1. A note on: Group Behavior Open Clusters: contain a few hundred to a few thousands of stars. Lie in the plane of the Milky Way Galaxy. A few parsecs in diameter. Generally young type stars Globular Clusters: Lie outside the plane of our galaxy. Very far away, thousands of parsec. Contain no main Sequence stars with masses > 0.8 Sun’s mass. Generally Older stars. Open Clusters Central Bulge

2. Open vs. Globular Page 507 Fig.19.8 A & B

3. More on Group Behavior Associations: T Type: Contain pre-main sequence stars. O-B Type: Contain O,B stars

4. Cluster evolution. page 532 Figure 20.17 (a). Initially stars in main seq. burning steadily while stars in lower end of the sequence are still forming. (b).O-Type stars left the main seq., some red giants visible. ©.Type B stars have evolved off the main seq. Stars in lower end of seq. catching up. (d).Main Seq. comprised of stars up to Type A. Sub and Red giants becoming apparent. Lower end completed. (e) Stars with < Sun’s mass remain on the sequence. White dwarfs have now formed.

5. Blue Stragglers Observed in many clusters. Main-Sequence beasts. Don’t make a whole lot of sense why they are what they are but believed to be the result of Mergers which are collisions between stars or the result of binary evolutions. Explains why such young stars amongst a cluster that may be at least 10 billion old.

6. Binary Stars 2 stars with a distance of 1000 star radii between them have evolved independently. Closer than that then gravity mates them.

7. Algol

8. Other class media Slides: H-R diagrams for open cluster, cluster evolution. Video: Stellar Formation part 1. Tomorrow video: Stellar formation, part 2.

9. The life of a Star The life cycle of a star. 1. Collapsing cloud (done that) 2. Proto-star (done that) 3. Star (main sequence) 4. Dying star (main sequence turn-off) Lets review and look at some pretty pictures..

10. Stages in the life of a star 1.A gas cloud in the Orion nebula 2. A collapsing cloud, proto-star Region. 3. A genuine star 1. -> 2. -> 3.

11. Stages, continued. 4. Red Giant phase, turned off of the main sequence. 5a. A planetary nebula, from a smaller mass star like our sun. 4 -> 5a

12. Stages again… 5b. Larger mass stars will supernova…. 6. Leaving behind filamentary clouds rich in heavy elements. 4 5a5b 6

13. 3. Main Sequence Star a. Stars enter main sequence through the Hayashi Track, a proto-star evolutionary path. b. Once on the main sequence track, a stable star spends most of its life here. What does it do there? p-p fusion (e.g. 4 H 1 => He 4 + energy) Stays in Hydrostatic equilibrium Energy produced in core through p-p chain accounts for most of energy production c. Dying Star

14. When H is depleted, inner core (now mostly He) contracts & heats up.  The high temperature ignites the shell of H around the core.  Increased pressure drives the envelope of the star outward.  Creating a giant or Supergiant. Figure 20.2c, page 517 Figure 20.3, page516

15. He fusion 2 He -> Be + energy He + Be -> C + energy Much higher temperatures (10 8 K) needed for He fusion. The Helium Flash: When T core ~ 10 8 K, He begins to burn. 1. He 4 + He 4 => Be 8, He + Be => C 2. He fusion in the core and H shell burning 3. Eventually He in core is exhausted 4. Contraction of the core raises the temperature further 5. Both He and H shells now burning.

16. Giants and Super Giants Our Sun and others like it.. Temp. never high enough to attain carbon fusion in core. Some conversion at He shell: C + He => O + energy. Through subsequent temperature and density fluctuations, He shell flashes occur. Surface layers (envelope) become unstable => blow off-> planetary nebula High mass stars > 12 solar masses.. High temperatures insure creation of heavier elements at a rapid pace. No He flash occurs. Die explosively. Red giant Lets look at the big picture =>

17. H-R diagrams A.Main sequence star B.Sub Giant branch C.He Flash D.Horizontal branch E.C core/planetary nebula D C B A E

18. Some terms White dwarf: remainder of low mass star evolution Shines only by the light of it’s stored energy. Eventually becomes a black dwarf. A black dwarf is a cold burned out star. Table 20.3, page 531.

19. Some low mass endings; planetary nebula Eskimo nebula, constellation Gemini 1500 pc away NGC 3132 nebula, 2 stars, both unrelated, Material receding from small star ½ ly

20. High mass endings