Stellar Evolution Astronomy 315 Professor Lee Carkner Lecture 13

Changes   This is an illusion due to the fact that stellar lifetimes are very long   We can’t watch as any one star changes, so we have to examine different stars at different stages

Keys to Evolution  Stars change properties as they go through their lives  The evolution of a star is based on two basic things:    The star will change so that it can get back into hydrostatic equilibrium    The mass of the star determines the evolutionary path it will follow

Fusion and Evolution  The outward force for a star is the thermal pressure   If the rate of energy generation in the core changes it will change the point at which hydrostatic equilibrium occurs   Each star follows an evolutionary path on the HR diagram (as T and L change)

Pre-Main Sequence Evolution   H.E. is not reached again until fusion begins   Starts above the main sequence and moves down

The Main Sequence   As hydrogen is converted to helium the core gets a little denser and reactions speed up raising the luminosity   What happens when all the hydrogen is gone?

Post Main Sequence   The star then begins a series of other types of fusion reactions   Star leaves the main sequence and becomes a giant

Mass and Evolution   Few million years   Billions of years  Main sequence lifetime (T) is inversely proportional to mass: T = 1/M 2.5

Cluster Evolution   All members of the cluster were born at the same time but have different masses   High mass stars first

Cluster Evolution

Cluster Ages   The higher mass the stars the lower the age  The point at which the cluster diverges from the main sequence is called the turn-off point 

The Pleiades

NGC 3293

Evolution of a Cluster

Extrapolation  If A0 stars live for 440 million years and F0 stars live for 3 billion year, how long do A3 stars live?   3X10 9 – 4.40X10 8 = 2.56X10 9   2.56X10 9 /10 = 2.56X10 8   (2.56X10 8 )(3) = 7.68X10 8   (4.40X10 8 )(7.68X10 8 ) = 1.21X10 9 = 1.21 billion years

Post Main Sequence Evolution   Core becomes denser and contracts   Called the shell hydrogen burning  Star burns from the inside out

Above the Main Sequence   This energy expands the outer layers of the star   The expansion cools the outer layers as well   The star moves up and right above the main sequence becoming a giant

Becoming a Giant

Structure of a Giant

Helium Burning   In some stars this happens very rapidly in a helium flash   Star becomes hotter and less luminous as the core readjusts

Burning Other Elements  Helium burning happens very rapidly and soon the core is full of carbon and oxygen   If the star is massive enough it will burn C and O into other elements   This is where everything heavier than He comes from

The End   Number of elements a star processes depends on mass  Elements end up in layers around the core   A star spends only about 10% of its life as a giant (for solar mass star about 1 billion years)

7)Main Sequence  8)Red Giant  9)Helium Flash  Star rapidly burns He in core Evolution of a Solar Mass Star

10)Horizontal Branch  11)Asymptotic Giant Branch  C and O core contracts, He and H burns in shell, star expands and cools Evolution of a Solar Mass Star

Which Way Does the Star Go?  Up and to the right (8-9, 10-11)  Contracting core and shell burning cause move to higher L, lower T  Down and to the left (9-10) 

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