VariableStarsPulsation,Evolutionandapplicationsto解析.ppt

Variable Stars: Pulsation, Evolution and applications to Cosmology Shashi M. Kanbur SUNY Oswego, June 2007 Stellar Evolution Recall equations of stellar structure. Nuclear energy generation rate ε(ρ,T) slowly changes the composition decreasing the amount of Hydrogen and increasing the amount of Helium on a nuclear time scale – the characteristic time for stellar properties to change as a result of nuclear burning: tnuc ~ 1010M/L years. This timescale is much longer than those involved in stellar pulsation. So solve equations of stellar structure, get structure of star at t=0. Δt ~ tnuc, so X, Y, Z change due to nuclear burning. Find X(t+Δt), Y(t+Δt), Z(t+Δt), recalculate stellar stucture with these new values. Now there is a new ε = ε(ρ,T); recompute changes to X, Y, Z over the next time interval Δt. Continue and develop a stellar evolutionary track. Hertzsprung –Russell Diagram Plot of T on X axis against L on Y axis. Or, plot of color on X axis against magnitude on Y axis. At each t, star has a certain surface temperature and luminosity. Plot of (T,L) or ((B-V),V) as a function of time is a stellar evolutionary track on a HR diagram. HR diagram is the most important diagram in Astronomy. Aspects of the HR diagram Main Sequence (ms): all stars are nuclear burning hydrogen to helium in their core. Mass increases as you go up the ms, but stars do not travel “too much” on the ms. Upper ms: high mass, hot stars (why?), convective core, radiative envelope, H burning by CNO cycle: M 4-8Msun Lower ms: low mass, cooler stars (why?), radiative core, convective envelope, H burning by proton-proton chain: M 4 Msun. Sun is a lower main sequence star. Partially understand in terms of Stefan Boltzmann law: L ~ R2T4 Luminosity Classes and Spectral Types Luminsoity Class I: Supergiants, II: Bright giants, III: Giants, IV: Subgiants, V: Main Sequence Spectral Type: classifcation of stellar spectra, essentially a temperature sequence, OBAFGKM Sun is a G2V, Betelgeuse is a M5V Exe