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The Stars HNRT 227 Chapter 14 22 October 2015 Great Idea: The Sun and other stars use nuclear fusion reactions to convert mass into energy. Eventually, when a star’s nuclear fuel is depleted, the star must burn out. 1 Chapter Outline • • • • The Nature of Stars The Anatomy of Stars The Variety of Stars The Life Cycles of Stars 2 iClicker Question • How often do you look up at the night time sky? – – – – A B C D Always (when possible) Often Sometimes Never 3 iClicker Question • Do you enjoy looking at the stars? – – A B yes no 4 The Nature of Stars 5 The Nature of Stars • Astronomy – Oldest science (?) • Star – Ball of gas – Fusion reactor • All stars have a beginning and an ending 6 Measuring the Stars with Telescopes and Satellites • Electromagnetic radiation • Measurement of photons – – – – Wavelength Intensity Direction Variation 7 Telescopes 8 Orbiting Observatories • Great Observatories Program – Hubble Space Telescope – Spitzer Infrared Telescope – Chandra X-Ray Observatory 9 iClicker Question • What source of data is used to analyze stars? – A – – B C – – D E chemical testing of actual samples of stellar matter electromagnetic radiation spacecraft in orbit around distant stars both B and C above no data is used 10 iClicker Question • Telescopes are devices that: – – – – A B C D focus and concentrate radiation magnify only visible light all use mirrors both B and C above 11 The Anatomy of Stars 12 The Structure of the Sun • Structure – – – – – – Stellar core Radiative zone Convection zone Photosphere Chromosphere Corona • Solar Wind – Stream of particles 13 Our Sun 14 More On Solar Structure • Hydrogen fusion takes place in a core extending from the Sun’s center to about 0.25 solar radius • The core is surrounded by a radiative zone extending to about 0.71 solar radius – In this zone, energy travels outward through radiative diffusion • The radiative zone is surrounded by a rather opaque convective zone of gas at relatively low temperature and pressure – In this zone, energy travels outward primarily through convection 15 How do we know the Sun’s interior? • Helioseismology is the study of how the Sun vibrates • These vibrations have been used to infer pressures, densities, chemical compositions, and rotation rates within the Sun 16 Magnetic Fields 17 Northern Lights (Aurora Borealis) 18 The Sun’s Energy Source: Fusion • • • Sun’s Energy Source – – Historical Current • hydrogen Fusion – 3-steps-hydrogen burning 1) P + P D + e+ + neutrino + energy 2) D + P 3He + photon + energy 3) 3He + 3He 4He + 2protons + photon + energy Life expectancy – 11-12 billion years 19 The Variety of Stars • Differences – Color – Brightness • Distance • Absolute brightness – Energy output – luminosity • Apparent brightness • Behavior – Total mass – age 20 The Cosmic Distance Ladder • Distance – Light-years • Measurement – Triangulation (parallax) – Spectroscopic parallax – Cepheid variable – Tully-Fisher – Supernovae Type Ia – Hubble’s Law 21 The Hertzsprung-Russel Diagram • Star Groupings – Main-sequence stars – Red giants – White dwarfs 22 Step 1 to an H-R Diagram 20 15 10 Absolute Magnitude • Plot for the 20 nearest and brightest stars to Earth Not an H-R Diagram 5 0 0 5000 10000 15000 20000 25000 30000 -5 -10 Temperature 23 Step 2 to an H-R Diagram • Reversing the y-axis Getting Closer to an H-R Diagram Temperature (K) -10 -5 Absolute Magnitude 0 5 10 15 20 0 5000 10000 15000 20000 25000 2430000 Step 3 to an H-R Diagram • Reversing the x-axis – Lowest temperature to the right Now it's looking Like an H-R Diagram Temperature (K) -10 -5 5 10 15 20 30000 25000 20000 15000 10000 5000 0 25 Absolute Magnitude 0 A Standard H-R Diagram 26 iClicker Question • Compared with other stars, our Sun is: – – – A B C an unusually large star not a star at all a rather ordinary star 27 iClicker Question • The outer part of the Sun, the part that actually emits most of the light we see, is called the: – – – – – A B C D E chromosphere convective zone photosphere core radiative zone 28 iClicker Question • The solar wind is composed of: – – – – A B C D and bits of neutral gas all kinds of organic substances air charged particles including hydrogen helium ions 29 iClicker Question • Northern lights result from interactions of: – A – B – C the Sun’s magnetic field with Earth’s gravity UV light from the Sun with Earth’s ozone layer the solar wind with the Earth’s magnetic field 30 iClicker Question • The Sun’s peak output of energy is in: – A – B – C the ultraviolet portion of the electromagnetic spectrum the visible portion of the electromagnetic spectrum the infrared portion of the electromagnetic spectrum 31 iClicker Question • What is the Sun’s energy source? – A – – – – B C D E combustion of hydrogen rich chemical fuels fusion of hydrogen fission of hydrogen radioactive decay gravitational collapse 32 The Life Cycles of Stars 33 The Birth of Stars • Nebular Hypothesis – Laplace 34 The Main Sequence and the Death of Stars • Stars much less massive than the Sun – Glows 100 billion years • No change in size, temperature, energy output – Brown dwarfs • Some don’t consider these stars 35 The Main Sequence and the Death of Stars • Stars about the mass of the sun – Hydrogen burning at faster rate – – – – • Move off main sequence Helium burning Red giant Begin collapse White dwarf 36 The Life Cycle of a Star Like the Sun • • • • • • • • Gas cloud Fragmentation Protostar Kelvin-Helmholz contraction Hayashi Track Ignition Adjustment to Main Sequence Hydrogen Core Depletion • • • • • • • • Hydrogen shell burning Helium flash Helium core burning Helium core depletion Helium shell burning Helium shell flashes Planetary nebula White Dwarf 37 Sun’s Life Cycle on H-R Diagram H-R Diagram with annotations for Stellar Stages Temperature (log) 100000 10000 1000 -10 Planetary Nebula Asymptotic Branch Horizontal Branch Helium Flash -5 Kelvin-Helmholtz Contraction Adjust to Main Sequence 5 Absolute Magnitude 0 Sub Giant Hayashi Core Track Contraction 10 White Dwarf 15 38 20 The Main Sequence and the Death of Stars • Very Large Stars – Successive collapses and burnings – Iron core – Catastrophic collapse • supernova 39 Layers of Massive Star H -> He Layers of He -> C C -> O O -> Ne Ne -> Mg Nuclear Fusion Mg -> in High Mass Stars Si Si -> Fe Fe 40 Neutron Stars and Pulsars • Neutron Star – Dense and small – High rotation rate – Little light • Pulsar – Special neutron star – Electromagnetic radiation – End state of supernova 41 Black Holes • Black Hole – Result of collapsed large star – Nothing escapes from surface – Cannot “see” them • See impact on other stars, dust, etc. • Detect x-rays, gamma rays 42 iClicker Question • It is estimated that the total lifetime of our Sun is 12 billion years. Roughly, how far is it through its hydrogenburning phase now? – – – – A B C D 10% 25% 50% 90% 43 iClicker Question • Triangulation and Cepheid variables are methods to measure: – – – – A B C D distances to stars energy output of stars lifetime of stars composition of stars 44 iClicker Question • A Hertzsprung-Russell diagram plots a star’s temperature versus its: – – – – – A B C D E color distance age size energy output 45 iClicker Question • Large dust and gas clouds are commonly found throughout space. They are called: – – – – – A B C D E red giants galaxies nebulae supernovae white dwarfs 46 iClicker Question • The fusion process in very large stars produces chemical elements up to: – – – – – A B C D E He C Fe U Pb 47 iClicker Question • All natural elements beyond iron are created in: – – – A B C supernova explosions fusion of very large stars the big bang 48 iClicker Question • An object that is so dense and massive that nothing, including light can escape from its surface is called: – – – – – A B C D E a a a a a supernova red giant white dwarf black hole neutron star 49 iClicker Question • Are you surprised that the chemical elements about you were made in a supernova? – – A B Yes No 50