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Announcements ● ● ● All students: email me this week ([email protected]) telling me whether you want to: – Do a presentation Mar 12 – Do a presentation at end of term – Do a research paper – Do an art project And a couple possible topics you are interested in covering. List of some possible topics on course web page (http://flash.uchicago.edu/~ljdursi/SETI) under blog. Marks – Reading Quizzes and Assignments ● Reading Quiz: – ● 5 NCRs, 9 CRs, 1 CR+ Assignments: Review: The Distance Ladder ● ● ● Different `realms' of distance in Universe, each requiring different units, techniques of measurement: – Solar System – Nearby stars – Galactic distances – Extra-galactic distances Measurement for each realm depends on knowing distances from the nearer realms `Rungs' on Distance Ladder Review: The Distance Ladder ● Sun Moon Earth Geometric measurement of distance to Sun depends on knowing distance to Moon – Solar system `rung' depends on Earth-Moon `rung' Review: The Distance Ladder ● Parallax distance measures of nearby stars REQUIRES knowing how big an AU is – `nearby star' rung depends on `solar system' rung Summary of Last Class: Light ● Light is a form of electromagnetic radiation ● All EM radiation ● – Dims with distance as the inverse square law – Forms a broad spectrum Dense, opaque material glows when hot as a blackbody – ● Hotter glows more, and at shorter (blue-er) wavelengths Other processes give rise to distinctive line spectrum which can be used to determine – Composition – Speed (by Doppler shift) Summary of Last Class: Galaxies ● ● ● ● ● Galaxies are `island universes' which contain most of the matter, stars in the Universe Can be spiral, elliptical, or irregular Star formation continues in galaxies, particular in spiral galaxies Galaxies also contain gas clouds, dust Galaxies are separating over time: expanding universe Feedback: ● Most unclear item from last week's readings? What we're going to cover today ● The Stellar Cycle: Birth, Life, and Death of the Stars – Birth: turbulent collapse of clouds of gas – Life: ignition; burning; balance between gravity and pressure – Death: gravity begins to win; but burning has one last hurrah. Stars are crucial for life ● Stars are the main engines in the Universe ● Stars are where planets are found ● Stars produce energy that can power life ● Stars produce all the heavy elements (eg Carbon) that build life Stellar Cycle The Birth of Stars ● At end of this, we'll know: – Where stars are formed – How they form – What has to happen for a star to `turn on' – How planets form around stars Turbulence ● Happens when flow velocities are too large to be kept smooth by viscosity Turbulence ● Gas clouds in the galaxy are turbulent, too – Very wispy, tenuous gas – No viscosity to speak of – `Stirred' by energetic events in the galaxy Gas Clouds ● Two broad types of clouds: – – Gas clouds ● Warm ● Very wispy Molecular clouds ● ● ● Colder Much denser Gas has condensed enough that complex molecules have formed Molecular Clouds ● Because molecular clouds are cooler and denser, atoms collide more often ● Can form complex molecules ● Greatly helped by presence of grains ● Provides sites for atoms to latch onto ● Region of high atom density; atoms more easily find other atoms to interact with Gas Clouds ● All of these gas clouds are turbulent ● Random motions, eddies ● ● Where fluid comes together, dense regions Fluid is moving fast enough that can compress very dense spots Gas Clouds ● ● Gravity acts to try to pull these dense spots together However, – Pressure in gas clouds – Rotation Gas Clouds ● ● ● If a large enough, dense enough region is formed, gravity can start to win and core starts collapsing inward Nearby material can also start falling in As collapses, `spins up' and disk can form Gas Clouds ● ● ● Collapse will usually happen in many places throughout the cloud at the same time This is why stars tend to be clustered Amount of stars depends on size of gas cloud producing stars Gas Clouds ● ● ● ● As core collapses, gets hotter and denser Begins to glow Begins to evaporate nearby complex molecules Any particularly dense regions can (for a while) protect columns in their shadow Gas Clouds ● ● ● Nearby gas evaporates, but disk remains Flattens out at spinning increases with collapse Can begin to coalesce as star begins to form Protoplanetary Disks ● These protoplanetary disks can be seen around very young protostars Protoplanetary Disks Jets and Outflows ● ● ● ● As core collapses further, heat increases and so gas pressure increases If core is small enough, this ends the process If core is large enough, burning can `turn on' and begins rather violently Under some circumstances, enormous jet can form perpendicular to disk Summary The Life of Stars ● At end of this, we'll know: – The structure of stars – How stars burn – How stars age – Our Sun's life story Failed Stars ● ● ● ● `Stars' that are too small (~8% of the mass of the Sun, or ~80 Jupiter masses) never ``turn on'' Central temperatures never get hot enough for nuclear burning to begin in earnest Nuclear burning is what powers the star through its life Star sits around as a brown dwarf – too big and hot to be a planet, too small and cold to be a real star Failed Stars ● ● ● Such brown dwarfs have been observed ~100,000 times fainter than Sun Stars, failed or otherwise, often observed in binary systems – ● (~30% of all stars in binary systems?) Turbulent collapse makes it very likely that two cores form nearby or large core splits into two. Hydrostatic Equilibrium ● ● ● ● Once collapse has halted in a star, force inward (gravity) must be balanced by force outward (gas pressure) (Much of the rotation has been taken away by the planetary disk by this point) Central region is hottest because pressure from the entire star is pushing down on it Star as a whole is hot enough that no molecules are left; everything is broken into components Nuclear Reactions ● ● ● ● ● ● Nuclei of atoms themselves interact Change the elements: alchemy The star, like the cloud it came from, is mostly hydrogen So hot the electrons are stripped off; left with bare protons (hydrogen nuclei) Under extreme heat, protons can fuse together to produce helium: and more heat! Higher temperatures – faster reactions Question ● What happens if an external force `squishes' the star a little bit? Built In Thermostat ● If star is squished in, – Central region gets hotter – Reactions speed up – Star gets hotter – Gas pressure increases – Star fluffs out – Central temperature returns to normal Built In Thermostat ● ● If star is pulled out a little, – Central region gets cooler – Reactions slow down – Star gets cooler – Gas pressure decreases – Star falls back – Central temperature returns to normal Star is STABLE Given that burning is stable, ● What effects how hot a star is? Given that burning is stable, ● What effects how hot a star is? – ● ● MASS The bigger the star that forms from the collapse – More pressure on the central region – More burning – Hotter – Brighter What color are more massive stars? HR diagram and Main Sequence ● ● ● From previous, expect that hotter stars should be brighter – Blackbody – More massive -> bigger When temperature vs brightness is plotted, see `Main Sequence' Other populated regions show later stages in stellar evolution Stellar Evolution ● Nuclear reactions are very sensitive to temperature – ● ● Massive stars burn MUCH faster than smaller stars Even though massive stars have more fuel (hydrogen) to begin with, it is exhausted more quickly Everything happens faster with more massive stars because pressure is higher Stellar Evolution ● ● ● ● As burning in core progresses, Hydrogen in center becomes depleted (Sun: ~10 billion years) Core of Helium `ash' left behind Shell of Hydrogen burning slowly moves outwards As heat source moves further out, star `puffs out' ● Outer regions cool, redden ● Red Giant (Sun: 1 billion years) Stellar Evolution ● ● ● Eventually Helium core gets so hot that even it can burn, to Carbon New energy source: star gets hotter and bluer, and shrinks back to more normal size Burning happens faster with heavier elements; soon Helium becomes exhausted, a Carbon core forms; becomes giant again Low Mass stars: envelope ejection ● ● ● ● ● Helium burning can be very unstable Outer layers begin pulsing; blows most of the envelope off of the star (so called) `Planetary nebula' forms Only the core is left behind, still glowing (because hot) but inert White dwarf High Mass Stars: Continue Burning ● Slightly more massive stars (4 to 8 solar masses): – Everything happens faster – Carbon can burn, as well; one more stage of burning – Then again leave (larger) white dwarf and planetary nebula behind Very High Mass Stars: Continue Burning ● Very massive stars burn VERY fast – ● ● Main sequence stage – 10 million years Burning happens so quickly that outer layer can't go unstable Burning progresses faster and faster through higher and higher elements until Iron ● No further burning is possible ● Left with a large envelope and very heavy core Life Story of Our Sun ● Formed in ~50 million years ● Began life about 5 billion years ago ● – A little dimmer (¾ current brightness) – A little cooler, smaller Slowly getting bigger and hotter: – 1 billion yrs from now: 10% brighter – Greenhouse effect – 5 billion years from now: 40% brighter – Earth like Venus today – Still main sequence Life Story of Our Sun ● ● ● ● Red giant branch begins Next 700 million years; sun doubles in energy output Doubles in size, gets little redder Next 600 million years; very strong wind; planets pushed somewhat outwards ● At biggest, sun almost out to Venus' orbit ● Helium Flash!! ● ● Helium begins burning, process repeats itself but 10x faster Ends with ½ of suns mass blown away; white dwarf remains The Old Age and Death of Stars ● At end of this, we'll know: – The final stages of stellar life – How stars of different mass die – How they feed back material into the interstellar environment, to be made into new stars The Old Age and Death of Stars ● Small stars end their life quietly – ● ● White dwarf remnants Massive stars continue burning in outer layers even when they have burned all the way to iron in the core. New ash from burning continues to pile onto iron core until pressure cannot support it any more Type II Supernova ● ● ● The result is a collapse to a different form of matter – a neutron star, or a black hole -- and a release of energy Energy release can be equal to the entire energy of the host galaxy Entire envelope is blown apart – Heavy elements from burning blown into surrounding gas Type Ia Supernova ● ● ● Almost as much energy can come from another kind of supernova If a star which ended up as a white dwarf has a companion, matter can `rain in' on the inert white dwarf until it gets hot enough to burn Can burn catastrophically, exploding and releasing heat, heavy elements into surrounding gas Supernova Feedback ● Originally, gas was all hydrogen and helium – ● ● ● No planets, life Generations of stars produced all the heavy elements which make up planets and living things Supernova explosions release these heavy elements into the galaxy – New stars are formed – Can make planets, life Supernova energy contributes to the turbulence in the gas clouds, and can compress gas to start new cycle of star formation Stellar Cycle Revisited Reading for Next Week ● Chapter 7, 8 – origins of life on earth