Stars and Stellar Evolution

Size: px

Start display at page:

Download "Stars and Stellar Evolution"

Benjamin Nicholson
5 years ago
Views:

1 Stars and Stellar Evolution

3 Stars and Stellar Evolution K.S. de Boer and W. Seggewiss 17 avenue du Hoggar Parc d activités de Courtabeuf, B.P Les Ulis Cedex A, France

4 Cover image: The stellar association LH 95 in the Large Magellanic Cloud showing star formation, young stars and old stars. HST-ACS image, courtesy of D. Gouliermis and NASA/ESA ISBN This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, re-use of illustrations, recitation, broad-casting, reproduction on microfilms or in other ways, and storage in data banks. Duplication of this publication or parts thereof is only permitted under the provisions of the French and German Copyright laws of March 11, 1957 and September 9, 1965, respectively. Violations fall under the prosecution act of the French and German Copyright Laws. c EDP Sciences, 2008

5 Contents 1 Introduction Historical background History of the characterization of stars History of the ideas about the evolution of stars Stellar evolution - the importance of gravity Relevance of stars for astrophysics Elementary astronomy and classical physics Classical observations The Planck function Spectral lines, metallicity, and gas conditions The surface parameters of stars The Hertzsprung-Russell Diagram, HRD Observational HRDs: M V with SpT or B V Physical HRD: luminosity L and effective temperature T eff Spectral energy distributions Relation between M V, M bol, and L Caution with mass - luminosity - temperature relations Surface parameters and size of a star Names of star types from location in the HRD Summary Stellar atmosphere: Continuum radiation + structure Introduction Radiation theory Definitions Radiative intensity Mean intensity, radiative flux Radiation density and radiation pressure The equation of radiation transport Exploring the equation of radiation transport a: No background intensity: Iν 0 = b: background intensity: Iν Graphic representation of the cases Thermodynamic equilibrium The radiative transfer in stellar atmospheres Effects of geometry Including all frequencies Continuity equation Special cases and approximations Atmospheres in LTE Plane parallel atmosphere iii

6 iv Limb darkening Gray atmosphere; Rosseland mean Structure of stellar atmospheres Temperature structure Pressure structure Opacity and the absorption coefficients Absorption due to ionization Total absorption cross section for hydrogen Absorption due to ionization of helium Absorption due to ionization of metals The H ion Absorption due to dissociation Free-free transitions Scattering Total absorption coefficient Effects of gas density on opacity Emission and the emission coefficient The spectral continuum and the Planck function Effects for the CMD Backwarming, blanketing Electron density and opacity effects Stellar atmosphere: Spectral structure Spectral lines Line profile Lorentz profile Pressure broadening Doppler broadening The Voigt profile Shape and strength of spectral lines and curve of growth Small optical depth in the line (τ 1 and/or α 1) Very large optical depth in the line (τ 1 and/or α 1) Intermediate α and/or τ Shape of curve of growth Statistics Boltzmann statistics and excitation equation Ionization and Saha equation Statistics and structure in stellar spectra Excitation Ionization Spectrophotometric methods Balmer jump and Balmer Series T eff and log g from Strömgren photometry Metallicity from Strömgren photometry Spectroscopy and the curve of growth Excitation Ionization Depth structure of atmosphere Abundance of elements Special features The G-Band Quasi-molecular absorption: H 2 and H Molecular absorption in cool atmospheres

7 v 3.5 Magnetic fields and Zeeman effect Gravitational settling and radiation levitation Stellar rotation Rotation broadening of spectral lines Rotation and average surface parameters T, M V, B V Stellar classification: the MKK system and newer methods Development of stellar classification towards the MKK system Quality of the MK classification process New classification methods Stellar structure: Basic equations Four basic equations for the internal structure Mass continuity Hydrostatic equilibrium Energy conservation Temperature gradient Radiative energy transport Convective energy transport Conductive energy transport Stability and time scales Virial theorem Kelvin-Helmholtz time scale Nuclear time scale Dynamical time scale Convection versus radiation Schwarzschild s criterion for convection Ledoux s criterion for convection Estimates for ad < rad Adiabatic gradient ad Radiative gradient rad Absorption-driven or radiation-driven convection? Large absorption coefficient κ Large flux F (r) Convective overshoot Mixing length theory Material functions Opacity Equation of state Ideal gas law Radiation pressure Degenerate gas Energy production functions: nuclear fusion and gravity Stellar winds Coronal models Radiative winds Line driven winds Continuum-driven winds Dust-driven winds Bi-stability winds: fast and dilute or slow and dense Winds enhanced due to stellar rotation Pulsation-driven winds

8 vi 5 Nuclear fusion in stars Energy production: fusion of H and He Binding energy of nuclei Estimates for the occurrence of hydrogen fusion The Gamow peak proton proton chain CNO cycle Temperature dependence of H-fusion energy production He fusion: the triple alpha process Nucleosynthesis Carbon and oxygen burning; α-capture Nitrogen burning Fusion to heavier elements General considerations (NSE); s-, r- and p-process s-process r-process p-process Nucleosynthesis and the Universe; Yields The burning of Lithium Neutrinos Mean free path for neutrinos Solar neutrinos Neutrino experiments The solar neutrino problem Neutrino oscillations The Sudbury Neutrino Observatory and solution of the problem Relevant neutrino reactions Advantages of heavy water The solution of the solar neutrino problem Nobel prize 2002 for neutrino research Stellar structure: Making star models The equations of state and their complications Polytropes; Consequences of differing equations of state The general polytropic equation Special polytropes Polytrope for ideal gas Completely convective stars Non-relativistic degenerate electron gas Relativistic completely degenerate electron gas Balance between internal pressure and gravitation The maximum mass of a normal star The minimum mass of a star Methods for solving the differential equations Numerical solutions Differential equations against mass shell Adding stellar evolution A model using gaussian functions Vocabulary for stellar structure: definitions Zero-age-main-sequence star parameters from models ZAMS: structure as a function of mass shell ZAMS: parameters along the ZAMS - a star as a leaky ball Similarity along the MS; homology; thermostat; luminosity and mass 96

9 vii A star as a leaky ball: general behaviour, effects of chemical composition Internal structure and chemical composition Consequences of nuclear enrichment for stellar structure Non-hydrogen stars Central temperature and density of He and C stars Summary Star formation, proto-stars, very young stars Evidence of star formation, populations, IMF Signs of present star formation Star-formation processes and results of star formation Molecular clouds: places of star formation Discovery and importance of interstellar molecules Characteristics of molecular clouds Observed phenomena in star forming regions Instabilities in the interstellar gas Gravitational instability (Jeans instability) Thermal instabilities Energy input and energy loss Density fluctuations and their growth Stability and ambipolar diffusion in molecular clouds Low efficiency of star formation Cloud support mechanisms Ambipolar diffusion Theoretical scenario of star formation Pre-main-sequence evolution (PMS evolution) Energy source of PMS stars Theory of pre main-sequence stars Contraction along the Hayashi line in the earliest phase The accretion rate Ṁ Bipolar outflows, jets, Herbig-Haro objects, disks Definition of bipolar outflows and Herbig-Haro objects Some physical characteristics of bipolar flows Circumstellar disks Origin of outflows Very young stars General characteristics of T Tauri stars T Tau stars and X-ray emission T Tauri stars as young objects Herbig Ae and Be stars Summary The almost stars: Brown Dwarfs Introduction and naming problems Nuclear fusion in brown dwarfs Deuterium burning Lithium burning Evolution and surface parameters of BDs How ubiquitous are BDs? Deuterium, litium and cosmology The limit to giant planets Summary

10 viii 9 Stars out of balance: from MS star to red giant Main-sequence stars Changes in the main-sequence phase Evolution due to the changing composition of the interior The end of the main-sequence phase Effects of convection on the MS phase Stars without inner convection (M init < 1.15 M ) Stars with inner convection (M init > 1.15 M ) Why and how does a star become red giant? A gedankenexperiment : the gravothermal hysteresis cycle The hysteresis cycle and real stars A second red giant phase The overall stellar thermal equilibrium (STE) Isothermal He core and Schönberg-Chandrasekhar limit Luminosity evolution of red giants Red giant luminosity depends on M init Effects of metallicity The core drives the evolution, the envelope follows Duration of the main-sequence phase Stellar evolution: Stars in the lower mass range Defining the low mass range The MS-mass limit of 1.15 M The MS-mass limit of 0.5 M H shell burning: the red giant phase Evolution of the RG core and of the H-burning shell The RG surface: spectral lines, mass loss and dust The end of the RG phase: He ignition, He flash Core He-burning stars The end of core He burning and on to the AGB General aspects The end of core He burning Envelope thickness, pulses, dredge-up, hot bottom burning, s-process fusion Low mass core He burners (M init <2 M ): Horizontal-Branch stars HB stars and the various types Metal content and age of HB stars, morphology of HBs Evolution of stars on the HB and toward the AGB AGB stars: structure and evolution AGB star evolution and the CMD He-shell flashes (thermal pulses) and convection Third dredge-up: nuclear fusion and s-process Flashes and mass loss of fusion enriched material All happenings in a very thin layer Higher mass core He burners: blue loop stars and the AGB Stars with M init larger than 7 to 8 M Stars with M init = 2 to 7 M AGB stars and hot bottom burning Timescales The end of the AGB phase Massive AGB stars: OH/IR stars and pagb stars Low mass AGB stars: pagb stars and planetary nebulae

11 ix 10.6 The end phase: white dwarfs Classification of WDs Ultimate fate of WDs Born-again stars Initial to final mass relation for lower MS stars Some special stars Pulsational variables: RR Lyrae, δ Cepheids, PG 1159 and ZZ Ceti stars RR Lyrae stars δ Cepheid stars PG 1159 stars ZZ Ceti stars (pulsating WDs) λ Bootes stars Cool subdwarf stars Blue stragglers Gaps and bumps in the MS, HB, AGB Gap on the main sequence Gaps on the HB The RGB and AGB bumps The Red clump Summary Stellar pulsation and vibration Describing a star with oscillations The formalism Oscillations and limiting frequencies The driving forces of oscillations Spherically symmetric radial pulsations Formalism for radial pulsation Atmospheric radial pulsations Details of the κ mechanism Types of pulsational variables The instability strip: δ Cep, W Vir, RR Lyr, δ Sct, DA variables Cepheids RR Lyr δ Sct DA variables or ZZ Cet stars Main-sequence variables Red variables: Miras Massive variables (LBVs) Vibrations Helioseismology Asteroseismology Doppler imaging and spotted stars Doppler-shift asteroseismology Photometric asteroseismology PG 1159, sdb, and DB variables The Solar cycle of 11 years; effects on climate Stellar coronae, magnetic fields and sunspots Stellar coronae Effects of radiation transport Magnetic fields Sunspots Prominences and flares

12 x 12.6 Relevance of the structures for stellar evolution Stellar evolution: Stars in the higher mass range Defining the high mass range Types of high mass stars The O and Of-type stars Determining the temperature of O stars Determining the mass of O stars Oe/Be stars Summary O type stars B type stars Wolf-Rayet (WR) stars Luminous blue variables: LBVs; P-Cygni stars Red supergiant stars Expanding envelopes, luminous winds Processes of radiation acceleration Radiative acceleration by the continuum Radiative acceleration through spectral lines Making a P-Cyg profile Mass loss Velocity profile Density profile Evolution and the HRD General nature of evolution of high mass stars Evolution of stars of M When does a star evolve with a blue loop? Evolution of a 60 M star Evolution and effects of metallicity Evolution and effects of rotation See a star evolve: P Cygni Nuclear fusion times and endphases of high mass stars Rotation and stellar evolution General aspects of rotation Rotation and effects of deformation Rotation and variation in T eff Rotation and effective gravity Possible effects of rotation on structure Rotation and meridional circulation Rotation driven instabilities Brunt-Väisälä oscillations Solberg-Høiland instability Baroclinic instability Shear instability Rotation of the Sun Convective flows will be turbulent Braking internal rotation Stabilizing forces Redistribution of angular momentum with evolution Magnetic field and rotation Rotation makes a magnetic field stronger Rotation braking by magnetic fields Loosing angular momentum

13 xi 14.6 Rotation and mass loss Mass loss disks Mass loss and loss of angular momentum Chemical effects of rotation: mixing Rotation and mass loss affect high mass star evolution Rotation and mass accretion affect WD evolution The first stars First stars have very low metal content Making a star in metal-free gas Evolution of first stars Nucleosynthesis in Population III stars Lithium in first stars Models and variation of free input parameters Effects on models and evolution Complications with convection Effects of metal content Effects of mass loss Effects of rotation Effects of combined parameters Degenerate stars: WD, NS, BH White dwarfs Internal structure of WDs Atmosphere of a WD Cooling and crystallization of a WD; cooling time Chandrasekhar limit, maximum mass of a WD Transfer of mass onto a WD; Eruptions Can a WD become NS? Neutron stars Two ways for stars to become NS Structure and mass of neutron stars The surface layers of a NS Behaviour of neutron stars: pulsars Strange (quark) stars Black holes Schwarzschild radius Observational evidence for the presence of stellar black holes Nobel prize 2002 for X-ray astrophysics Supernovae Historical supernovae, supernova rate Observed types of supernovae Theories about supernovae Hydrodynamic (core collapse) supernovae Onset of the collapse The collapse End of the collapse and rebounce The explosion Decay of luminosity Endothermic nuclear reactions and light curve bump Deceleration Thermonuclear supernovae

14 xii Other mechanisms to make SNe Supernovae and their progenitors Hypernovae / Gamma-ray bursts Initial mass of stars becoming super- or hypernova SN Type Ia and cosmology SN 1987A in the LMC SN 1987A itself Effects of SN 1987A on its environment Endproduct of first stars: M init to M final Evolution of binary stars Introduction Equipotential surfaces Mathematical formulation Graphical representation of equipotential surfaces Mass exchange General case Conservative mass exchange Classification scheme for close binary systems Complications Non-conservative mass exchange Accretion disks Common envelopes; merging stars Evolution of binary stars Towards massive X-ray binaries and beyond Towards low-mass X-ray binaries Microquasars Low mass binary systems: towards cataclysmic binaries, SN Ia WDs and rotation: Nova and SN Ia phenomena Variety of binary evolution; special objects explained Multiple branching in binary evolution Special objects now explained by binary evolution Cataclysmic variables; Novae; Supersoft X-ray sources Type Ia supernovae Type Ib and Ic supernovae X-ray binaries (HMXB, LMXB) Binary pulsars High speed OB stars Merged stars Summary Luminosity and mass function The luminosity function The stellar initial mass function Power law mass functions; equivalences Salpeter mass function Relation between the luminosity and mass functions Determinations of the mass function Star clusters Open clusters Globular clusters Mass segregation Field stars Completeness of the photometry

15 xiii Results for mass functions The high-mass end of the IMF The IMF and its universality The mass function for the first stars Isochrones Definition Examples Effects of metal content of stars Transforming (L,T )-isochrones to (M V,B V )-isochrones Difference between isochrones and evolutionary tracks Using isochrones in CMDs Synthetic CMDs Special CMD-regions to find the age of star groups Star formation history (SFH) Photometric SFH SFH and synthetic spectral energy distributions Stars influence their environment Star formation and IS cloud metal content Effects of first stars Chemical evolution Consumption of primordial D, Li, He Metal production and yield Radioactive decay and nucleochronometry What comes of all evolution? Stars and their light Stellar remnants Gas returned to IS space Summary; Questions, Constants, Acronyms, Lists Stars and their structure Stars and their evolution Stellar evolution in comparison Stars and effects for their environment List of questions Acronyms, Constants, Abbreviations List of Figures List of Tables Index

Stellar Astronomy Sample Questions for Exam 4

Stellar Astronomy Sample Questions for Exam 4 Chapter 15 1. Emission nebulas emit light because a) they absorb high energy radiation (mostly UV) from nearby bright hot stars and re-emit it in visible wavelengths.