1. Unit 11: Stellar Evolution Mr. Ross Brown Brooklyn School for Law and Technology

2. In this unit we will learn about: Principles of stellar evolution models How mass plays a role in the evolution and fate of a star The lifetimes of low- & high-mass stars, & how to calculate lifetime using luminosity & mass The changes that occur as stars consume hydrogen Why a star’s core shrinks as its atmosphere expands as it leaves the main sequence The sequence of fusion reactions that take place in a high-mass star as it nears the end of its life

3. What are the primary factors that influence a star’s lifecycle? 23 March 2016 Do now: What are the forces that will influence a star’s lifecycle?

4. What are the primary factors that influence a star’s lifecycle? Gravity – From the moment of a star’s birth, gravity is the force that drives a star towards its own collapse – Resisted by generating energy and pressure Hydrostatic equilibrium

5. What are the primary factors that influence a star’s lifecycle? Solar Nebula Theory – How our Sun began – Interstellar cloud

6. What are the primary factors that influence a star’s lifecycle? When the Sun consumes about 90% of its hydrogen……….. (after about 10 billion years) – Core will shrink and grow hotter – Remaining hydrogen burns faster – That radiating energy make the Sun grow huge – But its outer edges will cool, turning red – After another billion years, Sun will shrink some but grow hotter still, a yellow giant

7. What are the primary factors that influence a star’s lifecycle? 24 March 2016 Do now: How will a massive star’s lifecycle be different from an average star such as our Sun?

8. What are the primary factors that influence a star’s lifecycle? Lifecycle of a low mass star

9. What are the primary factors that influence a star’s lifecycle? Massive stars – >8M ☉ – May only live 100 million years – Increased heat fuses elements into heavier elements: He  C, C  O, Si  Fe. – Fe can’t release energy, star instantly collapses – Boom!

10. What are the primary factors that influence a star’s lifecycle? Lifecycle of a high mass star

11. What are the primary factors that influence a star’s lifecycle? Stellar Recycling – All the energy radiating out, and all the matter after the Boom! – Each time through the cycle, more heavy elements are generated – We wouldn’t have terrestrial planets if not for this process

12. What are the primary factors that influence a star’s lifecycle?

13. What are the stages of star formation? 29 March 2016 Do now: Based on what we’ve discussed, what are the early stages of star formation?

14. What are the stages of star formation? Interstellar gas clouds – Gas and dust – Hydrogen (71%) and Helium (27%) – Dust is silicates, carbon, and iron compounds – Cold; 10°K – Gravity  collapse  disk – Protostar

15. What are the stages of star formation?

16. Protostar – Hotter; 1500°K – As gravity increases, so too does temperature – When temperature reaches ~1,000,000°K, nuclear reactions begin H  He

17. What are the stages of star formation?

18. What are the stages in the life of a star? 30 March 2016 Do now: What are the limits on the sizes of stars?

19. What are the stages in the life of a star? Stellar mass limits – 0.1 and 30M ☉ – Stars smaller than 0.1 are very rare. Why? – These low mass stars are very dim and are called “Brown Dwarf” stars due to their dim red light Too little gravity

20. What are the stages in the life of a star? Very high mass stars – Very rare. Why? – They emit so much radiation that their pressure prevents additional material from falling into them – As such, if there were a 200M ☉ star it would “shine” itself apart Too much gravity

21. What are the stages in the life of a star? The lifetime of a main-sequence star – Determined by _____________ & _____________ – Formula: t=10 10 m / l years mass luminosity

22. What are the later stages in the life of a star? 31 March 2016 Do now: How do stars die?

23. What are the later stages in the life of a star? The Death of a Star – The core continues to shrink. H is converted to He, He to C. – Burns hotter, grows, becomes more luminous – Due to size, outer layers cool considerably – Carbon and silicon atoms condense into “flakes” – “Mira” star streams gas through interstellar space

24. What are the later stages in the life of a star? As the star dies, there’s a shell of glowing gas, called a planetary nebula Astronomers thought it was a bubble, ejecting gas in all directions With better pictures (thanks Hubble) we now know it ejects gas in line with its axis …but it’s not a planet! Nope, not a bubble

25. What are the later stages in the life of a star?

27. 1 April 2016 Do now: Where are the heaviest elements of the periodic table produced?

28. What are the later stages in the life of a star? The old age of massive stars – As heavier and heavier elements are produced, and as the core compresses down further and further, ever heavier elements are created

29. What are the later stages in the life of a star?

30. For example: – 12 C + 16 O fuse into….. – 28 Si – As the necessary temperatures for these reactions increase, the star develops a layered effect, like an onion

31. What are the later stages in the life of a star? Core collapse! – Iron cannot burn, tightly bound nuclei – Fusion stops – Under this great gravity and pressure, protons and electrons “merge” becoming neutrons, and the core changes from a sphere of iron to a sphere of neutrons

32. What are the later stages in the life of a star? In less than a second, core shrinks from iron ball size of the Earth to a ball of neutrons about 6 miles across All this collapsing generates heat to billions of degrees, and in a sudden rush it explodes in a supernova

33. What are the later stages in the life of a star?

34. Supernova explosion propels material at 10,000 km/ sec Supernova remnant continues to expand

35. What are the later stages in the life of a star? How did astrophysical theories of stellar evolution develop? – 1850s, Robert Boyle, Jacques A. C. Charles, and Joseph Gay‐Lussac et al develop ideal gas law – 1924, Sir Eddington develops mass-luminosity relationship