1. STARS AND THE HR DIAGRAM

2. STAR IS A LARGE SELF-LUMINOUS, GASEOUS SPHERE IN STEADY STATE EQULIBRIUM STELLAR STRUCTURE CONTROLLED BY 1.HYDROSTATIC EQUILIBRIUM – PRESSURE BALANCES GRAVITY AT EVERY LEVEL –P=nk B T – IDEAL GAS LAW 2.ENERGY TRANSFER – HEAT GENERATED INSIDE IS TRANSPORTED AT THE SAME RATE OUTSIDE a.Radiative transport b.Convective transport c.Heat conduction 3.ENERGY GENERATION – FUSION a.Efficiency 7x10 -3 mc 2 b.p-p cycle – ignition 5x10 6 K c.CNO cycle – ignition 2x10 7 K

8. Stability of Stars Reaction rate ~ T 4 for proton-proton chain and T 15 for CNO chain If fusion rate speeds up for some reason would the star explode ? Thermal pressure increases  Star expands till gravity balances thermal pressure ; Expansion lowers temperature  reduces fusion rate  pressure decreases  Stars shrinks  temperature increases…. Stability is always established between nuclear reaction rates and gravity compression. Similar if fusion rate slows down Cepheid variables

10. HR DIAGRAM Effective Temperature

11. NEARBY STARS OR STARS IN A CLUSTER

12. Temperature, Size, Luminosity L=4  R 2  T 4 Assume R=constant All stars have the size of the Sun but some are hotter and some colder

13. Test R=constant Hypothesis If hypothesis OK all data on blue line Hypothesis wrong But.. We learned that Hotter are big Colder are small

14. Mass-Luminosity Diagram Theory L~M 3 Use this to label HR diagram Measure Mass of Star independently – How?

16. Stars have different masses Stars with same mass have similar properties The more massive a star the more luminous Most important property in Star is Mass – composition a smaller factor

17. Conclusions Stars spend most of their lifetime in main sequence During their lifetime T and L almost constant Something else for birth and death More Mass Hotter More luminous Bigger t~M/L L~M p p=3-4 t~1/M p-1 1/M 2-3 Star with 5 solar masses has a lifetime 625 times smaller than Sun