Announcements

1. Announcements • All students: email me this week (jdursi@artic.edu) 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.

2. Marks – Reading Quizzes and Assignments • Reading Quiz: • 5 NCRs, 9 CRs, 1 CR+ • Assignments:

3. 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

4. Review: The Distance Ladder • Geometric measurement of distance to Sun depends on knowing distance to Moon • Solar system `rung' depends on Earth-Moon `rung' Sun Moon Earth

5. 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

6. 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)

7. 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

8. Feedback: • Most unclear item from last week's readings?

9. 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.

10. 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

11. Stellar Cycle

12. 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

13. Turbulence • Happens when flow velocities are too large to be kept smooth by viscosity

14. 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

15. 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

16. 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

17. 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

18. Gas Clouds • Gravity acts to try to pull these dense spots together • However, • Pressure in gas clouds • Rotation

19. 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

20. 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

21. 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

22. Gas Clouds • Nearby gas evaporates, but disk remains • Flattens out at spinning increases with collapse • Can begin to coalesce as star begins to form

23. Protoplanetary Disks • These protoplanetary disks can be seen around very young protostars

24. Protoplanetary Disks

25. 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

26. Summary

27. 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

28. 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

29. 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.

30. 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

31. 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

32. Question • What happens if an external force `squishes' the star a little bit?

33. 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

34. 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

35. Given that burning is stable, • What effects how hot a star is?

36. 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?

37. 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

38. 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

39. 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)

40. 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

41. 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

42. 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

43. 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

44. 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

45. 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

46. 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

47. 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

48. 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

49. 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

50. 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