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Lecture

Wednesday January 16th astro.docx

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Department
Astronomy
Course
ASTR 1P02
Professor
Bozidar Mitrovic
Semester
Winter

Description
Wednesday January 16 , 2013h Astro Lecture 5: - First Red Giant stage of a medium mass star (M= 0.5 M-8M) o Non-fusing He core which shrinks and heats up o Hydrogen is fused into helium (Hydrogen fusing shell) o Non-fusing hydrogen over layer - Because of its proximity to very hot He-core the rate of hydrogen fusion in hydrogen fusing shell is greater than the fusion rate during the main sequence phase - The energy transport to the surface cannot catch up with all that energy produced in the fusing shell and the thermal pressure builds in the interior of the star - This pressure causes the star to expand - Increase in the surface area causes the surface temperature to drop and its color changes from white – yellow to orange or red - When the temperature of the shrinking helium core increases to 100 million K the helium nuclei start fusing into C12 (and some O16) nuclei with the release of energy (so-called helium flash) - This fusion occurs via so-called triple alpha process o He4 + He4 + He4 (“Alpha particle” = nucleus of 4he)= C12 + gamma rays (energy) - C12 + He4 = O16 (8 protons and 8 neutrons) - The star has entered the yellow giant phase o He fusing core o H fusing shell o Non-fusing top layers - Now more energy is produced by fusing core and fusing shell, the star is again near the equilibrium and it shrinks in size - More energy producing and smaller surface imply higher surface temperature and the colour reaches yellow - Since the energy produced in He fusion is less (per fusion reaction) than in H fusion the rate of He “burning” must be high in order to produce enough energy to support the star - As a result this phase does not last very long (about 1 million years in the case of 1 solar mass star) - When all of the helium in the core is used the star enters the final red giant stage o Non-fusing carbon core (with some O16 nuclei) o He fusing shell o H fusing shell o Non-fusing top layers - The medium mass star does not have enough mass for the contracting carbon core to start fusing carbon into silicon,… What happens next? Friday January 18 , 2013 Remnants of a medium mass star: - planetary nebula and the white dwarf - planetary nebulae: their name comes from the fact that they often have blue- green color (i.e the ring nebula) similar to that of Uranus and Neptune, but they have nothing to do with planets - they are formed from the star’s material pushed out by radiation pressure (the central core is hot (10,000 – 30,000 k) and produces a lot of high energy UV photons) - the material in the ejected shell of gas glows due to fluorescence - White Dwarf is the dead core of the star o Consists of carbon and some oxygen nuclei in a sea of negatively charged electrons o Density of about 1000 kg/cm3 o The object emits light because it is still very hot (carbon and oxygen nuclei move with high speeds) - The gravitational collapse of this very dense object is prevented by the pressure created by degenerate electrons - So called Pauli principle: one can put at most 2 electrons into a given energy state - The degenerate matter such as white dwarf has some unusual properties o 1) The pressure depends only on density but not on temperature o 2) The bigger the mass the smaller the radius of the object (R= 1/3√ M) - The maximum mass that and object composed of degenerate matter can have is called chandrasekmass limit - For a white dwarf (composed of mainly C12 in a sea of degenerate electrons) is 1.4M o Note: more massive white dwarf has smaller radius - For the white dwarf that the sun will produce one day it is estimated that it would have a radius compared to that of earth - Many white dwarfs have been observed (i.e Sirius B) Monday January 21 , 2013 Astor lecture 7: Life story of a high mass star (M>8M): - every stage in life (protostar, main sequence) is shorter as a result of high mass (faster gravitational contraction, main sequence lifetime = M/L= (L=M3.5) 1/M2.5 - while on main sequence the hydrogen is fused He via so-called CNO cycle - once the hydrogen in the core is fused into helium, the fusion of helium into carbon starts right away since the temperature of the he-core is high enough because of the high mass of the star – the star becomes a yellow giant - once the helium in the core is fused into carbon, the high core temperature results in the fusion of carbon into heavier nuclei with release of energy - stellar nucleosynthesis: H-> He, He -> C, C->Ne Na Mg Al, Ne -> O Mg, O -> Si S Ar Ca, Si -> Fe (iron) = last stop, in all these reactions energy is released (exoteric) - to fuse nuclei heavier than iron (such as silver and gold) the energy needs to be supplied - the nucleus of iron has the largest binding energy per nucleon (proton or neutron) – it is the most stable nucleus - to fuse nuclei which are heavier than iron the energy must be supplied - the structure of the core of a massive red giant o iron core o Si fusion into iron o He fusion into carbon o H is fusing into He - Each successive fusion reaction releases less energy than the previous one - As a result the reaction rate speed up (more Si nuclei must be consumed per unit time than, say carbon nuclei) - As a result each phase in the fusion chain lasts a shorter period of time - Example: o M= 25SM  Fusion reaction: H -> He  Duration = 10 million years  He -> C  Duration = 1 mill
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