Main sequence stars. Haris Ðapo. Antalya Lecture 3. 1 Akdeniz University, Antalya

Transcription

1 Main sequence stars Haris Ðapo 1 Akdeniz University, Antalya Antalya Lecture 3. Haris Ðapo (Akdeniz University) Main sequence stars Main sequence stars 1 / 22

2 Outline 1 Introduction 2 Hydrogen burning pp chain CNO cycle 3 Energy transport 4 Surface 5 Summary Haris Ðapo (Akdeniz University) Main sequence stars Main sequence stars 2 / 22

3 Outline 1 Introduction 2 Hydrogen burning pp chain CNO cycle 3 Energy transport 4 Surface 5 Summary Haris Ðapo (Akdeniz University) Main sequence stars Main sequence stars 3 / 22

4 Main sequence star a main sequence star burns hydrogen Proxima Centauri and Sun Procyon Celestia and Archenar Haris Ðapo (Akdeniz University) Main sequence stars Main sequence stars 4 / 22

5 Hertzsprung - Russell diagram Haris Ðapo (Akdeniz University) Main sequence stars Main sequence stars 5 / 22

6 Basics After the collapsing phase to a main-sequence star, along the path on the H-R diagram, a star starts to burn hydrogen the longest phase in the stars life M (M ) MAIN SEQUENCE (10 6 yrs) , , , Haris Ðapo (Akdeniz University) Main sequence stars Main sequence stars 6 / 22

7 Outline 1 Introduction 2 Hydrogen burning pp chain CNO cycle 3 Energy transport 4 Surface 5 Summary Haris Ðapo (Akdeniz University) Main sequence stars Main sequence stars 7 / 22

8 pp chain - Idea proton - proton chain reaction (pp chain) is how stars converts hydrogen to helium pp chain dominates in stars the size of the Sun or smaller and was the only way Pop. III stars could burn hydrogen pp fusion if kinetic energy of the protons is high enough to overcome Coulomb barrier the reaction proceeds via tunneling and would not be practical otherwise however helium-2 is unstable!; we need beta decay to change one proton into a neutron immediately In the Sun, deuterium-producing events are rare enough that a complete conversion of its hydrogen would take more than years Haris Ðapo (Akdeniz University) Main sequence stars Main sequence stars 8 / 22

9 pp chain - Network Haris Ðapo (Akdeniz University) Main sequence stars Main sequence stars 9 / 22

10 pp chain e + e + 2γ(1.02MeV) ppi chain releases a net energy of 26.7 MeV. the pp I branch is dominant at temperatures of 10 to 14 MK the pp II branch is dominant at temperatures of 14 to 23 MK 90% of the neutrinos in the reaction 7 Be(e,ν e ) 7 Li have energy of MeV, while the rest 10% carry MeV the pp III chain is dominant if the temperature exceeds 23 MK pp III chain generates high energy neutrinos (up to MeV) hep has never been observed due to its great rarity the total energy yield of one whole chain is MeV only energy released as gamma rays supports a star against gravity, neutrinos do not the neutrinos in the ppi, ppii and ppiii chains carry away 2.0%, 4.0% and 28.3% of the energy in those reactions in more massive stars this ratios will change in favor of neutrinos Haris Ðapo (Akdeniz University) Main sequence stars Main sequence stars 10 / 22

11 Sun s neutrino spectrum Haris Ðapo (Akdeniz University) Main sequence stars Main sequence stars 11 / 22

12 CNO cycle I (the other hydrogen burning) CNO for carbon-nitrogen-oxygen CNO cycle dominates in stars > 1.3M pp-chain starts at , CNO chain starts at K because of stronger temperature dependence CNO starts to dominate at K (Sun: K) carbon, nitrogen and oxygen are catalyst for conversion of hydrogen into helium there is also and extended CNO cycle (with fluorine), but it is rare while in CNO no new C or N or O nuclei are produced in run up to equilibrium, relative proportions of the nuclei are altered star with CNO are limited to max mass of about 150 M while primordial starts with only hydrogen and helium could go up-to 250 M Haris Ðapo (Akdeniz University) Main sequence stars Main sequence stars 12 / 22

13 CNO cycle II Haris Ðapo (Akdeniz University) Main sequence stars Main sequence stars 13 / 22

14 Helium core As it converts hydrogen to helium star forms a helium core. Haris Ðapo (Akdeniz University) Main sequence stars Main sequence stars 14 / 22

15 Outline 1 Introduction 2 Hydrogen burning pp chain CNO cycle 3 Energy transport 4 Surface 5 Summary Haris Ðapo (Akdeniz University) Main sequence stars Main sequence stars 15 / 22

16 Radiation and Convection zones convection zone of a star is the range of radii in which energy is transported primarily by convection in the radiation zone, energy is transported by radiation stellar convection consists of mass movement of plasma convection only occurs if the temperature changes rapidly (i.e. there is a large difference between their bottom and top temperatures because of CNO cycle strong dependence on temperature stars slightly more massive than the sun will have a convective core in sun and similar stars the core will be in radiative zone however in very low mass stars and red giants the entire star will be in convection zone Haris Ðapo (Akdeniz University) Main sequence stars Main sequence stars 16 / 22

17 Convection Haris Ðapo (Akdeniz University) Main sequence stars Main sequence stars 17 / 22

18 Outline 1 Introduction 2 Hydrogen burning pp chain CNO cycle 3 Energy transport 4 Surface 5 Summary Haris Ðapo (Akdeniz University) Main sequence stars Main sequence stars 18 / 22

19 Photosphere the photosphere is the region from which externally received light originates a stars spectral type and temperature originate in the photosphere of course the atmosphere of a star is partially transparent so the last scattering point for photons coming out of the star lies in a region and is not a sharp surface it also depends on the energy of the photon this leads to various definitions of where you define the surface and thus the temperature we can only investigate the photosphere of sun in detail the sun: K; effective temperature 5800 K material constantly rises to the surface and falls down (boiling) above photosphere are chromosome and corona features: solar flares and sunspots Haris Ðapo (Akdeniz University) Main sequence stars Main sequence stars 19 / 22

20 Outline 1 Introduction 2 Hydrogen burning pp chain CNO cycle 3 Energy transport 4 Surface 5 Summary Haris Ðapo (Akdeniz University) Main sequence stars Main sequence stars 20 / 22

21 Summary main sequence stars burn hydrogen via pp chain reaction of CNO cycle stars spend most of there life as a main sequence star convective and radiative zones what astronomers observe is the photosphere of a star Haris Ðapo (Akdeniz University) Main sequence stars Main sequence stars 21 / 22

22 Îlginiz için teşekkür ederim! Haris Ðapo (Akdeniz University) Main sequence stars Main sequence stars 22 / 22