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?

Evolution of 1 Solar Mass Star

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

IC HST

Mz3 -- HST

Ring Nebula -- HST

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

Sirius A and B

High Mass Stars    Star will become a supergiant with a huge radius (up to 5 AU) but most of its mass in a small earth-sized core of layered elements

Evolutionary Paths

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

Supernova 1987a -- Before & After

Supernova 1987a -- Remnant

Anasazi Depiction of 1054 SN?

Crab Nebula -- Optical & X-ray

Post Main Sequence Paths

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 

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