STARS AND THE HR DIAGRAM
STAR IS A LARGE SELF-LUMINOUS, GASEOUS SPHERE IN STEADY STATE EQULIBRIUM
STELLAR STRUCTURE CONTROLLED BY1. HYDROSTATIC EQUILIBRIUM – PRESSURE BALANCES GRAVITY AT EVERY LEVEL –P=nkBT – IDEAL GAS LAW2. ENERGY TRANSFER – HEAT GENERATED INSIDE ISTRANSPORTED AT THE SAME RATE OUTSIDE
a. Radiative transportb. Convective transportc. Heat conduction
3. ENERGY GENERATION – FUSION a. Efficiency 7x10-3 mc2
b. p-p cycle – ignition 5x106Kc. CNO cycle – ignition 2x107 K
Stability of Stars• Reaction rate ~ T4 for proton-proton chain and T15 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 shrinkstemperature 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=4R2T4
AssumeR=constantAll stars have the size of the Sunbut some are hotterand some colder
Test R=constant Hypothesis
If hypothesis OKall data onblue line
Hypothesis wrongBut..
We learned thatHotter are bigColder are small
Mass-Luminosity Diagram
TheoryL~M3
Use this tolabel HRdiagram
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~Mp p=3-4t~1/Mp-1 1/M2-3
Star with 5 solar masses has a lifetime 625 times smaller than Sun
