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The Most Astonishing Fact About the Universe: The Life Cycle of Stars The Sun Andrew Rivers, Northwestern University Range of star properties dramatized: Which is the closest star in this picture? Alpha Centauri and companions Beta and Proxima Centauri. The stars are at similar distances; Proxima is closest, but is M-class and therefore far fainter. Where does a star’s energy come from? • How much energy? – 3.901026 Watts for the sun – Largest earth power plants=5 109 Watts. • Possibilities – Chemical reactions? • Most efficient: 2H + O = H20 +energy. • Sun would last 18,000 years. – Gravitational “settling”. • If Sun still contracting, convert PE to KE to light • 30 million years worth of energy! • Contradicts biology, geology, astronomy Solution: Nuclear Fusion • Energy by combining light nuclei like hydrogen to make heavy nuclei. – In the sun 4 Hydrogen nuclei are fused into a helium nucleus: Efficiency 0.08% • Lifetime 10 billion years= O.K. Why is fusion of two nuclei difficult? long range, repulsive Coulomb force Short range, attractive strong nuclear force Charged nuclei must get close enough, then SN force can overcome Coulomb repulsion How do two positive nuclei get close enough for fusion? Like charges repel If traveling fast enough, they get close Attractive SN force overcomes repulsive Coulomb force Foundation Principle 1: Power from Fusion P-P chain, Fusion in Main Sequence stars http://www.cartage.org.lb/en/themes/sciences/Astronomy/ Binding Energy Curve Iron (Fe) most stable element As more nucleons are in nucleus, they are more tightly bound by SN force Energy released through Fusion Foundation Principle 2: Hydrostatic Equilibrium Balance Fusion in core = outward pressure http://www.cartage.org.lb/en/themes/sciences/Astronomy/ Image credit: NASA / CXC / M. Weiss. Hydrostatic Equilibrium: outward photon pressure from fusion reactions in core balances inward gravitational pressure. Mass Luminosity Law Principle: Hydrostatic Balance Principle: Fusion in Core Larger mass stars have greater internal pressure from gravity Fusion rate must be greater for more massive stars in order to balance greater inward gravity pressure. More massive stars have a greater fusion burn rate (luminosity) Why are more massive stars brighter? Conclusion: Though more massive stars have more nuclear fuel (M), they have greater burn rates (L) and therefore have shorter lifetimes Tool of the Trade: Hertzsprung-Russell Diagram Purpose: classify stars, explore stellar evolution Where do we expect to find stars on this plot? Anywhere? In some places and not others? Regions of the HR diagram, from http://zebu.uoregon.edu/~soper/Stars/hrdiagram.html Recall: Stars are approximate Blackbodies A Star’s color depends on its surface temperature. Cooler=redder, hotter=bluer A cool, bright star Some faint, hot stars HR diagram “scatter-plot of the nearest stars. http://www.vb-tech.co.za/astronomy/From http://zebu.uoregon.edu/~soper/Stars/hrdiagram.html 25 closest stars, from “O” Stars “M” Stars Physical Basis for Stellar Evolution • Hydrostatic equilibrium – Inward gravitational pressure and outward pressure (usually radiation pressure) must balance in any stable star – Gravity, though the weakest force is always attractive and omnipresent. • If there is no outward pressure, the star must collapse. • “The War Against Gravity” Why should stars have a “life cycle”? • Only set amount of Hydrogen gas to use in nuclear fusion. – Must find some other way to counteract gravitational pressure • Initial mass determines how quickly fuel will burn (The luminosity) to maintain equilibrium – L~M3.5 More massive stars, even with more fuel to start should burn quicker Life-story of a Sun-like Star, Stage 1: Formation from a molecular cloud/solar nebula Stage 4 Stage 1 Molecular cloud Stage 2 Stage 3 Protostar Main-sequence Red giant Stage 5 Stage 6 Planetary nebula White dwarf • Raw material for star formation strewn throughout the galaxy – Giant Molecular Clouds. Cool clouds of H2 (molecular hydrogen) and some CO. – When do they collapse? • Pressure wave from Supernova or or collision between clouds or other compressing event.. • Gas must be slow moving (little KE) to collapse Observation: Dust in Molecular Clouds Blocks Visible Radiation Reddening: red wavelengths pass through dust more readily than blue. Star forming regions often cold (easier to collapse) therefore they do not radiate visible light (dark clouds). Molecular Cloud Barnard 68 Observation: Molecular Clouds from the Spitzer Infrared Space Telescope Optical Longer wavelength IR not blocked by dust How Can We See Molecular Clouds? The Observation: Carbon Monoxide emission line in the Milky Way Emission Line Spectrum The Physics: Molecules have rotational transition, releasing radio waves Life Story Stage 2: Protostar Stage 1 Molecular cloud Stage 2 Protostar • Fusion has not yet begun, but clump of gas has condensed. – Radiates in red and infrared (Temperature is 2000 to 3000K) – Optical light is blocked by dust – Infrared gets through (wavelength large compared to size of dust grains. – Protostars only around for short time (few million years) Protostar stage: forming planets Proto-planet accretion To final solar system Proto-planets Condensing into dust particles which build up Collapsing gas & dust cloud “pancaking” Artist’s rendition of a forming proto-planetary disk with newborn planets Life Story, Stage 3: The Main Sequence Stage 1 Molecular cloud Stage 2 Stage 3 Protostar Main-sequence • As cloud collapses, temp rises in the core until fusion is possible – When fusion “turns on” the protostar becomes a star. – The stars stay on the main sequence for 90% of their lifetimes – Stars form in groups (open clusters) Q: What do all stars on the main sequence have in common? A: PP-Chain. All are burning Hydrogen into Helium in their cores. NGC 3603 Star Birth Sequence Q: Why are the stars we see in this newborn cluster all blue? Consider the Blackbody spectrum. Which stars are we biased to see? Sun Formation Life-story told by HR diagram Evolution of the sun onto the Main Sequence Pleiades Image Credit & Copyright: Rogelio Bernal Andreo Q: Which cluster is older? Hyades HR Diagram of Pleiades Cluster Bright, blue O stars Faint, red M stars Sun Life Story Stage 4: Red Giant Expansion Stage 1 Molecular cloud Stage 2 Stage 3 Protostar Main-sequence Stage 4 Red giant • Star runs out of hydrogen fuel in core – Gravity doesn’t go away – Star collapses and heats up – Core inert, Shell burning begins Artist’s rendition of Earth’s future with red giant Sun • Fusion is rapid because the shell layer is still compressing • Luminosity of star increases • Radius of star increases, becoming giant Shell Hydrogen burning after core of Hydrogen has been exhausted. New source of fusion means the core of the star becomes hotter causing the star size to grow. • As core temp rises, Helium atoms eventually reach speeds required for Helium fusion • Sudden onset Helium flash – Extra outward pressure from the core star expands – Hydrostatic equilibrium means that the star will then contract – Variable stars: the pulsations are not damped, but periodic. Hydrogen shell burning Temp rises, Helium ignites in core Triple Helium fusion process Q: Why is He fusion harder, than H fusion, requiring greater temperatures? Life Story, Stage 5: Red Giant rundown Stage 1 Molecular cloud Stage 2 Stage 3 Protostar Main-sequence Stage 4 Red giant Stage 5 Planetary nebula • What happens when no more Helium? – Star center compresses, Burn Carbon + Helium= Oxygen • Planetary Nebula – Low mass stars the increased number of photons from core push on outer carbon/silicon in layers puff off the star – Much of mass returned to Interstellar Medium and is heated by the star Ring Nebula as seen by the Hubble Space Telescope. Image Credit: NASA, ESA, and C.R. O'Dell (Vanderbilt University) Cats Eye Nebula Image Credit: NASA,ESA, HEIC, Hubble Heritage Team Pictorial story of the evolution of Sun from proto-star birth to white dwarf cool-down. Story of the sun as told on an HR diagram. Note that the longest time in the normal evolution is on the Main Sequence An HR diagram story of the evolution of the sun after the main sequence. Theoretical evolution of a star cluster: Birth to Main Sequence Theoretical evolution of a star cluster: Main Sequence to Red Giants Theoretical evolution of a star cluster: Red Giants and the Turn-off Point Stage 4B: The Alternate Ending Red Giant rundown, high mass Stage 2 Molecular cloud Protostar Stage 3 MS Blue Giant • Running out of fuel Red giant – Star center compresses – Burn Carbon + Helium to get Oxygen – Shell burning with other reactions • Reactions become less efficient. – Hydrogen burning most efficient – Other reactions less so – Iron=most stable element, no energy available • But Gravity remains! Shell burning in a star at the end of its days ( Freedman Geller & Kaufmann, Universe) Recall: Binding Energy Curve Iron (Fe) most stable element As more nucleons are in nucleus, they are more tightly bound by SN force Energy released through Fusion Less tightly bound as nuclei get bigger, short range SN not as effective Image credit: Wikipedia user Sakurambo. Supernova Molecular cloud Stage 4 Stage 2 Protostar Stage 3 MS Blue Giant Red giant Supernova • Iron core No more reactions can produce energy to hold out core • Star begins to collapse due to gravity • What stops the collapse? – Electron degeneracy pressure – Electrons resist when we try to place them in the same place (not the same thing as electrostatic repulsion) – As soon as the collapsing core reaches the density where electrons “see” each other, the star becomes stable and stops collapsing • Inner layers are still moving inward and hit the “solid wall” of the new white dwarf and…. • Bounce! • The supernova emits as much energy per second as all stars in the galaxy combined (for awhile). Supernova 1987A: Closest supernova during age of telescopes (in Large Magellanic Cloud) Before….. After! Supernova 1994D outshines its entire host galaxy Life Story end-game: White Dwarf stars Stage 4 Stage 1 Molecular cloud Stage 2 Stage 3 Protostar Main-sequence Red giant Stage 5 Stage 6 Planetary nebula White dwarf • A white dwarf star remains after the supernova and as a product of evolution of low mass stars • Theoretical calculations the maximum white dwarf = 1.4Msun – Chandrasekar mass • What then? The Old open cluster NGC 6971. The current HR diagram reveals age and stages of evolution. Fighting the War Against Gravity • Normal Stars – Fusion Hydrogen Helium +energy. Outward pressure from escaping photons balances gravity • Massive stars, after hydrogen – Fusion of heavier elements, pressure is balanced same as above – Iron is the most stable, no energy available to maintain equilibrium through fusion • White Dwarf stars – Gravitational force balanced by “electron degeneracy pressure” Degeneracy pressure • Fermions (electrons, protons, neutrons) cannot occupy the same energy level • If all available energy states are filled, an electron must go to a higher energy level – this takes work! The gas resists compression A White Dwarf Star Summary: Life-story of Stars Stage 1 Molecular cloud Stage 2 Stage 3 Protostar Main-sequence Life story of sun told as a track on the H-R diagram Stage 4 Red giant Stage 5 Stage 6 Planetary nebula White dwarf High mass stars have different HR tracks and can make heavier elements before going supernova. What is this picture? Jocelyn Bell Burnell, first to notice a radio source, identified as a pulsar Fast rotating neutron star generates strong magnetic field and a “flashlight beamed” periodic source of radio waves, first known as LGM’s Pulsars can rotate at a rate of approx 1000 times per second Alternative Ending: Neutron Stars A rapidly rotating neutron star lies at the heart of the crab nebula The Stellar Evolution Cycle Summary of star evolution stages Grand Unification We are stardust! All heavy elements are cooked in stars Spread New stars through formgalaxy from the by rich supernovae ashes