Study Guide for Quiz 3: ---------------------- Cosmic Distances: ----------------- Trigonometric Parallaxes RR Lyrae Variables Cepheid Variables Period-Luminosity Relation The Milky Way Galaxy -------------------- The Milky Way is our Galaxy Diffuse band of light crossing the sky Galileo: Milky Way consists of many faint stars The Nature of the Milky Way Philosophical Speculations: Wright & Kant What is the geometry of the Milky Way? A spherical shell, or a disk? Size of the MW from Star Counts: Herschels Star Gauges Kapteyn Model Globular Cluster Distribution: Shapley, RR Lyrae stars First to locate the Sun outside the center of the Galaxy. The importance of dust obscuration in calculating the luminosity distance. Nature of the "Spiral Nebulae" The Great Debate! ------------------------------ Two hypotheses: spiral nebulae are external, or internal to Milky Way? Island Universe Hypothesis (Kant & Humboldt) Nebular Hypothesis (Laplace) Role of finding distances in resolving the debate Leavitt: Cepheid Period-Luminosity Relation, distances Shapley-Curtis Debate (1920) Hubble: Cepheids in Andromeda The Milky Way & Andromeda ------------------------- Common Properties of the Milky Way & Andromeda Galaxies Disk & Spheroid Structure of the Galaxy Pop I Stars: Young, metal-rich, disk stars Ordered, nearly circular orbits in the disk Pop II Stars: Old, metal-poor, spheroid stars Disordered, elliptical orbits in all directions Chemical Evolution, connection to stellar populations Supermassive Blackholes Spiral Galaxies --------------- Disk & Spheroid Components Thick disk of stars, thin disk of dust, spiral arms Spheroid: bright central Bulge and faint extended Halo Rotation of the Disk Orbital period of the Sun in the Galaxy Measurement of Galaxy Masses from Rotation Curve, Doppler effect Spiral Arms: Outlined by O&B Stars, Gas & Dust clouds Sites of recent star formation. Why O&B stars don't move very far from their birthplaces? Types of Galaxies ----------------- Three basic types of Galaxies: Spirals Ellipticals Irregulars Dwarf Galaxies Differences between the types of galaxies in terms of Relative Gas content Star Formation History Internal Motions of stars: ordered rotation vs. disordered. Structure Groups & Clusters of Galaxies ----------------------------- Galaxies tend to group into Clusters The Milky Way is part of the Local Group Hierarchy of Structure: Groups: < 30 bright galaxies, many dwarfs Clusters: 30 - 100's of bright galaxies, many dwarfs Where Ellipticals & Spirals are found in Rich Clusters Superclusters: Clusters of Clusters Voids, Filaments, & Walls, porous structure of the universe Large scale structure reflects the initial perturbations to the density and temperature of the universe. Interacting Galaxies -------------------- Tidal Interactions occur between Galaxies Frequency of occurence: the relative distance between galaxies is small, particularly in comparison with stars. Cause of most of the "peculiar" galaxies observed Tidal distortion in encounters Types of interactions Close Tidal Encounters Galaxy-Galaxy Collisions Starbursts induced by interactions Stars pass through during a collision, but the gas shocks and forms lots of stars. Drive metal-rich winds into the inter-galactic medium. Mergers & Galactic Cannibalism How we think massive ellipticals and clusters are made. Special Relativity: ------------------ First Postulate (uniformly moving observers) Second Postulate (speed of light) Newton's conception of absolute space & absolute time Einstein's conception of space & time as relative How time appears to different observers (the photon clock experiment) General Relativity: ------------------ Explanation of gravity as curved spacetime Matter tells spacetime how to curve, Curved spacetime tells matter how to move. Experimental verification of GR: Perihelion shift of Mercury Bending of Starlight near the Sun Strong gravitational lensing of galaxies by clusters Upcoming experiments to image the event horizon of a black hole and gravitational waves from merging neutron stars and black holes. Einstein's Cosmology: -------------------- Cosmological Principle: Universe is Homogeneous and Isotropic on Large Scales Cosmological Constant Evidence of large-scale homogeneity & isotropy Observational Cosmology: ----------------------- Hubble's Law Hubble Parameter, H0 How it is measured Uncertainties in measuring H0 Current rate of expansion of the Universe We are not at the center: raisins in expanding cake, points on expanding balloon, etc. Redshift distances Redshift maps Tests of the Big Bang, Cosmic Timeline, Early Universe Big Bang Theory: --------------- Basic features of the theory Expansion of the Universe (Hubble's Law) Density Parameter (Omega0) Critical Density: Omega0 determines the geometry of the Universe Age of the Universe (Hubble Time), road trip analogy, the 'unit' of 1/H0 is time, the age of the universe. Primordial (Big Bang) Nucleosynthesis Production of Deuterium, Helium, and light metals (Li,Be,B) Predictions for observed abundances Comparison with predictions Cosmic Background Radiation Blackbody Spectrum & Temperature Observed properties Conditions at the Epoch of Recombination Dark Matter ----------- Observational evidence for dark matter: Rotation curves of galaxies, M = V^2 R/G Hot gas bound to galaxy clusters Motions of galaxies in galaxy clusters Gravitational lensing of background galaxies by clusters of galaxies. The physical nature of dark matter: MACHOS: massive compact halo objects: ancient, cold white dwarfs, neutron stars, black holes, brown dwarfs WIMPS: weakly interacting massive particles: something like a neutrino: interacts weakly with normal matter, but also gravity Other possibilities? Observational evidence for dark energy: The deviation of the Hubble diagram from a straight line indicates acceleration of the expansion of the universe. Explanantion: possibly the vacuum energy of the universe: the 'cost of having space' The overall budget of the universe. Physics of the Early Universe ----------------------------- The Cosmic Timeline Unification of the forces just after the Big Biang Separation of the forces as the Universe cools and expands Inflationary Epoch explains the smoothness and flatness of the Universe Emergence of matter at 10^-6 seconds after the Big Bang Recombination and the emergence of the visible Universe