Class Notes (835,872)
Canada (509,458)
Brock University (12,091)
Astronomy (161)
ASTR 1P02 (68)

Wednesday January 16th astro.docx

6 Pages
Unlock Document

Bozidar Mitrovic

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
More Less

Related notes for ASTR 1P02

Log In


Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.