Class 17: Properties and evolution of stars Properties of stars Hertzsprung-Russell diagram Evolution of stars of various mass Supernovae and Hypernovae White dwarfs and neutron stars Black holes I : Stellar properties Stars have various… Luminosities Feeble star = 0.0001 x Lsun Powerful star =50,000 x Lsun Color Depends on surface temperature of star Cool star (RED) = 3000K Hot star (BLUE) = 30,000K Beta-Cygni Don’t bother copying… Stellar temperature/color also gives rise to “Spectral Classes” O (30,000K) B (20,000K) A (10,000K) F (7,000K) G (6,000K) – the sun! K (4,000K) M (3,000K) Knowing luminosity and temperature, we can calculate the radius using the “Stephan- Boltzmann law” L 4 R2T 4 LR T 2 4 Important points – =5.6710-8 W/m2/K4 is a number called the Stephan-Boltzmann constant Luminosity increases very rapidly with temperature (2 temperature gives 16 luminosity) and radius (2 radius gives 4 luminosity) II : The Hertzsprung-Russell diagram Very important plot in astronomy Plot of Luminosity against temperature for stars Find that stars fall into definite groups… III : Evolution of stars HR diagram gives clues to stellar evolution Main sequence Consists of stars living the “normal” part of their life… Producing energy via steady hydrogen burning (I.e. hydrogen converting into helium) Stars of different mass lie at different points on the main sequence Eventually, the hydrogen “fuel” runs out and the stars starts dying… it then leaves the main sequence. Life of a 0.05Msun “star” “Star” forms from rotating collapsing gas cloud (see formation of Solar System in class 3). Core heats up to few million K Trace Deuterium burns to form helium That’s it… Temperature never gets high enough to initiate hydrogen burning So never really becomes a proper star Object becomes a “brown dwarf” This is the case up to about 0.08Msun Evolution of the Sun Same beginning… cloud collapses This time, core is hot enough to initiate hydrogen burning (p-p chain) Steady hydrogen burning for 10 billion years (5 billion years more to go…) Then run out of hydrogen in core Nuclear reactions slow then stop Core gradually collapses; outer parts of Sun puff up tremendously – becomes Red Giant Series of explosions (Novae) then blow off outer layers of Red Giant Produces a “planetary nebula” Only the core of the star is left – becomes a white dwarf Evolution of a 10Msun star Star forms as before H-burning much faster (“CNO cycle”) only lasts few million years When H is exhausted, core contracts gets hot enough for helium-burning (makes Carbon) When He exhausted, core contracts and gets hot enough for carbon burning And so on… until the core is turned into Iron (the most stable element) Get shell or “onion” structure No more energy available when core becomes iron. Catastrophic core collapse… Core turns into neutron star Rest of star ejected in a supernova explosion Core-collapse (type-II) supernovae Very powerful explosion 1044 J released as radiation 100 more released in a neutrino pulse SN1987a (LMC) Neutron stars The remnant of the explosion… Typical mass of 1.5 Msun Typical radius of 10km Very dense – same density as atomic nuclei made of densely packed neutrons Extremely properties Very strong gravity on surface Very strong magnetic fields on surface Can spin very quickly (hundreds of times per second)… gives rise to pulsars.
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