Stellar Evolution & Galaxies: Birth to Death Cycle

1. Phys 1830: Lecture 29 Shapely 1, Malin • Previous Classes: • Radii, Mass, Lifetime on Main Sequence • This class: • Stellar evolution: star birth and death of a star with 1 solar mass. • Next Classes: • Stellar Evolution: more massive stars • Supernovae Type ! • Supernovae Type II • black holes • Galaxies – review lectures 12, 13 & 27.

2. Stars: Stellar Populations • When a massive star explodes as a supernova at the end of its life, it also pollutes the interstellar medium with elements, many fused during the explosion.

3. Stars: Stellar populations • New stars form out of the polluted interstellar medium. • These stars have more elements.

4. Stars: Stellar Populations - classification • Population I: • Age of our sun or younger. • Most enriched in chemical elements . • Population II: • Older, previous generation. • Less enriched. • Population III: • Have the chemical abundances of the early universe (only H, He, Li and traces of other elements). • Died long ago so not observed.

5. Review: • Stars that have only the elements that existed in the very early universe are called Population I stars. • True • False

6. Galaxies: • Definition: A large group of stars held together by the stars’ mutual gravitational attraction. • Sizes range from several million stars up to 100 billion stars.

7. Galaxies: • Mainly older Pop II stars.

8. Galaxies: • Spiral galaxies also contain significant amounts of gas and dust.

9. Galaxies: • Pop II and older Pop I stars: throughout the nucleus, bulge and disk. • Younger Pop I stars: in the Spiral arms.

10. The Story of Supernovae • It starts with star birth.

11. Star Formation in Galaxies: • These HII regions are sites of star formation in galaxies. The gas is being converted into stars. • Hot, young stars heat surrounding gas, ionizing it  HII regions.

12. Star Formation: Examples in our Milky Way Galaxy • Interstellar medium (ISM). • HI gas and dust between stars. • Where the gas is dense molecular clouds form.

13. At the end of this section you will describe to your neighbour the 5 main stages of evolution for a star like our sun, starting with star birth. • Star Birth • Main Sequence • Red Giant • Planetary Nebula • White Dwarf

14. Star Formation: • Pressure applied to dense, cold ISM clouds causes them to gravitationally collapse and form stars. • Blastwave of supernova. • Spiral density wave.

15. Orion by : Reinhold Wittich Narrow band and broad band filters.

16. Orion Nebula in IR • WISE: Near-IR (NIR) & Far-IR (FIR)

17. Star Formation: • Classic example of an HII region is the Orion Nebula. • To show the nebula, Malin masked out the bright light from the central hot young stars. • Several hundred stars are forming, along with protoplanetary disks around 1/3-1/2 of the stars.

18. Star Formation: Orion Nebula in IR young stars • left: Herschel Far IR • right: Spitzer Near IR

19. Star Formation – cold, dense dusty clouds: • Horsehead Nebula • Bok Globules

20. Star Formation: • Eagle Nebula. • Evaporating Gaseous Globules (EGGs)

21. Star Formation: • Herbig Haro Objects – the jets emanating from protostars.

22. Star Formation: • Note the jet coming out of the molecular cloud.

23. Star Formation: Herbig Haro Jets • Radius can be more than 0.5 to 15 ly long!

24. Star Formation: The Pleiades • Hot young stars  massive ones are blue. • Fusion occurring in the core. • Still surrounded by dust which reflects the blue light. • Form in a group, stars with different masses.

25. Star Formation and the H-R Diagram: • Large, cool protostar in the centre of proplyd nebula  in upper right. • Protostar contracts and temperature increases. • Fusion in core  on the Main Sequence.

26. Star Formation and the H-R Diagram: ZAMS • Zero Age Main Sequence at left edge of MS. • Stars live on MS for millions to billions of years. • Evolve off of MS when H in core is converted to Helium.

27. Star Formation and the H-R Diagram: MS Brown Dwarfs • Brown dwarfs are stars that • have small enough masses that they do not initiate nuclear fusion of H • do not reside on the MS

28. Stellar Evolution and the H-R Diagram: • Evolution of a 1 solar mass star. • Main Stages: • Red Giant • Planetary Nebula • White Dwarf

29. Red Giant Stage: Antares • Radius increases and surface temperature decreases. Almost as large as the orbit of Jupiter. • Fusion of He in core into Carbon (C) is initiated. • From MS to PN stage is roughly 10**8 years.

30. Planetary Nebula Model • Outer envelope of star is ejected. • Core of star is revealed. • Hot core ionizes the expanding envelope.

32. HST/WFC3

33. The Eskimo Nebula: Masking used to superimpose bright and faint structures.

34. Planetary Nebula Stage: • Lasts ~ 3 * 10**4 yrs. • Radius 0.25 to 2 ly.

35. White Dwarf: HST Binary system of Sirius A and Sirius B • E.g. Sirius B (Bond et al.). Dot in left corner. • High T, low L  R very small (e.g. size of Earth). • The stellar core after the nebula has dissipated.

36. White Dwarf Stage: HST Binary system of Sirius A and Sirius B • Contracts until it becomes a Degenerate Electron Gas: electrons packed as tightly as they can be. • Pressure support since negative charges repulse each other. • Fades over 100s * 10**9 yrs.

37. Stellar Evolution of a 1 solar mass star: • The position of a star on the HR diagram changes as the star evolves.

38. A planetary nebula is a) a contracting spherical cloud of gas surrounding a newly formed star, in which planets are forming. b) the expanding nebula formed by the supernova explosion of a massive star. c) an expanding gas shell surrounding a hot, white dwarf star. d) a disk-shaped nebula of dust and gas from which planets will eventually form, easily photographed around relatively young stars.

39. What does the star look like at that stage? What processes are occurring? Where is it on the HR diagram? • Describe to your neighbour the 5 main stages of evolution for a star like our sun, starting with star birth.

40. Star Formation and the H-R Diagram: ~1 * 10**6 yr < 50 * 10**6 yr ~1 * 10**9 yr • Large, cool protostar in the centre of proplyd nebula  in upper right. • Protostar contracts and temperature increases. • Fusion in core  on the Main Sequence.

41. Arriving at the MS: Herbig Haro Jets • Radius can be more than 0.5 to 15 ly long!

42. Stellar Evolution and the H-R Diagram: • Evolution of a 1 solar mass star. • Main Stages: • Red Giant • Planetary Nebula • White Dwarf

43. Red Giant Stage: Antares • Radius increases and surface temperature decreases. Almost as large as the orbit of Jupiter. • Fusion of He in core into Carbon (C) is initiated. • From MS to PN stage is roughly 10**8 years.

44. Planetary Nebula Stage: • Lasts ~ 3 * 10**4 yrs. • Radius 0.25 to 2 ly.

45. White Dwarf Stage: HST Binary system of Sirius A and Sirius B • Sirius B. Dot in left corner. • High T, low L  R very small (e.g. size of Earth). • The stellar core after the PN has dissipated. • Fades over 100s * 10**9 yrs.

46. At which stage does material flow from a star for a 100 million years? • Main Sequence • Planetary Nebula • Giant Star • White Dwarf