OPTION E - ASTROPHYSICS E5 Stellar Processes and Stellar Evolution AHL Nucleosynthesis.

OPTION E - ASTROPHYSICS

E5 Stellar Processes andStellar Evolution AHL

Nucleosynthesis



Nucleosynthesis

Stars have their origin in nebulae, like Orion dark nebula.

E.5.1 Describe the conditions that initiate fusion in a star.

A typical dark nebula has a temperature of about 100K and contains 1010 to 1019 particles. These consist of H (75%), He (24%) and dust (1%). The dust consists of atoms and molecules of many different elements.

The density is big enough for gravity to pull the individual particles together. As the particles move together under their mutual gravitational attraction they lose gravitational energy and gain kinetic energy. The temperature will increase, ionisation of the molecules will take place and the system will acquire its own luminosity.


At this point the so-called protostar is still very large and might have a surface temperature of 3000 K and therefore has considerable luminosity.

A protostar of mass equal to the Sun can have a surface area 5000x greater than the Sun and be 100x as luminous.

Potential solar system


As the gravitational contraction continues, the temperature of the core of the protostar continues to rise until it is at a sufficiently high temperature for all the electrons to be stripped from the atoms making up the core.

The core has now become plasma and nuclear fusion takes place in which H is converted into He and the protostar becomes a main sequence star on the H-R diagram.

The nuclear fusion process will eventually stop any further gravitational contraction and the star will have reached hydrostatic equilibrium in which gravitational pressure is balanced by the radiation pressure created by the nuclear fusion processes.


The formation of StarsThe formation of Stars

Whereabouts a protostar “lands” on the main sequence is determined by its initial mass.

The greater the initial mass, the higher will be the final surface temperature and the greater will be its luminosity.


More massive stars take less time to reach the main sequence than less massive stars.



Gravitational collapse puts a lower and a upper limit on the mass of matter that can form a star:

Protostars of mass less than 0.08 M

Not enough temperature and pressure to initiate nuclear fusion and the protostar will contract to a brown dwarf.

Protostars of mass more than 100 M

Internal pressure overcomes gravitational pressure and vast amounts of matter will be ejected from the outer layer of the protostar thereby disrupting the evolution of the star.

E.5.2 State the effect of a star’s mass on the end product of nuclear fusion.E.5.2 State the effect of a star’s mass on the end product of nuclear fusion.

At the end of a star’s lifetime as a main sequence star all the H in its core has been used up.

How long this takes and a star’s ultimate fate depends upon its initial mass.

The Sun will convert H into He for 5x109 years but a star of mass 25M will take only about 106 years to use up all its H.

• When all the H in the core has been used, no fusion processes will counteract the gravitational contraction so the core will now start contract.

• However there is sill some H in the outer layers.

• The core’s contraction will increase its temperature and release energy that will heat the outer layers.

• This makes the burning hydrogen extend further into the outer regions.

E.5.3 Outline the changes that take place in nucleosynthesis when a star leaves the main sequence and becomes a red giant.E.5.3 Outline the changes that take place in nucleosynthesis when a star leaves the main sequence and becomes a red giant.

• As the core contracts, the Sun as a hole expands.

• This causes the Sun’s surface temperature to drop and its luminosity to increase.

• The Sun will become a Red Giant:

• T=3500K

• R=0.5 AU

• L = 2000 L


• The He created by H burning in the outer layers adds to the mass of the core causing the core to further contract.

• Its T will rise enough for the He fusion to take place.

• He fusion will produce 12C and 16O.

• When all He has been used up the core further contracts.

• Its T rises and the energy radiated causes He burning in the outer layers: 2nd red giant phase

• In this phase the Sun will engulf the Earth.

• Its luminosity will be 10 000 L


• In this 2nd red giant phase the Sun undergoes bursts of luminosity in which a shell of its outer layers is ejected into space.

• In this process the core will be exposed. Because of its surface temperature (100 000K) the energy emitted will ionize the outer gas layers causing them to emit visible radiation producing a planetary nebula.

• The radius of the core is about that of the Earth and with no fusion reaction taking place within the core it will just simply cool down.

• The Sun has become a white dwarf star.

• Eventually it will radiate all its energy and become a black dwarf.




Evolutionary paths of stars and stellar processes



Evolutionary paths of stars and stellar processes

For stars in the main sequence, there is a relationship between the mass and the luminosity of the star:

Where the exponent α is between 3 and 4.

One application of the mass-luminosity relationship is to estimate the lifetime of the star on the main sequence.

E.5.4 Apply the mass–luminosity relation.E.5.4 Apply the mass–luminosity relation.

The more massive a protostar (> 4M) the more quickly its core will reach a temperature at which fusion takes place:

Protostars of mass 15 M 104 years

Protostars of mass 1 M 107 years

A massive protostar will have its luminosity quickly stabilised but temperature increases as it further contracts.

For small protostars because the energy is lost by convection and not radiation, the surface temperature remains constant.

The luminosity therefore will decrease as the protostar contracts.

E.5.4 Apply the mass–luminosity relation.E.5.4 Apply the mass–luminosity relation.

E.5.5 Explain how the Chandrasekhar and Oppenheimer–Volkoff limits are used to predict the fate of stars of different masses.E.5.5 Explain how the Chandrasekhar and Oppenheimer–Volkoff limits are used to predict the fate of stars of different masses.

Star’s mass Nuclear reaction OutcomeLess than 0.25

M

No further nuclear reactions take place. The core stays a core of He.

White dwarf with helium core

Mass between 0.25 and 4M

The T of the core reaches 108K. The reactions involving He produce C and O.

This process is called triple alpha process.

White dwarf with carbon/oxygen core

Mass between 4 and 8M

The fusion in the core will stop when O, Ne and Mg are formed.

White dwarf with oxygen/ neon/ magnesium core

Mass between 8 and 40M

The O, Ne and Mg core will contract further, reaching a T high enough for these elements to fuse, producing heavier elements until Fe is fused from silicon. Fusion cannot produce heavier elements than Fe (highest binding energy per nucleon. The star has become a red supergiant and will explode in a supernova.

Neutron star

Mass over 40M

Same as above Black hole

1. Evolution of a star of mass under 8 M 1. Evolution of a star of mass under 8 M

• The energy released from the core contraction and the H/He fusion in the outer layers may force the outer layers to be ejected in what is called a planetary nebula.

• This leaves behind the exposed core of the star.


• The evolution of a low mass star stops when the core is made mainly out of He or C/O or O/Ne/Mg.

Sirius B has a mass of 1.02 solar masses, R=5400km

and surface T 10000K.

http://en.wikipedia.org/wiki/Image:Sirius_A_and_B_Hubble_photo.jpg

Ring Nebula

White dwarf will eventually cool down

Outer layers

Projected timeline of the Sun's life (from wikipedia)

The fate of the SunThe fate of the Sun

The fate of the SunThe fate of the Sun

http://janus.astro.umd.edu/astro/stars/SunsLife.html



The fate of starsThe fate of stars

M < 8 M

The fate of a star depends on its initial mass. So, if:

White dwarf ejecting 60% of its mass as a planetary nebulae

4 M < M < 8 M The star is able to fuse Carbon and in this process produce Neon, Sodium, Magnesium and Oxygen during their final red giant phase

However, if:

Core H all burnt

Core contracts and T rises

Outer layer H burning

Expansion of outer

layer

Luminosity increases

RED GIANT

He adds to the core

Helium in core used up

Core contracts He burning in

core

Further contraction of core and expansion of outer layers

WHITE DWARF

He burning in outer layers

Ejection of mass via planetary nebulae

SummarySummary

• The electron pressure can stop the further collapse of the core and the star will become a stable white dwarf only if the mass of the core is less than 1.4 M

• This is known as the Chandrasekhar limit.• If the mass of the core is more than 1.4 solar

masses, the star will become a neutron star or a black hole.

Subrahmanyan Chandrasekhar


• The core of the Sun as a white dwarf does not keep contracting since there is a high density limit set by a quantum mechanical effect called electron degeneracy.

• This is the point where electrons cannot be packed any closer.

http://en.wikipedia.org/wiki/Image:ChandraNobel.png

2. Evolution of a star of mass over 8 M2. Evolution of a star of mass over 8 M

• Stars with a mass of 8M or more are able to fuse even more elements than carbon.

• After all the C in the core has been used the core undergoes a further contraction and its T rises o some 109 K.

• Neon is produced by the fusion of carbon.

• When all Neon has been fused the core contracts again until it reaches a temperature at which Oxygen can be fused.

• Between each period of thermonuclear fusion in the core is a period of shell burning in the outer layers and the star enter a new red giant phase.


• When only shell burning is taking place the radius and luminosity of the star increases such that the result is a supergiant with a luminosity and radius much greater than that of a lower mass red giant.


• Eventually a temperature is reached in the core of a supergiant at which the fusion of silicon can take place. The product of burning silicone is iron.

• As seen in Nuclear Physics topic, elements with an atomic number of 26 or greater cannot undergo fusion (because of a large coulomb repulsion).

• As the fusion within the core cease, the star reaches a critical state.

• The entire inner core contracts very rapidly and reaches a very high T (≈6x109K)


• The high energy gamma photons emitted collide with the iron nuclei breaking it into alpha-particles

• The core becomes very dense and electrons combine with protons producing neutrons and a vast flux of neutrinos

• As these carry a large amount of energy, the core cools and contracts.

• The rapid contraction produces an outward moving pressure wave.

• Because some material from the shells is collapsing, a colossal shock wave will be formed and it will rip the material of the star’s outer layers apart.


• The inner cores are now exposed and a vast amount of radiation floods out into space.

• The star has become a supernova.

• The star releases about 1046 J of energy and 96% of its mass.

• This energy is enough to produce elements with atomic numbers higher than iron.


• The material that is flung out in to space will eventually form dark nebulae from which new stars may be formed.

• And so the process repeats itself!

“We are made of star stuff.”Carl Sagan

“For you are dust, And to dust you shall return." Genesis 3,19


• If the core is more massive than the Chandrasekhar limit, the core will collapse further until electrons are driven into protons, turning them into neutrons.

• Neutron pressure now keeps the star from collapsing further and the star has become a neutron star.

• If the core is substancially more massive than the Oppenheimer-Volkoff limit of about 2-3 solar masses, neutron pressure will not be enough to oppose the gravitational collapse and the star and the star will become a black hole.


E.5.6 Compare the fate of a red giant and a red supergiant.E.5.6 Compare the fate of a red giant and a red supergiant.

Protostar formed from interstellar dust and gas

Main sequence less than 4M

Red Giant carbon-

oxygen core

Planetary nebulae and white dwarf

Super red giant iron core

Main sequence between 4M and 8M

Main sequence 8M or greater

Red Giant oxygen-neon

core

Supernova

Neutron Star

E.5.6 Compare the fate of a red giant and a red supergiant.E.5.6 Compare the fate of a red giant and a red supergiant.

E.5.7 Draw evolutionary paths of stars on an HR diagram.E.5.7 Draw evolutionary paths of stars on an HR diagram.

• The Evolution of Starshttps://www.youtube.com/watch?v=uCz-uXRf4rA

<iframe width="420" height="315" src="https://www.youtube.com/embed/uCz-uXRf4rA" frameborder="0" allowfullscreen></iframe>

• Stephen Hawking - Supernovashttps://www.youtube.com/watch?v=tXV9mtY1AoI

<iframe width="560" height="315" src="https://www.youtube.com/embed/tXV9mtY1AoI" frameborder="0" allowfullscreen></iframe>

• The Universe - Life & Death of a Star Full Documentaryhttps://www.youtube.com/watch?v=NuXPAQOLato

<iframe width="560" height="315" src="https://www.youtube.com/embed/NuXPAQOLato" frameborder="0" allowfullscreen></iframe>

• The Evolution of Starshttps://www.youtube.com/watch?v=uCz-uXRf4rA

<iframe width="420" height="315" src="https://www.youtube.com/embed/uCz-uXRf4rA" frameborder="0" allowfullscreen></iframe>

• Stephen Hawking - Supernovashttps://www.youtube.com/watch?v=tXV9mtY1AoI

<iframe width="560" height="315" src="https://www.youtube.com/embed/tXV9mtY1AoI" frameborder="0" allowfullscreen></iframe>

• The Universe - Life & Death of a Star Full Documentaryhttps://www.youtube.com/watch?v=NuXPAQOLato

<iframe width="560" height="315" src="https://www.youtube.com/embed/NuXPAQOLato" frameborder="0" allowfullscreen></iframe>

https://www.youtube.com/watch?v=uCz-uXRf4rA



https://www.youtube.com/watch?v=tXV9mtY1AoI

https://www.youtube.com/watch?v=NuXPAQOLato





https://www.youtube.com/watch?v=tXV9mtY1AoI



• It is thought that a neutron star is the result of a supernova.

• Their are extremelly dense objects made mostly out of neutrons and emit electromagnetic radiation in the radio part of the spectrum (they can also radiate X-rays).

• In such a star it is now degeneracy that which stops further contraction. Such star would have a density of 4x1017 kg/m3 (a neutron star is so dense that one teaspoon - 5 mL - of its material would have a mass over 5×1012 kg).

• Rotating neutron stars emit electromagnetic waves (in the radio part of the spectrum and also X-rays) and are called pulsars.

E.5.8 Outline the characteristics of pulsars. E.5.8 Outline the characteristics of pulsars.

A pulsar has been found in the centre of Crab Nebulae (supernovae remnant) emitting a visible light pulse.

E.5.8 Outline the characteristics of pulsars. E.5.8 Outline the characteristics of pulsars.

QuasarsQuasars• A Quasar (contraction of QUASi-stellAR radio source) is an extremely bright (10

000x the Milky Way’s luminosity) and distant active galactic nucleus.• They were first identified as being high redshift sources of electromagnetic

energy, including radio waves and visible light that were point-like, similar to stars, rather than extended sources similar to galaxies.

• While there was initially some controversy over the nature of these objects, there is now a scientific consensus that a quasar is a compact halo of matter surrounding the central supermassive black hole of a young galaxy.

• Some quasars are as far as 4700Mpc which make them the most distant objects.

The Chandra X-ray image is of the quasar PKS 1127-145, a highly luminous source of X-rays and visible light about 10 billion light years from Earth. An enormous X-ray jet extends at least a million light years from the quasar.

http://en.wikipedia.org/wiki/Chandra_X-ray_Observatory

Black HolesBlack Holes• A black hole is a region of space in which the gravitational field is

so powerful that nothing can escape after having fallen past the event horizon.

• The name comes from the fact that even electromagnetic radiation is unable to escape, rendering the interior invisible.

• However, black holes can be detected if they interact with matter outside the event horizon, for example by drawing in gas from an orbiting star.

• The gas spirals inward, heating up to very high temperatures and emitting large amounts of radiation in the process.

• The point near the black hole where the escape velocity equals the speed of light is know as event horizon (or Schwarzschild radius): nothing can be seen beyond this point.

• When a dying star contracts within its event horizon the entire mass of the star will shrink to a mathematical point at which its density will be infinite. Such point is known as singularity.

• A black hole therefore consists of an event horizon and a singularity.

Black HolesBlack Holes

OPTION E - ASTROPHYSICS E5 Stellar Processes and Stellar Evolution AHL Nucleosynthesis.

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