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Stellar End-States… Now, my suspicion is that the Universe is not only queerer than we suppose, but queerer than we can suppose. J. B. S. Haldane (1892 – 1964) from Possible Worlds, 1927 WHAT DO YOU THINK? 1. Will the Sun someday cease to shine brightly? 2. What is a nova? What is a supernova? 3. Where does carbon, silicon, oxygen, iron, uranium, & other heavy elements come from? 4. What is a pulsar? Essay Questions for the exam… How will our Sun evolve as a star? What will its final state be? Compare its predicted evolution to that of higher-mass stars. How do they end? How do we know? Essay Questions for the exam… (a) What is a pulsar? Where does it get its energy? How do we know? (b) Describe a black hole. How do astronomers detect them if they give off no light? Stellar Evolution Building models of what happens to stars Low mass (0.08 to 0.4 Msun Medium mass (0.4 to 4+ Msun Higher Mass (5+ to >100 Msun Low mass star evolution (~8% to ~40% of our Sun’s Mass) Slower fusion reaction rate Low Luminosity Cooler surface temp (K & M type red stars) Longer Lives “Economy models!” Medium Mass 40% to 400% of Sun’s Mass Life like our Sun – about 10 Billion years The Evolution of 1Mo Star 90% of Life as “Main Sequence” star Fuses Hydrogen to Helium The Evolution of 1Mo Star 90% of Life as “Main Sequence” star Fuses Hydrogen to Helium He collects in core & builds up over time The Evolution of 1Mo Star 90% of Life as “Main Sequence” star Fuses Hydrogen to Helium He collects in core & builds up over time Gradually gets larger & larger The Evolution of 1Mo Star He core collapses, triggers expansion to Red Giant Fuses Helium to Carbon in Core Fuses Hydrogen to Helium in shell Creates dust grains in outer edges Stellar Model of a Sun-Like Star A red giant! The Evolution of 1Mo Star Not massive enough to fuse Carbon to heavier elements like Iron Fusion in core stops Forms a planetary nebula “death shroud” Core collapse finally stops as white dwarf Planetary Nebulae “Death Shrouds” of ejected gas surrounding collapsed white dwarf corpse (Not “planets”!) Planetary Nebulae “Death Shrouds” of ejected gas surrounding collapsed white dwarf corpse (Not planets!) Model of Planetary Nebula seen almost edge-on The Evolution of 1Mo Star Forms a planetary nebula Core collapse finally stops as white dwarf Stellar “corpse” is stable, tiny, hot…... Supported by electron degeneracy pressure Sirius & White Dwarf Sirius & White Dwarf In X- Rays Note better Resolution! What supports weight of 1Mo star? As it forms (a protostar) Thermal pressure < gravity! (collapsing!) Pressure depends on temperature What supports weight of 1Mo star? As it fuses H to He (a main-sequence star) Radiation/Gas pressures = gravity (stable!) Pressure depends on temperature What supports weight of 1Mo star? As it fuses He to C & H to He (a red giant star) Radiation/Gas pressures = gravity (stable!) Pressure depends on temperature What supports weight of 1Mo star? After fusion stops (beyond red-giant stage) Electrons to the rescue! Degeneracy Pressure (Pressure no longer depends on temperature) Degeneracy Pressure • “Two particles cannot occupy same space with same momentum (energy)” – Pauli exclusion principle • For very dense solids, electrons cannot all be in ground states Degeneracy Pressure • Electrons become VERY energetic--- velocities approach speed of light. • Pressure holding up star no longer depends on temperature. White Dwarfs Very dense; 0.5 - 1.4 M packed into a sphere the size of the Earth! White Dwarf Stars Stable! Gravitational pressure in = electron degeneracy pressure out Not fusing: Generates no new energy Cooling off: Radiates heat into space, getting fainter over time White Dwarf Stars • Degenerate matter obeys different laws of physics • More massive star => smaller core becomes! Limit on White Dwarf Mass Predicted gravity will overcome electron degeneracy pressure if white dwarf mass greater than 1.4 M Chandrasekhar Limit Subrahmanyan Chandrasekhar (1910-1995) Subrahmanyan Chandrasekhar Indeed, I would feel that an appreciation of the arts in a conscious, disciplined way might help one to do science better. Nova! Peak Brightness 2 months later 50,000 times dimmer! Nova! If close binary star system: Gravity pulls matter from nearby star Accretion disk around white dwarf Heats from Friction Infalling matter can possibly fuse! White Dwarf suddenly, temporarily gets much brighter…. Recurrent Novae SUPERNova (Type 1A) If close binary star system and One star is a white dwarf and Companion transfers enough mass… White Dwarf can exceed Chandresekhar limit…. White Dwarf Supernovae What if a star’s end-state core is even larger? Degeneracy applies to nuclear particles, too! Collapses until neutron degeneracy pressure holds up the corpse (neutron star) If even neutron degeneracy can’t support the weight of the core… Black Hole! High Mass Stellar Evolution Much greater fusion rate MUCH brighter stars MUCH, MUCH shorter lives High-Mass Stars – Ferraris! Stars 10x larger than our Sun Fuse faster! Shine brighter!! Live very short lives… But… Make every element in your body after Helium! Even Larger Stars –Ferraris! Evidence Supporting Theories Periodic Table Abundances Multiples Neutrinos from Supernova SN of “4” match Helium fusion chain 1987a caught “early” in explosion Cosmic Rays The Periodic Table of Elements! Periodic Table Abundances Periodic Table Abundances Atomic Masses H=1 He = 4 C = 12 O = 16 N = 20 Mg = 24 Si = 28 S = 32 Fe = 56 Supermassive stars lose mass even before they blow up! SUPERNova (Type II) ! SUPERNova! Why so powerful? Lighter layers of the star have no support once core stops fusing Top layers fall onto heavier center & recoil fast Collapse pushes electrons into protons => form NEUTRONS & Neutrinos SUPERNova! How long do they take to explode? For a 20-Solar Mass star: Formation: Main Sequence: Red giant fusing He: Fusing Carbon: Fusing Silicon to Iron: Supernova Explosion: 100,000 years 10 Million years 1 Million years 1000 years 1 day seconds! A star explodes ….. 170,000 Light Years Away A star explodes ….. 12 Million Light Years Away A star explodes ….. 21 Million Light Years Away A star explodes ….. 60 Million Light Years Away A star explodes … 10 BILLIon Light Years Away Neutron Stars • What is a neutron star? (THEORY) • What is a pulsar? (OBSERVATION) • What evidence do we have that they are one in the same? Neutron Star THEORY • Leftover cores from supernova explosions • Supported by neutron degeneracy pressure • Very TINY 1.5 M with a diameter of 10 to 20 km Chandra X-ray image of the neutron star left behind by a supernova observed in A.D. 386. The remnant is known as G11.20.3. Neutron Star THEORY • Very DENSE: (1012 g/cm3 ) & HOT • Very rapid Rotation: Period = 0.03 to 4 sec • VERY strong Magnetic fields: 1013 x Earth’s. Chandra X-ray image of the neutron star left behind by a supernova observed in A.D. 386. The remnant is known as G11.20.3. Neutron Star THEORY Discovery of 1st Pulsar In 1967, Jocelyn Bell & Anthony Hewish accidentally discovered a pulsing radio source Sharp pulse every 1.3 sec. ~1000 light years away Called it a “pulsar”, but what was it? The Crab Pulsar The mystery was solved when another pulsar was discovered in the heart of the Crab Nebula. The Crab pulsar also pulses in visual light. Pulsar Observations Very tiny pulse “width” Object must be extremely small. Even white dwarf is too large! Very regular pulse of energy Occasional “Glitches” in signal A few seen in X-ray binary systems High temperatures, large masses Pulsar Observations Synchotron emission --- radiation emitted by charged particles moving around strong magnetic fields signal oscillation in “new” Supernova Remnants Fastest Pulses slow down over time Process of Science! Theory 1. Tiny 2. Rotating Fast 3. Strong Magnetic Field 4. Dense, Massive 5. Supernova Corpse 6. Energy From Rotation Neutron Star = Pulsar!! Theory Observation 1. Tiny Small Pulse Width 2. Rotating Fast Regular Pulse up to 1000 times a second 3. Strong Magnetic Field Synchrotron Radiation 4. Dense, Massive X-ray Binary accretion disks surround pulsar 5. Supernova Corpse See in SN Remnants 6. Energy From Rotation Slow Down over time Pulsar Model Model of Pulsar as Rotating Neutron Star Pulsars vs. Neutron Stars? All pulsars are neutron stars, but all neutron stars are not pulsars!! Seeing a pulsar depends on geometry. if beam sweeps by Earth’s direction each rotation, neutron star appears to be a pulsar “Lighthouse” Model of Pulsar Pulsars are the lighthouses of Galaxy! Pulsars as Celestial Beacons Neutron Stars as Gamma Ray Bursters! We sometimes see incredibly powerful, and INCREDIBLY short bursts of gamma ray radiation. GRBs > 2 seconds ~ supernova and collapse to a black hole GRBs < 1 second ~ collision of TWO merging neutron stars? KEY “Key Terms” Chandrasekhar limit cosmic ray glitch helium shell flash helium shell fusion lighthouse model neutron degeneracy pressure neutron star nova (plural novae) nucleosynthesis planetary nebula pulsar supernova white dwarf X-ray burster