1 The Stars 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. HNRT 227 Chapter October 2015

2 Chapter Outline The Nature of Stars The Anatomy of Stars The Variety of Stars The Life Cycles of Stars

3 The Nature of Stars

4 Astronomy –Oldest science (?) Star –Ball of gas –Fusion reactor All stars have a beginning and an ending

5 Measuring the Stars with Telescopes and Satellites Electromagnetic radiation Measurement of photons –Wavelength –Intensity –Direction –Variation

6 Telescopes

7 Orbiting Observatories Great Observatories Program –Hubble Space Telescope –Spitzer Infrared Telescope –Chandra X-Ray Observatory

8 The Anatomy of Stars

9 The Structure of the Sun Structure –Stellar core –Radiative zone –Convection zone –Photosphere –Chromosphere –Corona Solar Wind –Stream of particles

10 Our Sun

11 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

12 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

13 Magnetic Fields

14 Northern Lights (Aurora Borealis)

15 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  3 He + photon + energy 3) 3 He + 3 He  4 He + 2protons + photon + energy Life expectancy –11-12 billion years

16 The Variety of Stars Differences –Color –Brightness Distance Absolute brightness –Energy output –luminosity Apparent brightness Behavior –Total mass –age

17 The Cosmic Distance Ladder Distance –Light-years Measurement –Triangulation (parallax) –Spectroscopic parallax –Cepheid variable –Tully-Fisher –Supernovae Type Ia –Hubble’s Law

18 The Hertzsprung-Russel Diagram Star Groupings –Main-sequence stars –Red giants –White dwarfs

19 Step 1 to an H-R Diagram Plot for the 20 nearest and brightest stars to Earth

20 Step 2 to an H-R Diagram Reversing the y-axis

21 Step 3 to an H-R Diagram Reversing the x-axis –Lowest temperature to the right

22 A Standard H-R Diagram

23 The Life Cycles of Stars

24 The Birth of Stars Nebular Hypothesis –Laplace

25 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

26 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

27 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

28 Sun’s Life Cycle on H-R Diagram

29 The Main Sequence and the Death of Stars Very Large Stars –Successive collapses and burnings –Iron core –Catastrophic collapse supernova

30 Layers of Massive Star Si-> Fe Mg ->Si Ne -> Mg O ->Ne C -> O He -> CH -> He Fe Layers of Nuclear Fusion in High Mass Stars

31 Neutron Stars and Pulsars Neutron Star –Dense and small –High rotation rate –Little light Pulsar –Special neutron star –Electromagnetic radiation –End state of supernova

32 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