1101_21_lowmass_stars

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Ohio State University *

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1101

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Astronomy

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Oct 30, 2023

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Astronomy 1101 Lecture 13 The Evolution of Low-Mass Stars Astronomy 1101 163 How long a star lives on the Main Sequence depends on its mass . High-mass M-S Stars: 10M sun has t life ~10 Million years Live fast, die young… Low-mass M-S Stars: 0.1M sun has t life ~10 Trillion years! Live very long lives. The Sun: t life ~10 Billion years! t life,MS ~10Gyr (M sun /M) 3 164 Protostar phase H ignition MS Star formation from Proto-star to the Main Sequence is rapid. Proto-star Phase lasts ~10 4-5 years Ends with 100% of energy from gravity (Kelvin-Helmholz) and in Hydrostatic Equilibrium. Pre-MS Phase lasts ~30 Myr Ends with 100% of energy from core H fusion, and the star is in Thermal Equilibrium. The star settles onto the Main Sequence , fusing H into He in its core. 165 The Sun is 4.6 Billion years old and in equilibrium. Hydrostatic Equilibrium : Gas Pressure = Self Gravity Thermal Equilibrium : Energy Generation = Luminosity What happens when the core runs out of H? 100% of energy from core H fusion for ~10 Billion years. Has consumed ~50% of its available core Hydrogen. 166

Astronomy 1101 Lecture 13 Main Sequence stars slowly get more luminous and grow slightly in radius as they age. As Hydrogen fuses into Helium, Helium accumulates in the core but remains inert (non-fusing). To maintain the core Pressure, the core has to get hotter . Higher core temperature runs Hydrogen fusion faster . Star gets a little more luminous and grows in radius to compensate. today 167 Hydrogen Exhaustion When core Hydrogen runs out, the Helium core begins to contract, shoving H fusion into a shell. Inert He Core H Fusion Shell Envelope Star’s surface cools, and it grows in radius to ~2x larger than before. Core H Exhaustion H Shell Formation 168 Inert He Core Hydrogen Fusion Shell Cool, Extended Envelope With no source of fusion energy, the inert Helium core contracts and heats up. H fusion moves into a shell heated by the contracting inert He core . The star grows into a cool, luminous Red Giant Star. H shell fusion heated by hot He core. Extra energy produced means higher luminosity. The envelope to grows rapidly (bigger R) and gets cooler . 169 Maximum radius for the Sun will be ~0.8 AU (current orbit of Venus)!! It takes the Sun about 1 Billion years to climb the Red Giant Branch The helium core contracts and heats up, but it is still too “cold” to ignite Helium fusion. 2.3R sun 165R sun The Sun will become ~2000x brighter , with ~165x larger radius near the end of the Red Giant phase. 170

Astronomy 1101 Lecture 13 A secondary reaction of Helium and Carbon makes Oxygen: The Triple-alpha Process ignites in the Helium Flash . At the top of the Red Giant Branch, He fusion ignites in the core when T core ~100 Million K. Fuses 3 Helium into 1 Carbon in a 2-step reaction: 171 Core helium fusion begins and the star settles onto the Horizontal Branch or “Red Clump”. Star shrinks, gets less luminous and hotter . Gets energy from core He fusion with some H shell fusion He Fusion core H Fusion Shell Envelope 172 The Horizontal Branch Phase is stable but very short lived. He fusion is ~10x less efficient than H fusion, and the star is ~50 x more luminous than on the Main Sequence. t ~ E/L He fusion only generates enough energy for ~100 Million years . Carbon and Oxygen build up in the core as it is too cool to ignite Carbon fusion (<600 Million K). L~ 45L sun R~10R sun 173 With no source of fusion energy, the inert Carbon-Oxygen core contracts and heats up. Helium fusion moves into a shell heated by the contracting inert Carbon-Oxygen core. Extra energy increases luminosity, making the envelope rapidly grow and get cooler again . The star grows into a Red Asymptotic Giant Star. Surrounded by a H fusing shell. Inert C-O Core Helium Fusion Shell Cool, Extended Envelope Hydrogen Fusion Shell 174

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