9 Stellar Evolution Where do gold earrings come from?

9 Goals Explain why stars evolve off the main sequence. What happens when they leave the main sequence? How does mass affect what happens? How do stars die?

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9 The Main Sequence Stars characterized by what holds them up. 90% held up by heat of Hydrogen fusion 4H  He + Energy

9 M.S. Lifetime More Massive  Hotter Hotter  More luminous More luminous  shorter life

9 M13 M13 – Natalie Redfield ‘06

9 Age of M13 12 billion years old

9 The Main Sequence A star is a delicate balance between the force of gravity pulling in, and pressure pushing out. Stars on the main sequence fuse hydrogen in their core to produce thermal pressure. Longest phase of a star’s life.

9 What then? When the hydrogen in the core is almost consumed the balance between gravity thermal pressure pushing out and gravity pushing in is disturbed. The structure and appearance of the star changes dramatically. What happens then, depends on the star’s mass. Two cases: –Low-mass (< 8 x mass of Sun) –High-mass (> 8 x mass of Sun)

9 Helium Ash Heavier elements, sink to the “bottom.” After 10 billion years, core is “choked” with helium “ash”. H  He continues in shell around non-burning core.

9 The Red Giant Branch Without fusion pressure in core: –Helium core collapses (no counter to gravity) –Density in core increases. 3He  C + Energy in core 4H  He + Energy in shell Extra energy results in extra pressure. Star expands. The star gets bigger while its outside gets cooler.

9 The Onion Sun Red Giant Stars Layers of: –Non-fusing H –Fusing H –Fusing He –Non-fusing C “ash”

9 …And the Solar System? A few million years from now: –Sun becomes slightly brighter –Ocean’s begin to evaporate –“Hot House” Earth A few billion years from now: –Sun swells up –Engulfs the inner Solar System –Certain death for terrestrial planets –Possible “spring” on the Jovian ocean-moons!

9 Red Supergiant What happens when the Sun runs out of helium in its core? Same as before. Core shrinks, surface expands. Radius ~ 3 AU!

9 Death Core is contracting and heating. –Surface is cooling and expanding. Will it finally become hot enough in core for Carbon to fuse? For the Sun: No. Gravity keeps contracting the core: 1000 kg/cm 3 ! What stops it? Electron degeneracy pressure!

9 Electron Degeneracy Pressure from motion of atoms

9 Electron Degeneracy Pressure from electron shells

9 NGC3242 – HST – Bruce Balick

9 M57 – Ring Nebula

9 M27 – Dumbbell Nebula – copyright VLT, ESO

9 Cat’s Eye

9 Eskimo Nebula

9 Hourglass Nebula

9 NGC2440 – HST – Bruce Balick White Dwarf Mass of Sun Radius of Earth Hot as Sun’s core A million times denser than lead Slowly cool off

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9 High-Mass Stars Think back to the first carbon core. How they get from main sequence to the carbon core stage is a little different. Now however, there is enough mass that it becomes hot enough to fuse carbon? Hot enough to eventually fuse lots of elements.

9 The Iron Core 4H  He + Energy 3He  C + Energy C + He  O + Energy The ash of one reaction, becomes the fuel of the next. Fusion takes place in the core as long as the end result also yields energy. This energy causes pressure which counters gravity. But Iron doesn’t fuse.

9 Core-Collapse Iron core – no outward pressure. Gravity wins! Star collapses rapidly! Electron degeneracy can’t stop it. Atomic structure can’t stop it. Electrons and protons crushed together to produce neutrons. Neutrons pushed together by force of gravity.

9 Supernova

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9 The result of the catastrophic collapse is the rebound and explosion of the core. From start of collapse to now: 1 second! Matter thrown back into the interstellar medium. Matter rushing outwards, fuses with matter rushing inwards. Every element after Fe is made in the instant of a supernova!

9 M1 – Crab Nebula – copyright VLT

9 Veil Nebula – Lua Gregory (English ’05)

9 NGC 4526 – 6 Million parsecs away

9 Neutron Stars A giant ball of neutrons. Mass : at least 1.4 x mass of the Sun. Diameter: 20 km! Density: kg/m 3 –A thimble weighs as much as a mountain Day: 1 – seconds! Magnetic fields as strong as the Sun, but in the space of a city.

9 Pulsars Interstellar Lighthouses. See periodic bursts of radiation. Perfect clocks. While every pulsar is a neutron star, the opposite isn’t true.

9 Crab Nebula Pulsar

9 Neutron Degeneracy Neutron stars are held up by neutron degeneracy pressure. –Recall electron degeneracy pressure for white dwarfs. –For white dwarfs, maximum mass of 1.4 M sun For neutron stars, maximum mass ~3M sun What happens if a high-mass star is SO big that its central core is bigger than this? What happens when gravity is stronger than even neutron degeneracy pressure?

9 Density Density = mass per volume From Red Giant cores to White Dwarfs to Neutron Stars, density has been increasing. As density increases, the force of gravity on the surface increases. The greater the force, the higher the escape velocity: –How fast you need to go in order to escape the surface. How dense can something get? How strong can the force of gravity be? What if the escape velocity is faster than light?

9 Singularity When a high-mass star’s core is greater than ~3 x M sun, then, when it collapses, neutron degeneracy pressure can’t balance gravity. The star collapses to form a singularity. No size at all. Density infinite. Escape velocity > c

9 Black Hole Diagram. Singularity Event Horizon Schwarzschild Radius

9 Seeing Holes Can’t see black hole itself, but can see matter falling into a hole. Gravitational forces stretch and rip matter: heats up. Very hot objects emit in X-rays (interior of Sun) Cygnus X-1.

9 Binaries Gravitational tides pull matter off big low density objects towards small high density objects. Cygnus X-1