Cosmology The Life-Histories of Stars. Nuclear Fusion Stars produce light and heat because of the processes of nuclear fusion which take place within

CosmologyCosmology

The Life-Histories of StarsThe Life-Histories of Stars

Nuclear FusionNuclear Fusion

Stars produce light and heat because of Stars produce light and heat because of the processes of the processes of nuclear fusionnuclear fusion which take which take place within their massplace within their mass

Nuclear fusion releases energy because of Nuclear fusion releases energy because of the difference in the difference in binding energybinding energy per per nucleon of the fuels for fusionnucleon of the fuels for fusion

Binding energy is the energy required to Binding energy is the energy required to be supplied to a nucleus in order to pull be supplied to a nucleus in order to pull apart the nucleonsapart the nucleons

Nuclear Fusion Nuclear Fusion

Elements with low binding energies per Elements with low binding energies per nucleon combine to produce elements nucleon combine to produce elements with higher binding energies per nucleonwith higher binding energies per nucleon

This means that more energy is required This means that more energy is required to pull apart the nucleus of the product of to pull apart the nucleus of the product of fusion rather than the reactantsfusion rather than the reactants

So – energy must be given out when the So – energy must be given out when the reactants undergo the fusion process reactants undergo the fusion process


Hence we get Hence we get heat and heat and

light !light !


Nuclear fusion only takes place at Nuclear fusion only takes place at high temperatures – so hot that no high temperatures – so hot that no containment vessel can be used on containment vessel can be used on Earth to support this processEarth to support this process

On Earth the reactants have to be On Earth the reactants have to be held in ‘space’ by magnetic fields to held in ‘space’ by magnetic fields to avoid contact with the containment avoid contact with the containment vesselvessel


The The thermonuclearthermonuclear reaction which reaction which enables hydrogen nuclei to fuse to enables hydrogen nuclei to fuse to produce helium takes place at produce helium takes place at around 15 000 000 Karound 15 000 000 K

When two hydrogen nuclei fuse, the When two hydrogen nuclei fuse, the products are one deuterium atom products are one deuterium atom plus a neutrino and a positronplus a neutrino and a positron


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


The deuterium atom reacts with a The deuterium atom reacts with a further hydrogen atom and produces further hydrogen atom and produces a helium-3 atoma helium-3 atom

Two helium-3 atoms will then Two helium-3 atoms will then combine to produce Helium-4 with combine to produce Helium-4 with two additional hydrogen atomstwo additional hydrogen atoms

This process is know as This process is know as hydrogen hydrogen burningburning


Much higher temperatures are Much higher temperatures are required for other fusion processes required for other fusion processes to take place:to take place:

a) 100 000 000 K –helium burninga) 100 000 000 K –helium burning

b) 600 000 000 K –carbon burningb) 600 000 000 K –carbon burning

c) 1 000 000 000 K –neon burningc) 1 000 000 000 K –neon burning

d) 1 500 000 000 K –oxygen burningd) 1 500 000 000 K –oxygen burning

e) 3 000 000 000 K –silicon burninge) 3 000 000 000 K –silicon burning

EinsteinEinstein

All are familiar with the Einstein All are familiar with the Einstein equation: E = mcequation: E = mc22

The Sun loses mass in small The Sun loses mass in small quantities but this results in large quantities but this results in large amounts of energy being lost due to amounts of energy being lost due to the fact that c is a very large numberthe fact that c is a very large number

The Birth and Life of a StarThe Birth and Life of a Star

Most of what we understand as ‘space’ is Most of what we understand as ‘space’ is thought of as ‘empty’ space but it actually thought of as ‘empty’ space but it actually contains atoms of a variety of gases – contains atoms of a variety of gases – probably only one atom per hundred cubic probably only one atom per hundred cubic metres of spacemetres of space

In some parts of space the density of the In some parts of space the density of the gases is greater resulting in what is known gases is greater resulting in what is known as an as an inter-stellar gas cloudinter-stellar gas cloud

Hydrogen atoms (usually in pairs) and Hydrogen atoms (usually in pairs) and helium atoms exist in close proximity and helium atoms exist in close proximity and move slowly – i.e. the gases are coolmove slowly – i.e. the gases are cool


These hydrogen and helium atoms move These hydrogen and helium atoms move slowly enough to be drawn together by their slowly enough to be drawn together by their own gravity, thus creating a region of higher own gravity, thus creating a region of higher density gas which develops a greater density gas which develops a greater gravitational attraction for even more gravitational attraction for even more particles – this may produce a proto-star (a particles – this may produce a proto-star (a local concentration of atoms which is large local concentration of atoms which is large enough to create a star)enough to create a star)

As the gravitational attraction increases As the gravitational attraction increases then so does the kinetic energy of the then so does the kinetic energy of the atomsatoms


Greater and greater kinetic energies Greater and greater kinetic energies of the atoms mean that the of the atoms mean that the temperature increasestemperature increases

At 3 000 K orbiting electrons break At 3 000 K orbiting electrons break free from their atomsfree from their atoms

At several million Kelvin hydrogen At several million Kelvin hydrogen burning beginsburning begins


When hydrogen burning begins then When hydrogen burning begins then large quantities of energy are released large quantities of energy are released until a state of equilibrium is reached:until a state of equilibrium is reached:

a)a) Energy radiated by the star is balanced Energy radiated by the star is balanced by the energy released by the by the energy released by the thermonuclear fusionthermonuclear fusion

b)b) Gravity (pulling towards the core) is Gravity (pulling towards the core) is balanced by the thermal and radiation balanced by the thermal and radiation forces (pushing out from the core)forces (pushing out from the core)


All main sequence stars are in equilibrium All main sequence stars are in equilibrium as previously describedas previously described

Stars take a short time to form – between Stars take a short time to form – between 10 000 and 1 000 000 years (depending 10 000 and 1 000 000 years (depending on their size) but they stay stable for much on their size) but they stay stable for much longerlonger

The greater the mass of the star, the The greater the mass of the star, the greater its rate of hydrogen burning and greater its rate of hydrogen burning and the shorter its life as a main sequence starthe shorter its life as a main sequence star


Eventually most of the hydrogen is Eventually most of the hydrogen is used up and the star’s core will used up and the star’s core will contractcontract

The contraction of the core causes an The contraction of the core causes an increase in kinetic energies of the increase in kinetic energies of the atoms and the core temperature atoms and the core temperature rises which causes the rest of the rises which causes the rest of the star to expand and become a star to expand and become a red red giantgiant

The Birth and Life of a StarThe Birth and Life of a Star At the core of the red giant the At the core of the red giant the

temperature rises to the approximately temperature rises to the approximately 100 000 000 K and helium burning 100 000 000 K and helium burning begins:begins:

a)a) Two helium nuclei fuse to create a Two helium nuclei fuse to create a beryllium nucleusberyllium nucleus

b)b) A beryllium and a helium nucleus fuse to A beryllium and a helium nucleus fuse to produce a carbon nucleus with the produce a carbon nucleus with the emission of a gamma photonemission of a gamma photon

c)c) A carbon and a helium nucleus fuse to A carbon and a helium nucleus fuse to produce an oxygen nucleus and a produce an oxygen nucleus and a gamma photongamma photon


This helium burning process will This helium burning process will maintain the life of a red giant stable maintain the life of a red giant stable for a period equal to between 10% for a period equal to between 10% and 20% of the time that it was a and 20% of the time that it was a main sequence starmain sequence star

Eventually the core collapses once Eventually the core collapses once more and, depending on the mass of more and, depending on the mass of the star, a variety of different deaths the star, a variety of different deaths may ensuemay ensue

The Death of a StarThe Death of a Star

The death outcome of a star depends The death outcome of a star depends on its mass:on its mass:


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


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


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


Red giants which are massive Red giants which are massive enough to collapse beyond the stage enough to collapse beyond the stage of a white dwarf may develop further of a white dwarf may develop further thermonuclear reactions: carbon, thermonuclear reactions: carbon, neon, oxygen and silicon burningneon, oxygen and silicon burning

Eventually, when all stable reactions Eventually, when all stable reactions are finished and fuels are exhausted are finished and fuels are exhausted the collapsing core exceeds the the collapsing core exceeds the Chandrasekhar limitChandrasekhar limit


Collapse occurs until the neutrons Collapse occurs until the neutrons are as tightly packed as they can be, are as tightly packed as they can be, which produces a shock wave when which produces a shock wave when the collapse is suddenly haltedthe collapse is suddenly halted

The intense radiation from the core The intense radiation from the core causes the star to explode – a causes the star to explode – a supernovasupernova is produced is produced


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

Neutron Stars and Black HolesNeutron Stars and Black Holes

Originally predicted by computer Originally predicted by computer modelling but not confirmed by modelling but not confirmed by empirical evidence until pulsating empirical evidence until pulsating radio waves were detected in 1967radio waves were detected in 1967

These pulsating radio waves must be These pulsating radio waves must be transmitted by a rotating or vibrating transmitted by a rotating or vibrating body (pulsars) which must, therefore, body (pulsars) which must, therefore, be very small and very dense – be very small and very dense – possibly neutron starspossibly neutron stars

Neutron Stars and Black HolesNeutron Stars and Black Holes

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

What Do I Need to Learn ?What Do I Need to Learn ?

Describe nuclear fusion processesDescribe nuclear fusion processes Calculate energy released in fusion Calculate energy released in fusion

processesprocesses Describe how clouds of gas form into starsDescribe how clouds of gas form into stars Recall how main sequence stars may Recall how main sequence stars may

evolveevolve Recall the nuclear processes that occur in Recall the nuclear processes that occur in

a stara star Describe the probable evolution of the Sun Describe the probable evolution of the Sun

into a red giantinto a red giant

What Do I Need to Learn ?What Do I Need to Learn ?

Know about pulsars and quasarsKnow about pulsars and quasars Know about SETI – Search for Extra-Know about SETI – Search for Extra-

Terrestrial IntelligenceTerrestrial Intelligence

ActionAction

Read the chaptersRead the chapters Attempt all SAQsAttempt all SAQs Attempt all end of chapter questionsAttempt all end of chapter questions Ask for help at any time – not only Ask for help at any time – not only

during lessonsduring lessons Hand in your work for markingHand in your work for marking

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Cosmology The Life-Histories of Stars. Nuclear Fusion Stars produce light and heat because of the processes of nuclear fusion which take place within