Stellar Evolution Astronomy 315 Professor Lee Carkner Lecture 13

Changes  At first glance stars appear to be unchanging  This is an illusion due to the fact that stellar lifetimes are very long  Stars evolve, that is, their luminosity, temperature, radius, etc change over time  We can’t watch as any one star changes, so we have to examine different stars at different stages

Keys to Evolution  The evolution of a star is based on two basic things:  Hydrostatic Equilibrium  The star wants to be in a state where inward and outward forces balance  The star will change so that it can get back into hydrostatic equilibrium  Mass  Mass is the one property that does not change significantly  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  Heat is generated by fusion in the core  If the rate of energy generation in the core changes it will change the point at which hydrostatic equilibrium occurs  This is what causes stellar evolution

Evolutionary Paths  Each star follows an evolutionary path on the HR diagram  As the T and L for the star changes it changes position on the HR diagram  The exact path and the speed that the star moves along the path depends on mass  Most of the star’s life occurs on the main sequence where very little evolution occurs

Pre-Main Sequence Evolution  A cloud is in hydrostatic equilibrium until it is disturbed  H.E. is not reached again until fusion begins  PMS star is getting smaller and warmer as it contracts towards the zero age main sequence  Starts above the main sequence and moves down  Exact path depends on mass  Higher mass stars have larger T and L

Pre-Main Sequence Evolution

The Main Sequence  The star changes very little on the main sequence  As hydrogen is converted to helium the core gets a little denser and reactions speed up raising the luminosity  Stars slowly get a little more luminous during their main sequence lifetime  What happens when all the hydrogen is gone?

Main Sequence Evolution

Post Main Sequence  When the hydrogen is used up the fusion reactions in the core stop  The star then begins a series of other types of fusion reactions  These cause the star to go out of equilibrium  Star leaves the main sequence and becomes a giant

Mass and Evolution  High mass stars use their fuel up quickly and thus evolve off the main sequence quickly  Few million years  Low mass stars use their fuel up slowly and spend a long time on the man sequence  Billions of years  Main sequence lifetime (T) is inversely proportional to mass: T = 1/M 2.5

Cluster Evolution  Stars from in groups called clusters  All members of the cluster were born at the same time but have different masses  This means the cluster members will use their fuel up at different rates and leave the main sequence at different times  High mass stars first

Cluster Evolution

Cluster Ages  We can find the age of a cluster by looking at which stars have evolved off the main sequence  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 main sequence lifetime of the turn-off point is the age of the cluster

The Pleiades

NGC 3293

Evolution of a Cluster

Post Main Sequence Evolution  What happens to a star when the core becomes mostly helium?  Core becomes denser and contracts  Contraction releases heat, heat starts hydrogen fusion in the area around the core  Called the shell hydrogen burning  Star burns from the inside out

Above the Main Sequence  The contraction of the core and the hydrogen shell burning produce more energy than the star had before  This energy expands the outer layers of the star  Luminosity increases  The expansion cools the outer layers as well  Temperature decreases  The star moves up and right above the main sequence becoming a giant

Becoming a Giant

Structure of a Giant

Helium Burning  Eventually the helium core heats up enough to fuse helium into carbon and oxygen  In some stars this happens very rapidly in a helium flash  The burning of helium changes the equilibrium parameters again  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  This will cause the star to cool down and expand again  If the star is massive enough it will burn C and O into other elements  Fusion reactions get very complex and can produce elements up to iron (Fe)  This is where everything heavier than He comes from

The End  Eventually the star will be unable to burn any other elements and will die  Number of elements a star processes depends on mass  Elements end up in layers around the core  Material will be returned to the interstellar medium  A star spends only about 10% of its life as a giant (for solar mass star about 1 billion years)

Evolution of 1 Solar Mass Star

Evolution of a Solar Mass Star  H converted to He in core  He core contracts, H burns in shell, star expands and cools  He flash, star burns He in core, core expands, star contracts and cools  He converted into C and O  C and O core contracts, He and H burns in shell, star expands and cools  Contracting core and shell burning cause move to higher L lower T  Core burning causes move to lower L, higher T

Next Time  Read Chapter 14  Question of the Day:  How do stars die?  Monday April 15  List 2 due  Wednesday April 17  Quiz 2