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Chapter 9 Life and Evolution of Stars Outline I. II. III. IV. V. VI. Masses of Stars: Binary Stars Variable Stars Spectral Types of Stars H-R Diagram The Source of Stellar Energy Life Story of a Star Video Trailer: Birth of Stars I. Masses of Stars: Binary Stars 1. Binary Stars More than 50 % of all stars in our Milky Way are not single stars, but belong to binaries: Pairs or multiple systems of stars which orbit their common center of mass. If we can measure and understand their orbital motion, we can estimate the stellar masses. The Center of Mass (Active_Figure_12) center of mass = balance point of the system Both masses equal => center of mass is in the middle, rA = rB The more unequal the masses are, the more it shifts toward the more massive star. Types of Binaries: 1. Visual Binaries 2. Spectroscopic Binaries 3. Eclipsing Binaries Visual Binaries The ideal case: Both stars can be seen directly, and their separation and relative motion can be followed directly. (Video_Visual_Binaries) Sirius A and Siruis B (white dwarf) Spectroscopic Binaries (Video_Spectr_Binaries) Usually, binary separation “a” can not be measured directly because the stars are too close to each other. A limit on the separation and thus the masses can be inferred in the most common case: Spectroscopic Binaries Spectroscopic Binaries (2) The approaching star produces blue shifted lines; the receding star produces red shifted lines in the spectrum. Doppler shift Measurement of radial velocities Estimate of separation “a” Estimate of masses Spectroscopic Binaries (3) Typical sequence of spectra from a spectroscopic binary system Time Eclipsing Binaries (Animation) Usually, the inclination angle of binary systems is unknown uncertainty in mass estimates Special case: Eclipsing Binaries Here, we know that we are looking at the system edge-on! Eclipsing Binaries (2) Peculiar “double-dip” light curve Example: VW Cephei Eclipsing Binaries (3) Example: Algol in the constellation of Perseus From the light curve of Algol, we can infer that the system contains two stars of very different surface temperature, orbiting in a slightly inclined plane. The Light Curve of Algol II. Variable Stars Video Trailer: Variable Stars Chi Cygni expands and dims, and then contracts and brightens over 408 days Variable Stars • A variable star is a star that has lost its hydrostatic equilibrium. The brightness and size of a variable star change with time as it evolves. • Two Types of Variable Stars: – Pulsating stars: stars that appear to pulsate and change brightness. Examples are: Cepheid variables – RR Lyrae – Neutron stars (1 to 60 days - About 1 day - A couple of seconds) – Exploding stars: stars that show extreme brightness variability. Examples are: Nova – Supernova – T Tauri Nova outburst (Active_Figure_27) III. Spectral Types of Stars Spectral Classification of Stars (1) Temperature Different types of stars show different characteristic sets of absorption lines. Spectral Classification of Stars (2) Mnemonics to remember the spectral sequence: Oh Oh Only Be Boy, Bad A An Astronomers Fine F Forget Girl/Guy Grade Generally Kiss Kills Known Me Me Mnemonics Stellar Spectra F G K M Surface temperature O B A The Composition of Stars From the relative strength of absorption lines (carefully accounting for their temperature dependence), one can infer the composition of stars. IV. H-R Diagram Organizing the Family of Stars: The Hertzsprung-Russell Diagram We know: Stars have different temperatures, different luminosities, and different sizes. Absolute mag. or Luminosity To bring some order into that zoo of different types of stars: organize them in a diagram of Luminosity versus Temperature (or spectral type) Hertzsprung-Russell Diagram Spectral type: O Temperature B A F G K M The Hertzsprung-Russell Diagram (Simulation) The Hertzsprung-Russell Diagram (2) Same temperature, but much brighter than MS stars Lα 2 R x where, L = Luminosity of star R = Radius of star T = surface temperature of the star. 4 T , The Brightest Stars The open star cluster M39 The brightest stars are either blue (=> unusually hot) or red (=> unusually cold). (Is this a contradiction?) The Radii of Stars in the Hertzsprung-Russell Diagram Betelgeuse Rigel Polaris Sun The Relative Sizes of Stars in the HR Diagram V. The Source of Stellar Energy Energy of Stars • All stars are considered as huge balls of gases where nuclear fusion in their cores produces most of their energies. • It is possible to calculate an approximate star’s lifetime by determining its mass (tlife ~ 1/M2.5) • Cold (red ones) stars have longer lifetime than hot stars: – O star: ~ 1 million years – G star (Sun): ~ 10 billion years – M star : ~ 5,000 billion years • First stage: all stars start fusing hydrogen (H) to make helium (He) • This stage is considered to be the longest stage in a star’s lifetime ( 90% of its total lifespan) • Second stage: Fusing of helium (He) to make carbon (C) • The life of some stars (like our Sun) stops after this stage, but others will continue processing heavier and heavier elements than carbon in their cores. • For the massive stars (more than 8 solar masses), iron will be the last element that a star can form in its core. • Stars start their lifetime with a light element core (H) and end up with a heavy element core. Simulation VI. Life Story of a Star Life Story of a Star • Stars are born inside huge interstellar clouds following three stages: – Giant molecular cloud – Dense cores – Protostar T Tauri star • Stars are divided into two main groups: – Stars with masses less than 8 solar mass – Stars with masses larger than 8 solar mass • Stars with mass less than 8 solar mass – – – – – – – Giant molecular cloud Dense core T-Tauri star Main-sequence star: fusing H to make He Giant star: fusing He to make C Planetary Nebulae White Dwarf (with mass less than 1.4 solar mass) A T-tauri stage of a star: fast stellar winds While on the main sequence, a star is in “hydrostatic equilibrium”: inward pressure due to gravity balances the outward pressure due to heat. Simulation Do we see white dwarfs? A special binary system: a white dwarf and a regular star Outcome: a nova (or Supernova type Ia) March 1935 Nova Herculis 1934 May 1935 • Stars with mass larger than 8 solar mass – Giant molecular cloud – Dense core – T-Tauri star – Main-sequence star: fusing H to make He – Supergiant star: fusing He to make C, O, Ne, Mg, Si, ..Fe – Supernova explosion – Neutron star (with mass less than 3 solar mass), black hole (with different masses..) Betelgeuse: a supergiant star Do we see neutron stars? Neutron star: size no bigger than a city (10-15 km) النبّاض اإلشعاعي Pulsar (Video1) Pulsar: Lighthouse Model Crab Pulsar نباض السرطان 30نبضة في الثانية Do we see black holes? M87 Another special binary system: a black hole and a regular star Life Story of a Star: Summary (Simulation) Properties of WD, NS, and BH (Simulation) • White dwarfs (WD): – Size: Earth’s size – Mass: less than 1.4 solar mass (Chandrasekhar limit) • Neutron stars (NS): – Size: 10 to 15 km – Mass: less than 3 solar mass • Black holes (BH): – Size: depends on the mass (3 - ….. Km) (Simulation) – Mass: 1 – ……. Solar mass