Stellar Interior: Physical Processes

Transcription

1 Physics Focus on Astrophysics Focus on Astrophysics Stellar Interior: Physical Processes D. Fluri,

2 Content 1. Mechanical equilibrium: pressure gravity 2. Fusion: Main sequence stars: hydrogen helium Formation of heavy elements 3. Internal structure of stars

3 Mechanical equilibrium Stars in continuous battle with own gravity Gravity points inward, pressure force points outward Pressure built up due to energy release at center (fusion) Star loses energy at surface supplies needed! Stop of fusion star contracts (at least its core)

4 Mechanical equilibrium Described by a fundamental equation of stellar evolution: dp dr GM r 2 r where P pressure at radius r G gravitational constant M r mass of sphere with radius r density at radius r

5 Solar interior: temperature and pressure

6 Content 1. Mechanical equilibrium: pressure gravity 2. Fusion: Main sequence stars: hydrogen helium Formation of heavy elements 3. Internal structure of stars

7 H and He formed 3 minutes after Big Bang Today s abundances of chemical elements

8 Fusion in stars Stars are fusion reactors Main sequence stars fuse ( burn ) H to He in the center 2 processes for H-burning (i.e. fusion H He): pp-chain ( proton-proton-chain ) CNO-cycle ( carbon-nitrogen-oxygen-cycle )

9 H-burning: pp-chain (only possibility for 1 st stars) Energy production rate: T 4 Too small for massive stars Very slow: ~10 9 y (lifetime Sun!) Big Bang: no time to wait, but neutrons available!

10 H-burning: pp-chain 1 st step: Energy release per process MeV (minus MeV, lost by neutrino) Positron e + annihilates with electron 2 photons released: MeV Mean waiting time per proton: 14 bill. years 2 nd step: Energy release per process MeV Mean waiting time per deuterium 1.4 s 3 rd step: Energy release MeV Mean waiting time per 3 He-nucleus: 1 mill. years Total energy release: [2 ( ) ] MeV = 26.2 MeV

11 H-burning: pp-chain Two alternatives for 3 rd step Via Be and Li, or via Be and B In the sun: 91% according to first process Branching ratio of different processes temperature dependent Required temperature (at typical pressures in stars): > 10 Mio. K

12 Neutrinos Elementary particles: Very small mass (only upper boundary known) No electric charge Only subject to weak interaction and gravity

13 Detection of neutrinos Neutrino flux on Earth: cm 2 s 1 Earth transparent for neutrinos Detectors: 1 km below Earth s surface Huge water containers, e.g tons of water (Super-Kamiokande, Japan) Only 2 3 detections per hour By the way (movie 2012): Energy of neutrinos blocked by Earth not sufficient to heat glass of tea (by orders of magnitude!) times more energy released by radioactive decays inside Earth

14 Detection of neutrinos Sun, observed with neutrino detector (Super-Kamiokande, Japan), exposure time ": 500 days Direct proof of fusion as source of energy!

15 H-burning: CNO cycle More efficient than pp-chain at high temperatures C, N, O catalysts Controlled by 12 C abundance: initially not possible in Pop. III stars! Alternative branch possible Chemical composition changes: 1 H reduced, 4 He increased 12 C reduced, 14 N increased 18

16 H-burning: CNO cycle C, N, O as catalysts Energy released: MeV Not possible in first stars (no 12 C) Duration of 1 cycle: 340 Mio. years Long, but faster than pp-chain! Stars release more energy by CNO-cycle Required temperature: > 14 Mio. K Alternatives possible (T > 22 Mio. K) from 15 N via 16 O 14 N Chemical composition modified: 1 H reduced, 4 He increased 12 C reduced, 14 N increased

17 H-burning: pp-chain CNO-cycle 10 mio. K < T < 18 mio. K pp-chain dominates (low mass stars, e.g. sun) T > 18 mio. K CNO-cycle dominates (massive stars) 18

18 H-burning Formation of He core After 5 billion years (sun): 5% of total mass fused H He H and He abundances not modified at surface! No mixing of core and surface! Example: sun

19 Content 1. Mechanical equilibrium: pressure gravity 2. Fusion: Main sequence stars: hydrogen helium Formation of heavy elements 3. Internal structure of stars

20 He-burning: Triple- process 3 4 He 12 C 4 He + 4 He 8 Be 8 Be + 4 He 12 C + unstable, decay in s Excited state of 12 C near energy! Energy production rate (at T 10 8 K): T 40

21 Formation of heavy elements When H used up in core: shell burning Core collapses until temperature high enough for He-burning And so on for heavier elements... C, O, Ne, Mg, Fe (if star massive enough) Onion shells Burning of heavier elements faster (Si Fe, Ni lasts 1 d)

22 Formation of heavy elements Fe: further fusion eats energy supernova Elements heavier than Fe form only in supernovae

23 Nucleosynthesis Cycle Accumulation of heavy elements in stars Young stars Low-mass stars (~1 M ) Time scale:10 10 y Massive stars (~10 M ) Time scale: 10 7 y C,N,O,Fe Supernovae Planetary nebulae C,O Interstellar material He,C Fe White Dwarfs Neutron Stars Black Holes

24 Content 1. Mechanical equilibrium: pressure gravity 2. Fusion: Main sequence stars: hydrogen helium Formation of heavy elements 3. Internal structure of stars

25 Internal structure of the sun Core: Fusion Energy released Up to 0.25 R Radiation zone Up to 0.7 R Energy transport by radiation Stable layers Convection zone Above radiation zone Energy transport by convection Layering unstable Material mixed within convection zone

26 Internal structure of stars Depends on stellar mass and evolutionary state Main sequence stars:

27 Internal structure of stars Hot stars (left side): only core convective Cool stars (right side): outer convection zone Later than M4 stars: fully convective

28 Prerequisites for convection? Displace plasma bubble upwards (adiabatically): 1. Density smaller than in surroundings further ascent convection 2. Density greater than in surroundings descent stable (no convection) Pressure equilibrium: Pressure in bubble = pressure outside Necessary condition for convection: Density in bubble smaller than outside after displacement Temperature in bubble greater than outside after displacement Temperature gradient steeper than adiabatic Temperature gradient