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Flashcards in D.4 Stellar Evolution Deck (15):

How do stars form?

When large clouds of dust collapse under their own weight.


What are the conditions for a star to form?

The temperature of the cloud must be low enough fro the clouds to collapse under their own weight, otherwise the random motion of molecules would prevent the collapse. Mathematically this conditions is known as the Jeans criterion.


What is Jeans Criterion?

For a cloud to collapse its gravitational potential energy must be larger than the total random energy of its molecules.
GM^2/R > 3/2NkT
where M is the mass of the cloud, R its radius, N its number of molecules in it and T the temperature.


Using N = M/m what does the Jeans criterion become?

M > 3/2 kTR/mG where m is the average mass of a molecule in the cloud. This so called protostar then takes its place eon the main sequence of the HR diagram.


What are the nuclear reactions on the main sequence?

Main sequence stars fuse hydrogen into helium via the proton proton cycle. For stars more massive than our sun there is also the so called CNO cycle in which helium is again produced from hydrogen through the intermediate production of carbon, oxygen and nitrogen. This requires higher temperatures and hence massive stars - massive stars have greater gravitational pressure that heats up the core.


What are the nuclear reactions off the main sequence?

The nuclear reactions taking place after a star leaves the main sequence depend on the mass of the star. For stars with mass between 0.25 and 8 solar masses carbon is produced in the triple alpha process:
4He + 4He -----> 8Be + Gamma
2 2 4

4He + 8Be -----> 12C + Gamma
2 4 6

For even higher mass stars the temperature in the core is higher and this allows the fusing of heavier elements. Oxygen is the next element produced followed by neon, sodium, magnesium and silicon. The process ends with the production of iron which is near the peak of the binding energy curve. This gives a star an onion layered strutter with the heaviest elements closest to the core.


How are the rest of the elements produced?

Neutron absorption by nuclei. When a nucleus absorbs a neutron it will turn into an isotope of the original nucleus. This isotope is usually unstable and will decay. They sue is whether there is nog time for this decay to occur before the isotope absorbs yet another neutron.


What is the s process?

In stars where the number of neutrons is small the isotope does have time to decay. The isotope will undergo a series of decays including beats decays. In beta decay the atomic number is increased by 1, thus producing a new element. The process accounts for the production of about half the nuclei above iron it ends with the production of bismuth 209.


What is the r process?

In the presence of very large number of neutrons nuclei that absorb neutrons do not have time to decay. They keep absorbing neutrons one by one forming very heavy neutron rich isotopes. This happens during a supernova explosion. These neutron rich isotopes are then hurled into space by the supernova where they can no undergo beta decay producing nuclei of even higher atomic number.


What is the other way to turn a neutron into a proton and increase the atomic number?

In supernova explosions massive numbers of neutrinos are produced a neutron may absorb a neutrino and turn into a proton according to the reaction
1 n + v -----> 1p + 0 e
0 1 -1


How can we estimate the time a star spends on the main sequence?

Using the mass luminosity relation
L = kM^3.5 = E/T
where E is the energy produced and T is the lifetime not he main sequence.
But E = 0.12Mc^2 and so kM^3.5 = 0.12Mc^2/T ----> T = (0.12c^2/k)M^-2.5 thus T = 1/M^2.5


What is a type II supernova?

The explosion of a massive red supergiant star.


What is a type Ia supernova?

The explosion of a white dwarf star as mass falls into it and pushes the mass past the Chandrasekhar limit.


What is important for Type 1a supernova?

Do not have hydrogen lines in their spectra
Are produced when a mass form a companion star accretes onto a white dwarf forcing it to exceed the Chandrasekhar limit.
Have a luminosity which falls off sharply after the explosion


What is important for Type II supernova?

Have hydrogen lines in their spectra
Are produced when a massive red supergiant star explodes
Have a luminosity which falls off gently after the explosion.