Cosmology The Life-Histories of Stars

Nuclear Fusion  Stars produce light and heat because of the processes of nuclear fusion which take place within their mass  Nuclear fusion releases energy because of the difference in binding energy per nucleon of the fuels for fusion  Binding energy is the energy required to be supplied to a nucleus in order to pull apart the nucleons

Nuclear Fusion  Elements with low binding energies per nucleon combine to produce elements with higher binding energies per nucleon  This means that more energy is required to pull apart the nucleus of the product of fusion rather than the reactants  So – energy must be given out when the reactants undergo the fusion process

Nuclear Fusion Hence we get heat and light !

Nuclear Fusion  Nuclear fusion only takes place at high temperatures – so hot that no containment vessel can be used on Earth to support this process  On Earth the reactants have to be held in ‘space’ by magnetic fields to avoid contact with the containment vessel

Nuclear Fusion  The thermonuclear reaction which enables hydrogen nuclei to fuse to produce helium takes place at around K  When two hydrogen nuclei fuse, the products are one deuterium atom plus a neutrino and a positron

Nuclear Fusion  Positrons are anti-matter to electrons and when in contact they will annihilate each other – this quickly happens in stars due to the large number of unattached electrons

Nuclear Fusion  The deuterium atom reacts with a further hydrogen atom and produces a helium-3 atom  Two helium-3 atoms will then combine to produce Helium-4 with two additional hydrogen atoms  This process is know as hydrogen burning

Nuclear Fusion  Much higher temperatures are required for other fusion processes to take place: a) K –helium burning b) K –carbon burning c) K –neon burning d) K –oxygen burning e) K –silicon burning

Einstein  All are familiar with the Einstein equation: E = mc 2  The Sun loses mass in small quantities but this results in large amounts of energy being lost due to the fact that c is a very large number

The Birth and Life of a Star  Most of what we understand as ‘space’ is thought of as ‘empty’ space but it actually contains atoms of a variety of gases – probably only one atom per hundred cubic metres of space  In some parts of space the density of the gases is greater resulting in what is known as an inter-stellar gas cloud  Hydrogen atoms (usually in pairs) and helium atoms exist in close proximity and move slowly – i.e. the gases are cool

The Birth and Life of a Star  These hydrogen and helium atoms move slowly enough to be drawn together by their own gravity, thus creating a region of higher density gas which develops a greater gravitational attraction for even more particles – this may produce a proto-star (a local concentration of atoms which is large enough to create a star)  As the gravitational attraction increases then so does the kinetic energy of the atoms

The Birth and Life of a Star  Greater and greater kinetic energies of the atoms mean that the temperature increases  At K orbiting electrons break free from their atoms  At several million Kelvin hydrogen burning begins

The Birth and Life of a Star  When hydrogen burning begins then large quantities of energy are released until a state of equilibrium is reached: a) Energy radiated by the star is balanced by the energy released by the thermonuclear fusion b) Gravity (pulling towards the core) is balanced by the thermal and radiation forces (pushing out from the core)

The Birth and Life of a Star  All main sequence stars are in equilibrium as previously described  Stars take a short time to form – between and years (depending on their size) but they stay stable for much longer  The greater the mass of the star, the greater its rate of hydrogen burning and the shorter its life as a main sequence star

The Birth and Life of a Star  Eventually most of the hydrogen is used up and the star’s core will contract  The contraction of the core causes an increase in kinetic energies of the atoms and the core temperature rises which causes the rest of the star to expand and become a red giant

The Birth and Life of a Star  At the core of the red giant the temperature rises to the approximately K and helium burning begins: a) Two helium nuclei fuse to create a beryllium nucleus b) A beryllium and a helium nucleus fuse to produce a carbon nucleus with the emission of a gamma photon c) A carbon and a helium nucleus fuse to produce an oxygen nucleus and a gamma photon

The Birth and Life of a Star  This helium burning process will maintain the life of a red giant stable for a period equal to between 10% and 20% of the time that it was a main sequence star  Eventually the core collapses once more and, depending on the mass of the star, a variety of different deaths may ensue

The Death of a Star  The death outcome of a star depends on its mass:

The Death of a Star  Stars less than about three times the Sun’s mass will not develop temperatures high enough to ignite further nuclear reactions. The outer layers of gas escape and are ionized by radiation from the core, producing a planetary nebula. The core collapses and becomes more dense, packing the electrons close enough to generate Fermi pressure

The Death of a Star  Fermi pressure prevents further collapse but by this time the star is very small (about 1% of the diameter of the Sun) and very hot. These are white dwarfs and are not very bright in he night sky. They eventually cool.

The Death of a Star  If a white dwarf has a mass of greater than 1.4 x the mass of the Sun (the Chandrasekhar limit) the pressure in the core is so intense that electrons and protons combine to produce neutrons. These neutrons collapse rapidly – less than one second – with a rapid rise in temperature

The Death of a Star  Red giants which are massive enough to collapse beyond the stage of a white dwarf may develop further thermonuclear reactions: carbon, neon, oxygen and silicon burning  Eventually, when all stable reactions are finished and fuels are exhausted the collapsing core exceeds the Chandrasekhar limit

The Death of a Star  Collapse occurs until the neutrons are as tightly packed as they can be, which produces a shock wave when the collapse is suddenly halted  The intense radiation from the core causes the star to explode – a supernova is produced

The Death of a Star  The intense temperatures and pressures of supernovas create more thermonuclear fusion reactions which absorb energy. Viz. fusion between elements which create other elements more massive than iron. Iron has more binding energy per nucleon than any other element and so the creation of more massive elements must absorb energy

Neutron Stars and Black Holes  Originally predicted by computer modelling but not confirmed by empirical evidence until pulsating radio waves were detected in 1967  These pulsating radio waves must be transmitted by a rotating or vibrating body (pulsars) which must, therefore, be very small and very dense – possibly neutron stars

Neutron Stars and Black Holes  Theories developed by astrophysicists suggest that if the neutron star has a mass of more than 3 x the mass of the Sun then the core of the star would collapse to an infinitesimally small point so that for a radius of a few kilometres around the point the gravitational field would be so strong that not even light could escape

What Do I Need to Learn ?  Describe nuclear fusion processes  Calculate energy released in fusion processes  Describe how clouds of gas form into stars  Recall how main sequence stars may evolve  Recall the nuclear processes that occur in a star  Describe the probable evolution of the Sun into a red giant

What Do I Need to Learn ?  Know about pulsars and quasars  Know about SETI – Search for Extra- Terrestrial Intelligence

Action  Read the chapters  Attempt all SAQs  Attempt all end of chapter questions  Ask for help at any time – not only during lessons  Hand in your work for marking