Schedule

1. Astronomy 100Tuesday, Thursday 2:30 - 3:45 pmTom Burbinetburbine@mtholyoke.eduwww.xanga.com/astronomy100

2. Schedule • Today – Class • Tuesday-Class • Wednesday- Review Session • Hasbrouck 134 from 7-8 pm • Thursday-Exam #3 (Chapters 15, 16, 17, 18)

3. Homework Assignment(Due Today) • Make up a test question • Multiple Choice • A-E possible answers • 1 point for handing it in • 1 point for me using it on test • The question needs to be on material that will be on the 3rd exam • 15 people got extra HW credit for me using their question (or inspiring a question)

4. OWL assignment (Due Today) • There is be an OWL assignment due on Thursday March 31 at 11:59 pm. • There are 15 questions and a perfect score will give you 2 homework points.

5. OWL assignment (Due Tuesday) • There is be an OWL assignment due on Tuesday April 5 at 11:59 pm. • There are 15 questions and a perfect score will give you 2 homework points.

7. Things to remember • 90% of classified stars are on main sequence • Main sequence stars are “young” stars • If a star is leaving the main sequence, it is at the end of its lifespan of burning hydrogen into helium

8. Remember • Largest stars on main sequence are O stars • Largest stars that can exist are supergiants

9. You need to know stellar classifications • O, B, A, F, G, K, M • A0, A1, A2, … A9 in the order from the hottest to the coolest

10. wd white dwarfs

11. Classifications • Sun is a G2 V • Betelgeuse is a M2 I • Vega is a A0 V • Sirius is a A1 V • Arcturus is a K3 III

12. Binaries • About half of stars orbit a companion • 3 classes of binaries

13. Visual Binary • We can see the stars rotating around each other with a telescope

14. Eclipsing Binary The light from a star system drops as a star goes in front and behind another star.

15. Spectroscopic Binary The spectral lines of a star can be seen to be moving to shorter wavelengths and also to longer wavelengths

16. Importance of Binaries • It allows you to possibly determine a star’s mass if you know the orbital period and the separation of the two stars

17. Life of a Star • A star-forming cloud is called a molecular cloud because low temperatures allow Hydrogen to form Hydrogen molecules (H2) • Temperatures like 10-30 K • Denser than surrounding regions

18. Region is approximately 50 light years across

19. Condensing • Molecular clouds tends to be lumpy • These lumps tend to condense into stars • That is why stars tend to be found in clusters

20. Protostar • The dense cloud fragment gets hotter as it contracts • The cloud becomes denser and radiation cannot escape • The thermal pressure and gas temperature start to rise and rise • The dense cloud fragment becomes a protostar

21. When does a protostar become a star • When the core temperatures reaches 10 million K, hydrogen fusion can start occurring

22. 3 Basic Groups of Stars • Low-mass stars – born with less than 2 Solar Masses • Intermediate-mass stars – born between 2 and 8 solar masses • High-mass stars – born with masses greater than 8 solar masses

23. Things you need to know • Fusion rate increases with increasing temperature • There is a relation between thermal pressure and gravity

25. Animation • Death sequence of the Sun

26. Sun ends it time on the main sequence • When the core hydrogen is depleted, nuclear fusion stops • The core pressure can no longer resist the crush of gravity • Core shrinks

28. Why does the star expand? • The core is made of helium • The surrounding layers are made of hydrogen

29. And .. • Gravity shrinks the inert helium core and surrounding shell of hydrogen • The shell of hydrogen becomes hot for fusion • This is called hydrogen-shell burning

30. And … • The shell becomes so hot that its fusion rate is higher than the original core • This energy can not be transported fast enough to surface • Thermal pressure builds up and the star expands

31. And .. • More helium is being created • Mass of core increases • Increases its gravitational pull • Increasing the density and pressure of this region

33. When • When helium core reaches 100 million Kelvin, • Helium can fuse into a Carbon nucleus

35. Helium Flash • The rising temperature in the core causes the helium fusion rate to rocket upward • Creates a lot of new energy

36. However • The core expands • Which pushes the hydrogen-burning shell outwards • Lowering the hydrogen-burning shell’s temperature

37. And • Less energy is produced • Star starts to contract

39. Now • In the core, Helium becomes Carbon • Star contracts • Helium fusion occurs in a shell surrounding the carbon core • Hydrogen shell can fuse above the Helium shell • Inner regions become hotter • Star expands

40. Can Carbon undergo fusion? • Yes, but can’t reach the needed temperature (600 million Kelvin) in a low-mass star • Carbon on Earth is produced in the cores of stars

41. Planetary Nebulae • There is a carbon core and outer layers are ejected into space • The core is still hot and that ionizes the expanding gas

42. Planetary Nebulae

43. White Dwarf • The remaining core becomes a white dwarf

45. PRS Question • At the end of the Sun’s lifespan when it becomes a white dwarf, the white dwarf will be mostly composed of • A) Hydrogen • B) Helium • C) Carbon • D) Oxygen • E) Iron

46. PRS Question • At the end of the Sun’s lifespan and it becomes a white dwarf, the white dwarf will be mostly composed of • A) Hydrogen • B) Helium • C) Carbon • D) Oxygen • E) Iron

49. High-Mass Stars • The importance of high-mass stars is that they make elements heavier than carbon • You need really hot temperatures which only occur with the weight of a very high-mass star

50. Stages of High-Mass Star’s Life • Similar to low-mass star’s • Except a high-mass star can continue to fuse elements • When the fusion ceases, the star becomes a supernova • Supernova is a huge explosion