ASTR 1040: Stars & Galaxies Prof. Juri Toomre TA: Ryan Orvedahl Lecture 16 Thur 6 Mar 2014 zeus.colorado.edu/astr1040-toomre Blinking Eye Nebula Topics for Today & Tues Briefly revisit: planetary nebulae and their lovely shapes End of the line: a brilliant white dwarf star Maximum mass to WD: 1.4 solar masses Life tracks of massive stars: late stages allow fusion like layers of an onion Massive stars end life with supernova explosion, when iron core exceeds 1.4 M sun Pulsars fast spinning neutron stars with fierce magnetic fields; gradually slow down Things to do Review 17.3 Life as High-Mass Star Overview read Chap 18: Bizarre Stellar Graveyard and white dwarfs in detail, neutron stars next Homework #6 graded, Review Set 2 for Mid-Term Exam 2 next Thur (March 13) Observatory Night #6 on Monday (Mar 10), now Phase B with spectroscopy Clicker review red giants The main source of energy for a star as it grows in size to become a red giant is. B. A. gravitational contraction B. hydrogen fusion in a shell around core C. helium fusion in the core D. hydrogen fusion in the core REMINDER Life track in H-R diagram of solar-mass star Many meanders, but MS phase longest, red giant phase(s) shorter, finally white dwarf left to cool slowly blow lovely shell RG II? Red Giant I protostar white dwarf ZAMS 2: Subgiant to Red Giant (first visit) H burning in shell, makes much more energy Vast expansion, RG phase lasts ~ 500 MY Huge convective envelope 1
5. Red Supergiant Double-shell burning of H and He Phase could be very short if He burning is erratic (unstable) -- then lasts only a few MY, and blows off outer shells REMINDER REMINDER Upward and to the right in H-R: Giant phases in life track of low-mass star 6. Planetary Nebula Outer shells of red supergiant puffed off Great pictures! Helix Nebula Naked white dwarf emerges 10 NGC 7293: The Helix Nebula Planetary nebulae in many shapes: rotation and magnetic fields influence shapes Squid Etched Hourglass Stingray NGC 6369 2
More Planetary Nebulae Life after brief planetary nebula stage. Hot central core emerges as WHITE DWARF Ring Nebula 13 7. White Dwarf Inert C core, He & H shells Sizes of white dwarfs electron degeneracy pressure holds it up Very dense, size of Earth max mass of 1.4 M SUN More massive white dwarfs are SMALLER! Size Of A White Dwarf Final stage: NEBULA Cooling white dwarf --- snooze. But if WD has binary companion, much fun can begin! WHITE DWARF MS RG I RG II 3
Hubble Space Telescope spies 12-13 billion year old white dwarfs Formed less than 1 billion years after creation of the universe Crib Sheet Awards M-T Exam 1 13 Feb 2014 (6 March ceremony) Most Artful: Shannon Osborne Most Inscrutable: Dawson Stokley 10 6 Luminosity (solar units) Now consider evolution of MASSIVE STARS after MS 10-4 Evolution of massive stars Clock runs faster, can burn heavier elements First 4 steps pretty familiar, but no helium flash 1. 2. 3. 4. 40,000 ß Temperature 2,000 Successive core & shell fusion burning of C, O, Ne, Si.. all with alpha capture (or He) stars make many shallow H-R loops 5. 6. High-mass zigs and zags in H-R diagram Elements of higher mass fusion burn successively, releasing energy to support the star against gravity. Reactions may change too fast for outer layers to respond, so last zig/zags are small 4
BIG Evolutionary SWINGS Many layers of onion in massive star Cross the pulsational instability strip: Cepheids Core structure from successive burning stages: lesser elements on outside, heavier on inside Fusion by helium-capture (alpha-particles ) burns C, O, Ne, Mg, Si.. layers of onion Carbon (6), Oxygen (8), Neon (10) Magnesium (12). Helium nucleus (2 protons) is absorbed, energy is released Elements are created going up periodic table in steps of 2 Creation of elements from He-capture: evidence Even numbers favored! A few of many other fusion reactions also feasible in high-mass stars Mixture of elements in our near universe follows the pattern of He-capture fusion reactions, up to iron Even heavier elements are made by nucleosynthesis during supernova explosion 5
Onion-shell fusion burning stops with IRON (Fe, 26 protons ) CARTOON FUSION FISSION Iron does NOT release energy when it fuses! Binary Systems: The Algol Paradox Algol is a binary system consisting of a 3.7 solar mass main sequence star and a 0.8 solar mass red giant. Why is this strange? A. A. A 3.7 star should have become a red giant before a 0.8 solar mass star B. Binary stars usually have the same mass C. 0.8 solar mass stars usually never become red giants Clicker Puzzle: Algol Binary System A. Binary stars can have different masses but usually ARE formed at the same time. More massive star should have had a shorter main sequence lifetime What happened? Binary Mass Exchange The 0.8 solar mass star once was more massive (3.0), with a 1.5 mass companion As it became a red giant, it swelled and poured material onto its companion (lost 2.2) The red giant (0.8) is now less massive than its companion (3.7) Future: when the other star becomes red giant, it may pour gas back? early MS 3.0 1.5 now -2.2 0.8 3.7 6