Starry Monday at Otterbein

1. Welcome to Starry Monday at Otterbein Astronomy Lecture Series -every first Monday of the month- February 6, 2006 Dr. Uwe Trittmann

2. Today’s Topics • Lifecycle of Stars • The Night Sky in February

3. On the Web • To learn more about astronomy and physics at Otterbein, please visit • http://www.otterbein.edu/dept/PHYS/weitkamp.asp (Observatory) • http://www.otterbein.edu/dept/PHYS/ (Physics Dept.)

4. The Lifecycle of the Stars

5. Reminder: Hertzsprung-Russell-Diagrams • Hertzsprung-Russell diagram is luminosity vs. spectral type (or temperature) • To obtain a HR diagram: • get the luminosity. This is your y-coordinate. • Then take the spectral type as your x-coordinate. This may look strange, e.g. K5III for Aldebaran. Ignore the roman numbers ( III means a giant star, V means dwarf star, etc). First letter is the spectral type: K (one of OBAFGKM), the arab number (5) is like a second digit to the spectral type, so K0 is very close to G, K9 is very close to M.

6. Reminder: Spectral Classification of the Stars Class Temperature Color Examples O 30,000 K blue B 20,000 K bluish Rigel A 10,000 K white Vega, Sirius F 8,000 K white Canopus G 6,000 K yellowSun,  Centauri K 4,000 K orange Arcturus M 3,000 K red Betelgeuse Mnemotechnique: Oh, Be AFine Girl/Guy, Kiss Me

7. Constructing a HR-Diagram • Example: Aldebaran, spectral typeK5III, luminosity = 160 times that of the Sun L 1000 Aldebaran 160 100 10 1 Sun (G2V) O B A F GK M Type … 01234567890123456789 012345…

8. The Hertzprung-Russell Diagram • A plot of absolute luminosity (vertical scale) against spectral type or temperature (horizontal scale) • Most stars (90%) lie in a band known as the Main Sequence

9. Mass and the Main Sequence • The position of a star in the main sequence is determined by its mass All we need to know to predict luminosity and temperature! • Both radius and luminosity increase with mass

10. The Fundamental Problem in studying the stellar lifecycle • We study the subjects of our research for a tiny fraction of its lifetime • Sun’s life expectancy ~ 10 billion (1010) years • Careful study of the Sun ~ 370 years • We have studied the Sun for only 1/27 millionth of its lifetime!

11. Suppose we study human beings… • Human life expectancy ~ 75 years • 1/27 millionth of this is about 74 seconds • What can we learn about people when allowed to observe them for no more than 74 seconds?

12. Theory and Experiment • Theory: • Need a theory for star formation • Need a theory to understand the energy production in stars  make prediction how bight stars are when and for how long in their lifetimes • Experiment: observe how many stars are where when and for how long in the Hertzsprung-Russell diagram •  Compare prediction and observation

13. Nuclear Fusion is the energy source of the Stars • Atoms:electrons orbiting nuclei • Chemistry deals only with electron orbits (electron exchange glues atoms together to from molecules) • Nuclear power comes from the nucleus • Nuclei are very small • If electrons would orbit the statehouse on I-270, the nucleus would be a soccer ball in Gov. Bob Taft’s office • Nuclei: made out of protons (el. positive) and neutrons (neutral)

14. Nuclear fusion reaction • 4 hydrogen nuclei combine (fuse) to form a helium nucleus, plus some byproducts • Mass of products is less than the original mass • The missing mass is emitted in the form of energy, according to Einstein’s famous formulas: E = mc2 (the speed of light is very large, so there is a lot of energy in even a tiny mass)

15. Further Reactions – Heavier Elements Start: 4 + 2 protons End: Helium nucleus + neutrinos Hydrogen fuses to Helium

16. Fusion is NOT fission! • In nuclear fission one splits a large nucleus into pieces to gain energy • Build up larger nuclei Fusion • Decompose into smaller nuclei Fission

17. Check: Solar Neutrinos • We can detect the neutrinos coming from the fusion reaction at the core of the Sun • The results are 1/3 to 1/2 the predicted value! • Possible explanations: • Models of the solar interior are incorrect • Our understanding of the physics of neutrinos is incorrect • Something is horribly, horribly wrong with the Sun • #2 is the answer – neutrinos “oscillate”

18. Theory of Star Formation • A star’s existence is based on a competition between gravity (inward) and pressure due to energy production (outward) Heat Gravity Gravity

19. Star Formation & Lifecycle • Stage 1:Contraction of a cold interstellar cloud • Stage 2:Cloud contracts/warms, begins radiating; almost all radiated energy escapes • Stage 3:Cloud becomes dense  opaque to radiation  radiated energy trapped  core heats up

20. Example: Orion Nebula • Orion Nebula is a place where stars are being born

21. Orion Nebula (M42)

22. Protostellar Evolution • Stage 4: increasing temperature at core slows contraction • Luminosity about 1000 times that of the sun • Duration ~ 1 million years • Temperature ~ 1 million K at core, 3,000 K at surface • Still too cool for nuclear fusion! • Size ~ orbit of Mercury

23. The T Tauri Stage Stage 5 (T Tauri): • Violent surface activity • high solar wind blows out the remaining stellar nebula • Duration ~ 10 million years • Temperature ~ 5106 K at core, 4000 K at surface • Still too low for nuclear fusion • Luminosity drops to about 10  the Sun • Size ~ 10  the Sun

24. Path in the Hertzsprung-Russell Diagram Stages 1-5

25. Observational Confirmation • Preceding the result of theory and computer modeling • Can observe objects in various stages of development, but not the development itself

26. A Newborn Star • Stage 6:Temperature and density at core high enough to sustain nuclear fusion • Stage 7:Main-sequence star; pressure from nuclear fusion and gravity are in balance • Duration ~ 10 billion years (much longer than all other stages combined) • Temperature ~ 15 million K at core, 6000 K at surface • Size ~ Sun

27. Mass Matters • Larger masses • higher surface temperatures • higher luminosities • take less time to form • have shorter main sequence lifetimes • Smaller masses • lower surface temperatures • lower luminosities • take longer to form • have longer main sequence lifetimes

28. Failed Stars: Brown Dwarfs • Too small for nuclear fusion to ever begin • Less than about 0.08 solar masses • Give off heat from gravitational collapse • Luminosity ~ a few millionths that of the Sun

29. Main Sequence Lifetimes Mass(in solar masses)LuminosityLifetime 10 Suns 10,000 Suns 10 Million yrs 4 Suns 100 Suns 2 Billion yrs 1 Sun 1 Sun 10 Billion yrs ½ Sun 0.01 Sun 500 Billion yrs

30. Why Do Stars Leave the Main Sequence? • Running out of fuel

31. Stage 8: Hydrogen Shell Burning • Cooler core  imbalance between pressure and gravity core shrinks • hydrogen shell generates energy too fast  outer layers heat up star expands • Luminosity increases • Duration ~ 100 million years • Size ~ several Suns

32. Stage 9: The Red Giant Stage • Luminosity huge (~ 100 Suns) • Surface Temperature lower • Core Temperature higher • Size ~ 70 Suns (orbit of Mercury)

33. Lifecycle • Lifecycle of a main sequence G star • Most time is spent on the main-sequence (normal star)

34. The Helium Flash and Stage 10 • The core becomes hot and dense enough to overcome the barrier to fusing helium into carbon • Initial explosion followed by steady (but rapid) fusion of helium into carbon • Lasts: 50 million years • Temperature: 200 million K (core) to 5000 K (surface) • Size ~ 10  the Sun

35. Stage 11 • Helium burning continues • Carbon “ash” at the core forms, and the star becomes a Red Supergiant • Duration: 10 thousand years • Central Temperature: 250 million K • Size > orbit of Mars

36. Stage 12 • Inner carbon core becomes “dead” – it is out of fuel • Some helium and carbon burning continues in outer shells • The outer envelope of the star becomes cool and opaque • solar radiation pushes it outward from the star • A planetary nebula is formed Duration: 100,000 years Central Temperature: 300  106 K Surface Temperature: 100,000 K Size: 0.1  Sun

37. Planetary Nebulae “Eye of God” Nebula

38. “Cat’s Eye” Nebula

39. “Wings of the Butterfly” Nebula

40. The Ring Nebula (M57)

41. “Eskimo” Nebula

42. “Stingray” Nebula

43. “Ant” Nebula

44. Stage 13: White Dwarf • Core radiates only by stored heat, not by nuclear reactions • core continues to cool and contract • Size ~ Earth • Density: a million times that of Earth – 1 cubic cm has 1000 kg of mass!

45. Stage 14: Black Dwarf • Impossible to see in a telescope • About the size of Earth • Temperature very low  almost no radiation  black!

46. Evolution of More Massive Stars • Gravity is strong enough to overcome the electron pressure (Pauli Exclusion Principle) at the end of the helium-burning stage • The core contracts until its temperature is high enough to fuse carbon into oxygen • Elements consumed in core • new elements form while previous elements continue to burn in outer layers

47. Evolution of More Massive Stars • At each stage the temperature increases  reaction gets faster • Last stage: fusion of iron does not release energy, it absorbs energy  cools the core  “fire extinguisher”

48. Neutron Core Manhattan • The core cools and shrinks • nuclei and electrons are crushed together • protons combine with electrons to form neutrons • Ultimately the collapse is halted by neutron pressure • Most of the core is composed of neutrons at this point • Size ~ few km • Density ~ 1018 kg/m3; 1 cubic cm has a mass of 100 million kg!

49. Formation of the Elements • Light elements (hydrogen, helium) formed in Big Bang • Heavier elements formed by nuclear fusion in stars and thrown into space by supernovae • Condense into new stars and planets • Elements heavier than iron form during supernovae explosions • Evidence: • Theory predicts the observed elemental abundance in the universe very well • Spectra of supernovae show the presence of unstable isotopes like Nickel-56 • Older globular clusters are deficient in heavy elements

50. Review: The life of Stars