Transcript Week 11
Birth and Death in Stars Evidence of Star Formation • forming stars are usually embedded in clouds Evidence of Star Formation • forming stars are usually embedded in clouds The Life of a Star PHASES: WHAT CHANGES THINGS: GAS CLOUD • gravity pulls part of cloud together PROTOSTAR • nuclear reactions begin in star’s core MAIN SEQUENCE STAR • hydrogen fuel runs out in center of star LOW MASS HIGH MASS RED GIANT PLANETARY NEBULA WHITE DWARF SUPERGIANT • other fuels run out in center of star SUPERNOVA NEUTRON STAR BLACK HOLE . Luminosity The HR Diagram • position in diagram tells us what stage of life a star is in Temperature Protostar: “pre-star” without nuclear fusion PROTOSTAR SHRINKS AND HEATS UP PROTOSTAR BEING ASSEMBLED NUCLEAR FUSION STARTS Luminosity Main Sequence Stars highest mass SPICA: 107 yr lifetime SUN 10 10 yr lifetime lowest mass Temperature PROXIMA CENTAURI . lifetime greater than age of universe Thought Question Which of the cut-away views below best represents what is happening in the core of a star at the very end of its main sequence phase? (“” means nuclear reactions are happening, H = hydrogen, He = helium, C = carbon) Running Out… • nuclear fuel runs out at the center of the star first… where reactions happen fastest (and where it is hottest) Rules for Stars • pressure supports star and opposes gravity CROSSSECTION GRAVITY • nuclear reactions release energy that keeps gas hot and maintains pressure • stars use up nuclear fuel at center first (where gas is hottest) • when fuel runs out, gravity crushes star’s core and causes a temperature rise Becoming a Giant H fusion in core ends and core is crushed and heated… mostly H Fusion continues in a “shell” surrounding the He core … He Large energy release makes envelope expand CROSSSECTION Thought Question: As the Sun becomes a red giant, its… A. luminosity and temperature increase B. luminosity increases and temperature decreases C. luminosity decreases and temperature increases D. luminosity and temperature decrease Life Track of a Sun-Like Star He fusion H fusion Nuclear Reactions FUSION: small nuclei combine together IF they collide fast enough • example: hydrogen • higher temperature is needed for new nuclear fuels because: – nuclei have larger charges – nuclei are more massive (and slower) Thought Question Compared to the first step in H fusion, the electrical repulsion between nuclei in the following fusion reaction is how many times larger? =proton =neutron Life stages of a lowmass star like the Sun Planetary Nebulae • low-mass star pushes most of its gas into space, exposing hot core nuclear reactions stop star cools forever… • gas is illuminated by hot white dwarf The Ring Nebula (planetary nebula) • gas moving about 30 km/s • How old is this nebula? ABOUT 1.3 LIGHT-YEARS White Dwarfs Nearest example: Sirius B MANY others probably out there densely packed electrons resist being crushed nuclear reactions no longer needed to support star SIRIUS B “Degeneracy” Certain kinds of particles (like electrons and neutrons) don’t like looking EXACTLY like others… “THE EXCLUSION PRINCIPLE” example: electrons in atomic orbitals TWIN CLOSET if forced together, they must have: different spin orientation OR different motion Low-Mass Star Corpses: White Dwarfs neutron star . • pressure comes from “degenerate” electrons • star about same size as Earth • higher mass white dwarfs are smaller BUT… can’t be larger than 1.4 M or collapse! High-Mass Stars (M > 9 MSun) • massive stars zig-zag in HR Diagram using new fuels: new nuclear fuel ignites core of star shrinks, heats up fuel used up … until last fuel is used up SUPERGIANTS The End for a Massive Star Supergiant: “onion skin” structure in core: iron core gets too massive and collapses… BUT heat CANNOT be released by nuclear reactions using iron… Star Death Learn what happened by looking at: expanding gas cloud energy release star corpse Crab Nebula ABOUT 12 LIGHT-YEARS Thought Question: How high will the small superball bounce when I drop the stack of 4 superballs from 1 foot above the ground? A. It won’t bounce – it will come to a dead stop. B. It will bounce back to its start (1 foot). C. It will bounce twice as high (2 feet). D. It will bounce 4 times as high (4 feet). E. It will hit the ceiling. CUT-AWAY VIEW OF STAR CORE COLLAPSE “MAXIMUM SCRUNCH” EXPANSION Massive Star Corpses Iron core collapses and forms tiny neutron star: (protons, electrons crushed together to make neutrons) CITY SIZE!!! 10 km none found with more than 3 Msun collapsing gas “bounces” off to start supernova explosion The Crab Nebula pictures taken in 1973 and 2001 CORPSE Energy Release • energies can help reveal what happened… how much total energy is released? (how violent was the star’s death?) what kinds of energy are released? where could the energy have come from? SUPERNOVA Where Does Energy Come From? • Fusion? For H fusion over star’s whole life: Enuc » 0.0006 × M*c • Gravitational potential energy: 1 GM corpse EP » 2 Rcorpse 2 2 How Much Energy is Released? PLANETARY NEBULA (low-mass star) Light: total released SUPERNOVA (high-mass star) 1039 J 1043 J v » 20 km/s v »1500 km/s 38 10 J 1044 J 0 1046 J! Gas kinetic energy: 1 E K » mgasv 2 2 Neutrinos: electrons, protons combine to form neutrons and… Thought Question: Supernova explosions spread heavy elements like iron into space. If you find a star that has a smaller abundance of iron than the Sun, it is A. probably younger than the Sun. B. probably older than the Sun. C. not possible to tell the star’s age You are Star Stuff Dying stars give important chemicals back: • supernova explosions (main source of Fe) • planetary nebulas (source of C, N, O) STAR FORMATION LOW-MASS STAR DEATH HIGH-MASS STAR DEATH POLLUTION OF GAS CLOUDS Mass Very important in astronomy… v Mass measurement allows us to: • distinguish between different types of planets • predict future lives of stars • quickly identify how much “stuff” is in something very big (like the Milky Way) r M • find matter even when it is “dark” P m Listen: #1 #2 Two videos involving a car horn… Listen to the PITCH… Sound Waves You hear a certain pitch when a sound source and you are stationary… …imagine sound waves spreading out like ripples in a pond lobserve = lemit The Doppler Effect Applet Applet 2 Moving apart (“REDSHIFT”): Moving closer (“BLUESHIFT”): train’s motion adds a little train’s motion removes a little distance between each wave… length from each wave… larger wavelength/lower frequency smaller wavelength/higher frequency Measuring Star Speeds Radial velocity: rate of change of distance between two objects negative if distance is decreasing positive if distance is increasing distance covered during the time it takes to make one wave vr d = vr × t = f vr vr lemit lobs = lemit + = lemit + f c æ vr ö lobs = ç1+ ÷ lemit è cø Doppler shift tells us ONLY about the part of an object’s motion toward or away from us: vr pure “radial velocity” vt pure “transverse velocity” vr mixture vt Thought Question: The laboratory spectrum below shows emission lines from hydrogen. If the spectrums of the other objects also show hydrogen, which is moving away from us fastest? A. B. C. D. Thought Question: A policeman’s radar gun uses the Doppler effect, but it is not always able to measure total speed accurately. In which situation below would the cop be able to measure your speed using just the radar gun? A. B. C. D.