E5 Stellar Processes and Stellar Evolution

8/3/2019 E5 Stellar Processes and Stellar Evolution

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E5 stellar processes and stellar

evolution (HL only)


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Star formation


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Star formation

Interstellar space consists of gas (74% H, 25% He

by mass) and dust at a density of about 10-21

kg.m-3. This is about one hydrogen atom to every

cm3 of space.


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Star formation

When the gravitational energy of a given mass of

gas exceeds the average kinetic energy of the

molescules the gas cloud becomes unstable and

starts to collapse.

GM2/R > (3/2)NkT

Jeans criterion


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Star formation

As the cloud collapses, the particles get faster

and eventually clumps form that are hot enough

to emit light. Protostars are formed.


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Star formation

If the star is big enough the collapse will

continue until the star is hot enough for nuclear

fusion to occur. The radiation pressure produced

by the fusion balances the pull of gravity and

equilibrium is reached. The star is a main

sequence star (like our sun).


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Main sequence

41H 4He + 2e+ + 2 + 2e (26.7 MeV)


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Mass v luminosity relation

L Mwhere 3 < > 4


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Mass v luminosity relation

Since the luminosity could be the total energy

given out by the star (E) divided by the

lifetime of the star T we get

E/T M

Since E = Mc2 from Einsteins formula

Mc2

/T M

T M1-

Taking = 4 we get T M-3


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Lifetime of a star

T M-3The bigger the mass of a star, the shorter

its life (it burns out quicker)

A star with a mass 10x greater than the sun will

have a life time a factor 10-3 (1/1000) less than

the sun


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When the hydrogen runs out?


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Schnberg Chandrasekhar limit

After the star has used up about 12% of its

hydrogen, its core will contract but the outer

layers will expand substantially ()fusion

continues there). The star leaves the main

sequence and moves over to the Red Giant

branch


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Mstar < 0.25Msun

No further nuclear reactions

Core stays as Helium

After a Red giant it becomes a White Dwarf


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0.25Msun < Mstar < 4Msun

Core temperature reaches 108 K enabling

Helium fusion (higher temperature is needed

because Helium nuclei have 2 positive

charges)

Helium fuses to form oxygen and carbon

After a Red gaint a White Dwarf with a

carbon/oxygen core is formed


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4Msun < Mstar < 8Msun

Core temperature rises further enabling the

fusion of carbon and oxygen to take place

producing a core of oxygen, neon and

magnesium

After a Red giant a White Dwarf with an

oxygen/neon/magnesium core is formed


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8Msun < Mstar

Core temperature rises further so heavier

elements fuse. Helium in the outer layers

continues to fuse too. Eventually iron is

produced (which does not fuse see topic 7)

This is a RED SUPERGIANT

Will eventually become a NEUTRON STAR


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Anatomy of a RED SUPERGIANT

and neon


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Evolution of stars < 8Msun

Core contracts under its own weight

It stops when electrons have to be forced into

the same quantum state. This is not allowedso this electron degeneracy pressure stops

the star collapsing further

The outer layers are released to form a

planetary nebula

The resultant White dwarfhas no energy

source so is doomed to cool down to become

a Black dwarf.


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Evolution of stars > 8Msun

If the core is above 1.4 solar masses (the

Chandrasekhar limit) Electrons are forced into

protons producing neutrons.

The core is only made of neutrons and

contracting rapidly.


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The neutrons get too close to each other (this

time it is neutron degeneracy pressure

caused by neutrons not being allowed to

occupy the same quantum state) and the

entire core rebounds to a larger equilibrium

size.

The causes a catastophic shock wave whichexplodes the star in a SUPERNOVA


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The neutron star left over after the supernova

remains stable provided its has a mass of no

more than 3 solar masses (the Oppenheimer-

Volkoff limit)


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Neutron stars with masses substantially more

than the Oppenheimer-Volkoff limit continue

to collapse as the neutron pressure is

insufficient. They become Black holes

At the centre of the black hole is a singularity

The boundary around the singularity where

even light does not have sufficient escape

velocity to escape is called the event horizon

or gravitational radius.


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Stellar evolution


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Evolution of stars on the HR diagram


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Evolution of stars on the HR diagram


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Pulsars Another very important property of neutron star is

its strong magnetic field. When electrons move in

spirals around magnetic lines of force, radio waves

are produced and radiated out along the two

magnetic poles of the star.


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Pulsars Usually, the rotational axis of the neutron star does

not align with the magnetic axis. The radiation

beams will sweep around and create the light house

effect. What we observe on Earth will be pulses of

radio wave with very stable period. This is a pulsar.

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E5 Stellar Processes and Stellar Evolution