1. Stellar Evolution: The Life Cycle of Stars Dense, dark clouds, possibly forming stars in the future Young stars, still in their birth nebulae Aging supergiant Stars are not eternal. They are being born, live a finite life time, and die.

2. Parameters of Giant Molecular Clouds Size: r ~ 50 pc Mass: > 100,000 M sun Dense cores: Temp.: a few 0 K R ~ 0.1 pc M ~ 1 M sun Much too cold and too low density to ignite thermonuclear processes Clouds need to contract and heat up in order to form stars. Stars are formed during the collapse of the cores of Giant Molecular Clouds.

3. Contraction of Giant Molecular Cloud Cores Thermal Energy (pressure) Magnetic Fields Rotation (angular momentum)  External trigger required to initiate the collapse of clouds to form stars. Horse Head Nebula Turbulence Factors resisting the collapse of a gas cloud:

4. Shocks Triggering Star Formation Globules = sites where stars are being born right now! Trifid Nebula

5. Sources of Shock Waves Triggering Star Formation (1) Previous star formation can trigger further star formation through: a) Shocks from supernovae (explosions of massive stars): Massive stars die young => Supernovae tend to happen near sites of recent star formation

6. Sources of Shock Waves Triggering Star Formation (2) Previous star formation can trigger further star formation through: b) Ionization fronts of hot, massive O or B stars which produce a lot of UV radiation: Massive stars die young => O and B stars only exist near sites of recent star formation

7. Sources of Shock Waves Triggering Star Formation (3) Giant molecular clouds are very large and may occasionally collide with each other c) Collisions of giant molecular clouds

8. Sources of Shock Waves Triggering Star Formation (4) d) Spiral arms in galaxies like our Milky Way: Spiral arms are probably rotating shock- wave patterns.

9. Protostars Protostars = pre-birth state of stars: Hydrogen to Helium fusion not yet ignited Still enshrouded in opaque “cocoons” of dust => barely visible in the optical, but bright in the infrared

10. Heating By Contraction As a protostar contracts, it heats up: Free-fall contraction → Heating

11. From cloud to protostar: gravity is the key for the collapse Initial cloud with some rotation (even very, very small) Cloud spins up as it collapse A protostar

12. From a protostar to a true star Gas is heated when it is compressed The central part of a protostar is compressed the most, and when the temperature there reaches 10 million K, hot enough to ignite hydrogen fusion, the collapse is halted by the heated generated by the nuclear reaction A new star is born, and its internal structure is stabilized, because the energy produced in the center matches the amount of radiation from the surface

13. From Protostars to Stars Higher-mass stars evolve more rapidly from protostars to stars than less massive stars

14. From Protostars to Stars The Birth Line: Star emerges from the enshrouding dust cocoon

15. The Orion Nebula: Evidence of Star Formation

16. In the Orion Nebula The Becklin- Neugebauer Object (BN): Hot star, just reaching the main sequence Kleinmann-Low nebula (KL): Cluster of cool, young protostars detectable only in the infrared Visual image of the Orion Nebula Protostars with protoplanetary disks B3 B1 O6

17. Open Clusters of Stars Large masses of Giant Molecular Clouds => Stars do not form isolated, but in large groups, called Open Clusters of Stars.

18. Young Star Clusters Ultraviolet radiation and strong stellar winds from young, hot, massive stars in open star clusters are compressing the surrounding gas. 30 Doradus NGC 602

19. Protostellar Disks Conservation of angular momentum leads to the formation of protostellar disks  birth place of planets and moons

20. Protostellar Disks and Jets – Herbig-Haro Objects Disks of matter accreted onto the protostar (“accretion disks”) often lead to the formation of jets (directed outflows; bipolar outflows): Herbig-Haro Objects

21. Protostellar Disks and Jets – Herbig-Haro Objects (2) Herbig-Haro Object HH34

22. Protostellar Disks and Jets – Herbig-Haro Objects (3) Herbig-Haro Object HH30

23. Stellar Structure Temperature, density and pressure decreasing Energy generation via nuclear fusion Energy transport via radiation Energy transport via convection Flow of energy Basically the same structure for all stars with approx. 1 solar mass or less Sun

24. The Source of Stellar Energy In the sun, this happens primarily through the proton-proton (PP) chain. Recall from our discussion of the sun: Sun must produce energy, or else it would cool off quickly (≈10 4 yr) Stars produce energy by nuclear fusion of hydrogen into helium.

25. The CNO Cycle In stars slightly more massive than the sun, a more powerful energy generation mechanism than the PP chain takes over: The CNO Cycle.

26. 4 1 H --> 4 He + energy ( E = mc 2 ) Two ways to do this fusion reaction: In the Sun, about 500 million tons/sec are needed! If M<1.1Mo: p-p chain If M>1.1 Mo: CNO cycle Energy output of p-p cycle depends mildly on T: 10% ΔT  46% ΔE, with 50% of energy being generated in 11% of mass Energy output of CNO has steep dependence on T: 10% ΔT  340% ΔE, with 50% of energy being generated in 2% of mass p-p cycle is a “direct way to fuse 4 H into 1 He CNO cycle needs the help of C, N and O (catalysts) C, N and O simply assist the reaction, but do not partecipate Final output is the same: 4 H fuse into 1 He

27. Energy Transport Energy generated in the star’s center must be transported to the surface. Physicists know of three ways in which energy can be transported:

28. Energy Transport (2) However, in stars, only two energy transport mechanisms play a role: Inner layers: Radiative energy transport Outer layers (including photosphere): Convection Bubbles of hot gas rising up Cool gas sinking down Gas particles of solar interior  -rays

29. Energy Transport Structure Inner radiative, outer convective zone Inner convective, outer radiative zone CNO cycle dominantPP chain dominant

30. Balance happens thanks to flow (transport) of radiation from center (hotter) to surface (colder) Conduction, radiation, convection Opacity is key to efficiency of radiation transport p-p stars: radiative core, convective envelope CNO stars: convective core, radiative envelope Small stars (M<~0.4 Mo) all convective

31. Hydrostatic Equilibrium Imagine a star’s interior composed of individual shells… Within each shell, two forces have to be in equilibrium with each other: Outward pressure from the interior Gravity, i.e. the weight from all layers above

32. Hydrostatic Equilibrium (2) Outward pressure force must exactly balance the weight of all layers above everywhere in the star. This condition uniquely determines the interior structure of the star. This is why we find stable stars on such a narrow strip (Main Sequence) in the Hertzsprung-Russell diagram.

33. Pressure and Temperature of a Gas

34. Outward thermal pressure of core is larger than inward gravitational pressure Core expands Expanding core cools Nuclear fusion rate drops dramatically Outward thermal pressure of core drops (and becomes smaller than inward grav. pressure) Core contracts Contracting core heats up Nuclear fusion rate rises dramatically The Stellar Thermostat

35. Summary: Stellar Structure Mass Sun Radiative Core, convective envelope; Energy generation through PP Cycle Convective Core, radiative envelope; Energy generation through CNO Cycle

36. A main-sequence star can hold its structure for a very long time (depending on its mass), until it has H to burn into He And then???? Thermal Pressure Gravitational Contraction

37. Stellar Evolution in a Nutshell Mass controls the evolution of a star! M < 8 M Sun M > 8 M Sun M core < 3M Sun M core > 3M Sun

38. Review Questions 1.Where are the birth places of stars? 2.What are the main components of a protostar? 3.When and how is a new star born? 4.What prevents a star from collapsing?