The Night Sky The Universe Chapter 14 Homework: All the multiple choice questions in Applying the Concepts and Group A questions in Parallel Exercises. Celestial observation dates to ancient civilizations Stars Appear as point sources Twinkle from atmospheric turbulence Distance measured in light years (ly): 9.5x10 12 km Planets Visible by reflected light Extended sources The Celestial Sphere Stars Celestial objects projected onto imaginary sphere surrounding Earth Celestial equator, north and south poles align with Earth s equator and poles Altitude angle and azimuth angle determine location on celestial sphere Celestial meridian: east/west location of observer Objects appear to rotate about north/south poles Massive, dense balls of incandescent gas Powered by fusion reactions in their core Sun An average star Reference for understanding other stars Origin of stars Gaseous nebula Mostly hydrogen Shock waves induce gravitational collapse Gravitational energy released into higher temperatures and pressures Protostar Accumulation of gases that will become a star
Core Very hot, most dense region Nuclear fusion releases gamma and x-ray radiation Radiation zone Radiation diffuses outward over millions of years Convection zone Structured by hot material rising from the interior, cooling, and sinking Upper reaches: visible surface of star Sun surface temp. ~5,800 K Stellar Modeling Lifetime of the Sun Converts about 1.4x10 17 kg of matter to energy each year About 2,700 6000 lb SUVs! Born 5 billion years ago Enough hydrogen for another 5 billion years Lifetime depends on stellar mass Less massive stars have longer lifetimes More massive stars have shorter lifetimes Brightness of Stars Differences in stellar brightness 1. Amount of light produced by star 2. Size of star 3. Distance to star Apparent magnitude Scheme to quantify observed brightness First magnitude star 100 times brighter than sixth magnitude star Five uniform divisions in between Absolute Magnitude Brightness adjusted to a defined, standard distance Example: Sun Apparent magnitude = -26.7 Absolute magnitude = +4.8 Luminosity Total energy radiated into space per second Directly related to absolute magnitude Units correlated to Sun: 1 solar luminosity
Star Temperature Star Types Color variations apparent: red, yellow, bluish white Color related to surface temperature Blackbody radiation curves Red: cooler stars Blue: hotter stars Yellow: in between (Sun) Classification scheme Based on temperature: hottest to coolest O, B, A, F, G, K, M Hertzsprung-Russell diagram Plot of absolute magnitude versus stellar temperature Each dot = star Characteristic grouping Main sequence stars Red giants Novas White dwarfs Cepheid variables Standard star for distance calibrations (Hubble) Protostar stage Gravitational collapse Density, temperature and pressure increase 10 million K: fusion ignition temperature Dynamical equilibrium Inward force of gravity Outward pressure of fusion energy Star enters main sequence Life of a Star Stellar Evolution-1 Traces path across HR diagram Red giant stage Hydrogen in core exhausted Core collapses, heats Outer shell expands, cools Lifetimes O star ~ millions of years M star ~ trillions of years Mass determines ultimate fate of star
Stellar Evolution-2 The End - Massive Stars Late red giant stage Further core collapse and heating Helium fusion to carbon initiated Radius and luminosity decrease, moves back toward main sequence The end - less massive stars Helium fuel in core used up; helium and hydrogen fusion in shells exhausted Instabilities blow off outer layers into a planetary nebula Carbon core contracts to white dwarf; cools to black lump of carbon More mass: more gravitational contraction and heating Critical temperature: 600 million K Carbon fusion Heavier nuclei fuse, up to iron All fusion energy sources used up Energy expansion pressure lost Dynamic equilibrium disrupted Supernova Star collapses and rebounds from core Elements beyond iron created in explosion and distributed throughout Universe Subsequent events depend on mass of remaining core End States for Massive Stars Stellar Evolution - Summary Neutron star Remaining core between 1.4 and 3.0 solar masses Gravitational pressure fuses protons and electrons into neutrons Pulsar: rotating, magnetized neutron star Black hole Remaining core greater than 3 solar masses Gravitational collapse overwhelms all known forces Even light cannot escape the dense, compact object
Larger Scale Structures The Milky Way Binary systems Two gravitationally bound stars Most stars are in binary pairs, not ours Star clusters Tens to hundreds of thousands or more gravitationally bound stars Often share a common origin Galaxies Basic unit of the Universe Billions and billions of gravitationally bound stars Larger scale still Clusters of galaxies Superclusters of galaxies Billions and billions of galaxies! Visible as a diffuse band on a dark night Billions of stars, some bound in galactic clusters Structure 1. Galactic nucleus 2. Rotating galactic disk Diameter ~ 100,000 ly 3. Spherical galactic halo Contains ~150 globular clusters Other Galaxies Our nearest neighbors - the Local Group Dwarf galaxies ~1,000 light years in diameter Nearest dwarf being disrupted gravitationally by the Milky Way Andromeda 2 million light years away Very similar to Milky Way Classification scheme (Hubble) Elliptical, spiral, barred and irregular The Life of a Galaxy Big Bang Theory Universe evolved from an explosive beginning Supporting evidence 1. Microwave background radiation 2. Large scale expansion 3. Relative abundances of elements 4. Diffuse cosmic background radiation (COBE spacecraft) The end: Expansion forever or the big crunch?