Presentation on theme: "Stellar Death Astronomy 315 Professor Lee Carkner Lecture 14 “I am glad we do not have to try to kill the stars. … Imagine if a man each day should have."— Presentation transcript:
Stellar Death Astronomy 315 Professor Lee Carkner Lecture 14 “I am glad we do not have to try to kill the stars. … Imagine if a man each day should have to try to kill the sun? We were born lucky” --Earnest Hemingway, The Old Man and the Sea
Death Defined The star can no longer support itself by internal thermal pressure and so: The details depend on mass
Very Low Mass Red dwarfs (M < 0.4 M sun ) burn their fuel very slowly Take a very long time (10’s of billions of years) to use up all hydrogen Red dwarfs will fade away as they run out of fuel Never become giants since they produce no helium core
Solar Type Stars with between 0.4 and 4 M sun go through the following phases: Hydrogen and helium shell burning (asymptotic giant branch) What happens next?
Mass Loss All stars lose mass Mass loss is very low for main sequence stars Giants have higher mass loss rates, due to: Thermal pulses: changes in the core that cause bursts of energy which can push the outer layers away
Separation Core gets denser, outer layers get less dense If the core is hot enough, its radiation will make the ejected outer layers glow
Planetary Nebulae These glowing ejecta are known as planetary nebulae Have nothing to do with planets Composition: low density gas producing emission lines
Structure of Planetary Nebulae We would expect planetary nebulae to be spherical How does spherical star eject mater into a non-spherical shape? Blocked by companion stars or planets? Different waves of ejecta interacting?
White Dwarf The leftover core of the star becomes a white dwarf There is no fusion going on in a white dwarf so it slowly cools What supports a white dwarf?
Degeneracy Electrons obey the laws of quantum physics including the Pauli Exclusion Principle: Due to its high pressure the core becomes degenerate Degenerate gas resists compression because electrons cannot be forced any closer together due to the Pauli exclusion principle
White Dwarf Properties White dwarfs are very dense Start out hot and then cool White dwarfs obey the Chandrasekhar Limit Must be less than 1.4 M sun, or they cannot be supported by electron degeneracy pressure
Core Collapse In a short time (million years or less) the star burns through all elements up to iron There is no more thermal energy to support the very dense core Energy from the collapsing core rebounds to produce a supernova
Supernova A nova is a generic term for a sudden brightening of a star An exploding massive star is technically known as a Type II supernova Explosion is almost a billion times more luminous than the sun Leaves behind a supernova remnant
Stellar Corpses After a supernova (or the planetary nebula phase) the core of the star gets left behind Low and medium mass stars leave white dwarfs Higher mass stars produce neutron stars Very high mass stars produce black holes