to give water chemical bonds are broken and new ones are created. The process involves the rearrangement of electrons binding atoms together to form molecules.  Hydrogen occurs at room temperatures in the form of two atoms bound by two electrons to give a hydrogen molecule.  Oxygen also occurs at room temperatures as two atoms are bound together to form an oxygen molecule.  The oxygen atoms are bound by two pairs of electrons so chemists refer to it as a double bond.  If a stream of hydrogen is ignited in an atmosphere of oxygen a flame is formed and the electrons in the hydrogen and oxygen molecules undergo rearrangements in which molecules of water are produced.

2 H-H  +  O=O     gives    2 H-O-H

usually shortened to 2H2  +  O2   gives   2H2O

During the process energy is given out in the form of heat and light.

Ordinary chemical systems involve a rearrangement of atoms and the energy given out (or taken in) is relatively small when compared to thermonuclear reactions involving the NUCLEUS of the atom.  The energy changes are extremely large when compared to ordinary chemical reactions and occur at temperatures of millions or even hundreds of millions of degrees.

The enormity of the nuclear binding energy can be appreciated by comparing it to the binding energy of an electron in an atom.

The binding energy of an electron in an atom (usually referred to as the ionization energy) is 13.6 electron volts (eVs)

The binding energy of the protons and neutrons (the energy required to break apart) in a helium nucleus is 28,300.000 electron volts (28.3MeVs).

Hydrogen Burning

We have already seen in the section on 'Our Star the Sun' that there are five reaction pathways in which  hydrogen nuclei (protons) are converted to helium nuclei + energy.  Three of them are chain reactions known as the PP1, PP11, PP111 PP1V reaction pathways.  There is another pathway called the CNO Catalytic Process.  The most important fact to notice that the net effect in each case is

4 Hydrogen Nuclei   →    1 Helium-4 Nucleus   + energy and that the same amount of energy is given out whichever reaction pathway is followed.  

The percentage of each pathway differs from star to star.  Although all pathways are followed in the Sun, only a very small amount goes via the CNO catalytic pathway.   In some more massive stars this may be the principal pathway

The simplest (the PPI chain) takes place in three stages.

Proton    +   Proton         →   Deuteron   +   Positron   + neutrino   + energy as gamma ray photon     0.42 million electron volts

Proton    +   Deuteron     →   Helium 3     + energy as gamma ray photon                                          5.49 million electron volts

Helium 3 +   Helium 3     →   Helium 4    +  Proton      + Proton     + energy as gamma ray photon    12.86 million electron volts

The overall reaction is 4 Protons  →   Helium 4                              + energy as gamma ray photon    18.77 million electron volts

4 hydrogen nuclei  are converted into one helium 4 nucleus (helium 3 contains one neutron and two protons).

The complete PP I chain reaction releases a net energy of 18.77 MeV. The temperatures at which the PP I branch is dominant at temperatures of 10 to 14 megakelvins (MK). Temperature is usually stated in degrees Kelvin or megakelvin.   Below 10 MK, the only thermonuclear reactions that occur involve deuterium and lithium

The PP II branch is dominant at temperatures of 14 to 23 MK.

The PP III chain is dominant if the temperature exceeds 23 MK. (The pp III chain is not a major source of energy in the Sun (only 0.11%)).

The PP IV chain has been predicted  but has never been observed due to its great rarity. In this reaction, Helium-3 reacts directly with a proton to give helium-4.

3He + 1H → 4He + νe + e+   (Helium -3 +Hydrogen gives Helium-4 +a neutrino + an positron).

CNO Catalytic Process

The CNO catalytic cycle (for carbon nitrogen oxygen) or sometimes called Bethe-Weizsäcker-cycle, is a fusion reaction by which stars convert hydrogen to helium.

Overview of the CNO-I Cycle.

12C    +    1H        →        13N    +     γ

                            12N    +                →        13C   +     e+    +    ve (neutrino)

13C    +    1H        →         14N    +     γ

                            14N    +    1H        →         15O    +     γ

                            15O    +    1H        →         12C    +     4He

 Diagram by courtesy of Answers.com http://www.answers.com/topic/cno-cycle

The proton-proton chain is more important in stars the mass of the Sun or less. Only 1.7% of 4He nuclei being produced in the Sun are born in the CNO cycle. However theoretical models show that the CNO cycle is the dominant source of energy in heavier stars. The CNO process was proposed in 1938 by Hans Bethe.

The net result of the cycle is to convert four protons into an Alpha  particle plus positrons (annihilating with electrons and releasing energy in the form of gamma rays) plus two neutrinos which are escaping from the star with some part of the energy. The carbon, oxygen, and nitrogen nuclei serve as catalysts and are regenerated. However 14N is the rate limiting step and considerable quantities of nitrogen spills over from the cycle..

The Triple Alpha Reaction

Fast moving helium nuclei are called alpha particles.   Three alpha particles combine to form one carbon-12 nucleus.  This is a complicated process which takes place in four intermediate stages.  It only takes place at extremely high temperatures of the order of 100 million degrees Kelvin in the cores of sun-like stars in the red giant stage as already discussed and more importantly in very massive stars

Stage 1   Helium 4  +  Helium 4      →      Beryllium 8

Stage 2   Beryllium 8 has an extremely short half life 7x10-17 seconds and disintegrates back into 2 Helium nuclei

However at extremely high temperatures and pressures there are sufficient numbers of beryllium 8 nuclei present at any one time to react with Helium 4 nuclei to produce a doubly excited state of Carbon 12.  

Stage 3   Some of the doubly excited Carbon 12 nuclei emit a gamma ray and drop down to a single excited state

State 4    The single excited carbon 12 nuclei  finally drop down to the ground state and emit a second gamma ray

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

Carbon-Carbon Burning

12C +  12C          →        23Na    +    proton    +      γ

12C +  12C          →        24Mg    +        γ

12C +  12C          →        23Na    +    proton    +      γ

 12C +  12C          →        20Ne   +    44He      +      γ

Oxygen-Oxygen Burning

16O +  16O         →        32S    +         γ

16O +  16O         →        31P   +       proton    +    γ

Endergonic Reactions

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) 121622Ne 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)
1 Hydrogen-1 1H Big Bang 1 0 1
2 Helium-4 4He Big Bang  and pp1,2,3 and 4 and CNO 2 2 4
3 Oxygen-16 16O Helium 8 8 16
4 Carbon-12 12C Helium Triple Alpha process 6 6 12   
5 Neon-20 20Ne Carbon / Oxygen and He 10 10 20   
6 Iron-56 56Fe e process 26 30 56
7 Nitrogen-14 14N CNO Process 7 7 14
8 Silicon-28 28Si Oxygen 14 14 28   
9 Magnesium-24 24Mg Carbon 12 12 24   
10 Sulphur-32 32S Oxygen 16 16 32
11 Neon-22 22Ne Helium 10 12 22
12 Magnesium-26 26Mg Carbon 12 14 26
13 Argon-36 36Ar Oxygen Silicon 18 18 36
14 Iron-54 54Fe e Process and Silicon 26 28 54
15 Magnesium-25 25Mg Carbon 12 13 25
16 Calcium-40 40Ca Silicon and Oxygen 20 20 40
17 Aluminium-27 27Al Carbon 13 14 27
18 Nickel-58 58Ni e Process and Silicon 28 30 58
19 Carbon-13 13C CNO Process 6 7 13
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
22 Sodium-23 23Na Carbon 11 12 23
23 Iron-57 57Fe e Process 26 31 57
24 Silicon-30 30Si Carbon Neon 14 16 30
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.

 

Stars 

Types of Stars

Superstars and Supernovae

Isotopes