STARS AND THE HR DIAGRAM

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

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

HR DIAGRAM Effective Temperature

NEARBY STARS OR STARS IN A CLUSTER

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

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

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

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

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