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Transcript
Unit 11: Stellar Evolution Mr. Ross Brown Brooklyn School for Law and Technology 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 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? 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 What are the primary factors that influence a star’s lifecycle? • Solar Nebula Theory – How our Sun began – Interstellar cloud 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 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? What are the primary factors that influence a star’s lifecycle? • Lifecycle of a low mass star 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: HeC, CO, SiFe. – Fe can’t release energy, star instantly collapses – Boom! What are the primary factors that influence a star’s lifecycle? • Lifecycle of a high mass star 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 What are the primary factors that influence a star’s lifecycle? 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? 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 What are the stages of star formation? What are the stages of star formation? • Protostar – Hotter; 1500°K – As gravity increases, so too does temperature – When temperature reaches ~1,000,000°K, nuclear reactions begin • H He What are the stages of star formation? What are the stages in the life of a star? • 30 March 2016 • Do now: What are the limits on the sizes of stars? What are the stages in the life of a star? • Stellar mass limits – 0.1 and 30M☉ Too little gravity – 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 What are the stages in the life of a star? • Very high mass stars Too much gravity – 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 What are the stages in the life of a star? • The lifetime of a main-sequence star luminosity mass – Determined by _____________ & _____________ – Formula: t=1010m/l years What are the later stages in the life of a star? • 31 March 2016 • Do now: How do stars die? 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 What are the later stages in the life of a star? • As the star dies, there’s a shell of glowing gas, …but it’s not a planet! called a planetary nebula • Astronomers thought it was a bubble, ejecting Nope, not a bubble gas in all directions • With better pictures (thanks Hubble) we now know it ejects gas in line with its axis What are the later stages in the life of a star? What are the later stages in the life of a star? What are the later stages in the life of a star? • 1 April 2016 • Do now: Where are the heaviest elements of the periodic table produced? 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 What are the later stages in the life of a star? What are the later stages in the life of a star? • For example: – 12C + 16O fuse into….. –28Si – As the necessary temperatures for these reactions increase, the star develops a layered effect, like an onion 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 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 What are the later stages in the life of a star? What are the later stages in the life of a star? • Supernova explosion propels material at 10,000 km/ sec • Supernova remnant continues to expand 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