ASTRO 101

1. ASTRO 101 Principles of Astronomy

2. Instructor: Jerome A. Orosz (rhymes with “boris”)Contact: • Telephone: 594-7118 • E-mail: orosz@sciences.sdsu.edu • WWW: http://mintaka.sdsu.edu/faculty/orosz/web/ • Office: Physics 241, hours T TH 3:30-5:00

4. Homework/Announcements • Chapter 10 homework due April 30: Question 15 (Explain how and why the turnoff point on the H-R diagram of a cluster is related to the cluster’s age.) • For Chapter 11, skip sections 11.9, 11.11, 11.14, 11.16, 11.17, 11.18, 11.19 • Tuesday, May 7: wrap-up and review • Tuesday May 14, Final

5. Stellar Evolution • Observational aspects • Observations of clusters of stars • Theory • Outline of steps from birth to death

6. Stellar Models

7. Stellar Evolution • There are several distinct phases in the life cycle of a star. The evolutionary path depends on the initial mass of the star. • Although there is a continuous range of masses, there are 4 ranges of masses that capture all of the interesting features.

8. Stellar Evolution

9. Stellar Evolution • The basic steps are: • Gas cloud • Main sequence • Red giant • Rapid mass loss (planetary nebula or supernova explosion) • Remnant • The length of time spent in each stage, and the details of what happens at the end depend on the initial mass.

10. Points to Remember: • How to counter gravity: • Heat pressure from nuclear fusion in the core (no mass limit) • Gas pressure proportional to the temperature. • Electron “degeneracy” pressure (mass limit 1.4 solar masses) • Neutron “degeneracy” pressure (mass limit 3 solar masses) • Stars experience rapid mass loss near the end of their “lives”, so the final mass can be much less than the initial mass.

11. Points to Remember: • Sources of energy: • Nuclear fusion: • needs very high temperatures • about 0.7% efficiency for hydrogen into helium. • Gravitational “accretion” energy: • Drop matter from a high “potential” • About 10% efficient when falling onto massive bodies with very small radii.

12. Stellar Evolution

13. Star Formation • The starting point is a giant molecular cloud. The gas is relatively dense and cool, and usually contains dust. • A typical cloud is several light years across, and can contain up to one million solar masses of material. • Thousands of clouds are known.

14. Condensation Theory Image from Nick Strobel’s Astronomy Notes (http://www.astromynotes.com)

15. The Protostar • This diagram shows the steps as computed using a computer model.

16. The Protostar • This diagram shows how a star “moves” through the temperature-luminosity diagram as it forms.

17. The Protostar • This diagram shows how a star “moves” through the temperature-luminosity diagram as it forms.

18. The Protostar • High mass stars simply get bluer, whereas the lower mass stars contract and become dimmer.

19. The Protostar • An external disturbance can cause the cloud to collapse: • The material collapses to a rotating disk, and friction drives material into the center, where it builds up. • The central object heats up as the cloud collapses. Eventually, the temperature gets hot enough for nuclear fusion to occur. • We are left with a newly born star surrounded by a disk of material.

20. Young Star Systems • Many stars in the Orion nebula are surrounded by disks of material.

21. Young Star Systems • Many stars in the Orion nebula are surrounded by disks of material.

22. Young Star Systems • A collapsing cloud can form hundreds of stars. • Stars with small masses (less than a solar mass) are much more common than massive stars (stars more than about 15 to 20 solar masses). • The highest mass stars are very hot and luminous, and can alter the cloud environment.

23. Young Star Systems • Infrared images are useful since the infrared light penetrates deeper into the dark clouds, allowing one to see what is inside. Often one sees young stars.

24. Young Star Systems • Infrared observations often reveal hundreds of newly-formed stars embedded in molecular clouds.

25. Young Star Systems • Infrared observations often reveal hundreds of newly-formed stars embedded in molecular clouds. • In this particular case, many of the stars have not arrived on the main sequence.

26. Star Formation Summary

27. Stellar Evolution

28. Stellar Evolution • The basic steps are: • Gas cloud • Main sequence • Red giant • Rapid mass loss (planetary nebula or supernova explosion) • Remnant • The length of time spent in each stage, and the details of what happens at the end depend on the initial mass.

29. The Main Sequence • A star that is fusing hydrogen to helium in its core is said to be on the main sequence. • A star spends most of its “life” on the main sequence; the time spent is roughly proportional to 1/M3, where M is the initial mass.

30. Hydrostatic Equilibrium • The Sun (and other stars) does not collapse on itself, nor does it expand rapidly. Gravity and internal pressure balance. This is true at all layers of the Sun. • The energy from fusion in the core ultimately provides the pressure needed to stabilize the star.

31. Stellar Evolution

32. Stellar Evolution • The basic steps are: • Gas cloud • Main sequence • Red giant • Rapid mass loss (planetary nebula or supernova explosion) • Remnant • The length of time spent in each stage, and the details of what happens at the end depend on the initial mass.

33. After the Main Sequence • On the main sequence, the star is in hydrostatic equilibrium where internal pressure supports the star against gravitational collapse. Nuclear fusion (hydrogen to helium) is the energy source. • What happens when all of the hydrogen in the core is converted to helium? The details depend on the initial mass of the star…

34. Points to Remember: • Sources of energy: • Nuclear fusion: • needs very high temperatures • about 0.7% efficiency for hydrogen into helium. • Gravitational “accretion” energy: • Drop matter from a high “potential” • About 10% efficient when falling onto massive bodies with very small radii. • After a stage of nuclear fusion is complete in a stellar core, it will collapse and get hotter.

35. More Nuclear Fusion • Fusion of elements lighter than iron can release energy (leads to higher BE’s). • Fission of elements heaver than iron can release energy (leads to higher BE’s).

36. More Nuclear Fusion • Fusion of elements lighter than iron can release energy (leads to higher BE’s). • As you fuse heavier elements up to iron, higher and higher temperatures are needed since more and more electrical charge repulsion needs to be overcome. • Hydrogen nuclei have 1 proton each temperature ~ 10,000,000 K • Helium nuclei have 2 protons each temperature ~100,000,000 K • Carbon nuclei have 6 protons each temperature ~ 700,000,000 K • ….. • After each stage of fusion is complete, the core collapses and heats up. • More mass in the core --> higher core temperature --> fusion of heavier elements … • For a given core mass, there is a limit to how hot it can become.

37. After the Main Sequence: Low Mass • After the core hydrogen is used up, internal pressure can no longer support the core, so it starts to collapse. This releases energy, and additional hydrogen can fuse outside the core. • The excess energy causes the outer layers of the star to expand by a factor of 10 or more. The star will be large and cool: these are the red giants seen in the temperature-luminosity diagram.

38. After the Main Sequence: Low Mass • The red giants are stars that just finished up fusing hydrogen in their cores. Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com)

39. After the Main Sequence: Low Mass • Some red giants are as large as the orbit of Jupiter! • The Sun will reach approximately to the orbit of the Earth

40. After the Main Sequence: Low Mass • The excess energy causes the outer layers of the star to expand by a factor of 10 or more. The star will be large and cool: these are the red giants seen in the temperature-luminosity diagram. • The core continues to collapse, and helium can fuse into carbon for a short time. The star expands further.

41. After the Main Sequence: Low Mass • Helium fusion starts in a “shell” around the core, then after a “helium flash” the helium fusion starts in the core.

42. After the Main Sequence: Low Mass • Helium fusion starts in a “shell” around the core, then after a “helium flash” the helium fusion starts in the core.

43. After the Main Sequence: Low Mass • As core hydrogen fusion stops, low mass stars become more luminous and red (e.g. cooler), higher mass stars tend to just get redder while keeping the same luminosity. • In all cases, the star gets larger in size.

44. Next: • The “deaths” of stars.

45. After the Main Sequence: Low Mass • After the core hydrogen is used up, internal pressure can no longer support the core, so it starts to collapse. This releases energy, and additional hydrogen can fuse outside the core. • The excess energy causes the outer layers of the star to expand by a factor of 10 or more. The star will be large and cool: these are the red giants seen in the temperature-luminosity diagram.

46. After the Main Sequence: Low Mass • The excess energy causes the outer layers of the star to expand by a factor of 10 or more. The star will be large and cool: these are the red giants seen in the temperature-luminosity diagram. • The core continues to collapse, and helium can fuse into carbon for a short time. The star expands further.

47. After the Main Sequence: Low Mass • The core of a star like the Sun will not get hot enough to fuse carbon.

48. After the Main Sequence: Low Mass • The excess energy causes the outer layers of the star to expand by a factor of 10 or more. The star will be large and cool: these are the red giants seen in the temperature-luminosity diagram. • The core continues to collapse, and helium can fuse into carbon for a short time. The star expands further. The outer layers eventually may be ejected to form a “planetary nebula”.

49. After the Main Sequence: Low Mass • After hydrogen fusion is completed, the core collapses, and the outer parts of the star expand. • The core may fuse helium into carbon for a short time, after which the core collapses further. • The outer parts of the star expand by large amounts, and eventually escape into space, forming a planetary nebula. Matter is recycled back into space.

50. Planetary Nebulae • These objects resembled planets in crude telescopes, hence the name “planetary nebula.” • They are basically bubbles of glowing gas.