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