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Download Lecture 13 Main Sequence and Low Mass Evolution
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Transcript
Astronomy 101 Main Sequence Membership • For a star to be located on the Main Sequence in the H‐R diagram: – must fuse Hydrogen into Helium in its core. – must be in a state of Hydrosta)c Equilibrium. • Relax either of these and the star can no longer remain on the Main Sequence. The Main Sequence is a Mass Sequence. • The locaCon of a star along the M‐S is determined by its Mass. – Low‐Mass Stars: Cooler & Fainter – High‐Mass Stars: Ho9er & Brighter • Follows from the Mass‐Luminosity RelaCon: • Luminosity ~ Mass3.5 Main Sequence High Mass Luminosity (Lsun) 106 104 102 1 10-2 Low Mass 10-4 40,000 20,000 10,000 5,000 Temperature (K) 2,500 Internal Structure • Nuclear reacCon rates are very sensiCve to core temperature: – P‐P Chain: fusion rate ~ T4 – CNO Cycle: fusion rate ~ T18 ! • Leads to: – Differences in internal structure. – Division into Upper & Lower M‐S by mass. Upper Main Sequence • Upper Main‐Sequence stars: – M > 1.2 Msun – TCore > 18 Million K • Generate Energy by the CNO Cycle • Structure: – ConvecCve Cores – RadiaCve Envelopes Upper Main Sequence Star Radiative Envelope Convective Core Lower Main Sequence • Lower Main‐Sequence stars: – M < 1.2 Msun – TCore < 18 Million K • Generate Energy by the Proton‐Proton Chain • Structure: – RadiaCve Cores – ConvecCve Envelopes Lower Main Sequence Star Convective Envelope Radiative Core The Lowest Mass Stars • For 0.25 < M* < 0.08 Msun: • Generate energy by the P‐P Chain • Fully ConvecCve Interiors: ConvecCve Core and ConvecCve Envelope • Reddest Main Sequence Stars Red Main Sequence Star Convective Envelope Convective Core Structure along the Main Sequence Main Sequence LifeCme • How long a star can burn H to He depends on: – Amount of H available = MASS – How Fast it burns H to He = LUMINOSITY • LifeCme = Mass ÷ Luminosity • Recall: Mass‐Luminosity RelaConship: • Luminosity ~ Mass3.5 Main Sequence LifeCme • Therefore: • LifeCme ~ 1 / M2.5 • The higher the mass, the shorter its life. • Examples: Sun: ~ 10 Billion Years 30 Msun O‐star: ~ 2 Million years 0.1 Msun M‐star: ~ 3 Trillion years Consequences: • If you see an O or B dwarf star, it must be young as they only live for a few Million years. • You can’t tell how old an M dwarf is because their lives can be so long. • The Sun is ~ 5 Billion years old, so it will last only for ~ 5 Billion years longer. Structure & Mixing • Upper & Lower M‐S Stars: – Core & Envelope are separate. – No mixing of nuclear fusion products between the deep core and the envelope. – Surface composiCon is constant over lifeCme. • Red Main Sequence Stars: – Fully mixed: core & envelope are convecCve. – Enhances surface helium composiCon? Structure & Mixing • Upper & Lower M‐S Stars: – Core & Envelope are separate. – No mixing of nuclear fusion products between the deep core and the envelope. – Surface composiCon is constant over lifeCme. • Red Main Sequence Stars: – Fully mixed: core & envelope are convecCve. – Enhances surface helium composiCon? Main Sequence Phase • Energy Source: H fusion in the core • What happens to the He created by H fusion? – Too cool to ignite He fusion – Slowly build up an inert He core • LifeCme: ~10 Gyr for a 1 Msun star (e.g., Sun) ~10 Tyr for a 0.1 Msun star (red dwarf) Hydrogen ExhausCon • Inside: He core collapses & starts to heat up. H burning zone shoved into a shell. Collapsing core heats the H shell above it, driving the fusion faster. More fusion, more heaCng, so Pressure > Gravity • Outside: Envelope expands and cools Star gets brighter and redder. • Becomes a Red Giant Star Red Giant Star Inert He Core H Burning Shell Cool, Extended Envelope Climbing the Red Giant Branch Luminosity (Lsun) 106 104 Red Giant Branch 102 H-core exhaustion 1 10 -2 10 -4 40,000 20,000 10,000 5,000 Temperature (K) 2,500 Climbing the Red Giant Branch • Takes ~1 Gyr to climb the Red Giant Branch – He core contracCng & heaCng, but no fusion – H burning to He in a shell around the core – Huge, puffy envelope ~ size of orbit of Venus • Top of the Red Giant Branch: – Tcore reaches 100 Million K – Ignite He burning in the core in a flash. Helium Flash • Triple‐α Process: Fusion of 3 4He nuclei into 1 12C (Carbon): Secondary reaction with 12C makes 16O (Oxygen): Leaving the Giant Branch • Inside: – Primary energy from He burning core. – AddiConal energy from an H burning shell. • Outside: – Gets ho9er and bluer. – Star shrinks in radius, gelng fainter. • Moves onto the Horizontal Branch Horizontal Branch Star He Burning Core H Burning Shell Envelope Horizontal Branch Helium Flash Luminosity (Lsun) 106 104 Horizontal Branch 102 Red Giant Branch H-core exhaustion 1 10 -2 10 -4 40,000 20,000 10,000 5,000 Temperature (K) 2,500 Horizontal Branch Phase • Structure: – He‐burning core – H‐burning shell • Triple‐α Process is inefficient, can only last for ~100 Myr. • Build up a C‐O core, but too cool to ignite Carbon fusion AsymptoCc Giant Branch • Aner 100 Myr, core runs out of He – C‐O core collapses and heats up – He burning shell – H burning shell • Star swells and cools Climbs the Giant Branch again, but at higher T • Asympto)c Giant Branch Star AsymptoCc Giant Branch Star H Burning Shell He Burning Shell Inert C-O Core Cool, Extended Envelope The AsymptoCc Giant Branch Asymptotic Giant Branch Luminosity (Lsun) 106 104 Horizontal Branch 102 Red Giant Branch H-core exhaustion 1 10 -2 10 -4 40,000 20,000 10,000 5,000 Temperature (K) 2,500 The InstabiliCes of Old Age • He burning is very temperature sensiCve: • Triple‐α fusion rate ~ T40! • Consequences: – Small changes in T lead to – Large changes in fusion energy output • Star experiences huge Thermal Pulses that destabilize the outer envelope. Core‐Envelope SeparaCon • Rapid Process: takes ~105 years • Outer envelope gets slowly ejected (fast wind) • C‐O core conCnues to contract: – with weight of envelope taken off, heats up less – never reaches Carbon igniCon temperature of 600 Million K • Core and Envelope go their separate ways. Planetary Nebula Phase • Expanding envelope forms a ring nebula around the contracCng C‐O core. – Ionized and heated by the hot central core. – Expands away to nothing in ~104 years. • Planetary Nebula • Hot C‐O core is exposed, moves to the len on the H‐R Diagram Planetary Nebula Phase C-O Core Envelope Ejection Luminosity (Lsun) 106 104 102 1 10 -2 White Dwarf 10 -4 40,000 20,000 10,000 5,000 Temperature (K) 2,500 Core Collapse to White Dwarf • ContracCng C‐O core becomes so dense that a new gas law takes over. • Degenerate Electron Gas: – Pressure becomes independent of Temperature – P grows rapidly & soon counteracts Gravity • Collapse halts when R ~ 0.01 Rsun (~ Rearth) • White Dwarf Star Summary: • Main Sequence stars burn H into He in their cores. • The Main Sequence is a Mass Sequence. – Lower M‐S: p‐p chain, radiaCve cores & convecCve envelopes – Upper M‐S: CNO cycle, convecCve cores & radiaCve envelopes • Larger Mass = Shorter LifeCme Summary: • Stage: • Energy Source: • Main Sequence • Red Giant • Horizontal Branch • AsymptoCc Giant • White Dwarf • H Burning Core • H Burning Shell • He Core + H Shell • He Shell + H Shell • None! QuesCons • What happens when main sequence stars run out of fuel? • The Sun will end its main sequence lifeCme in ~5 billion years; then what? • What are red giants? • What are white dwarfs?
