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AST101 Recurring Themes in Astronomy

Explore the fundamental principles and concepts in astronomy, including the assumption of uniformity, Ockham's Razor, gravity, conservation laws, equilibrium, radiation, blackbodies, telescopes, distances, the Hubble Law, and the hierarchy of structure in the universe.

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AST101 Recurring Themes in Astronomy

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  1. AST101Recurring Themes in Astronomy

  2. Assumption of Uniformity Physical laws and constants do not change with time or location.

  3. Ockham’s Razor • Given two explanations for a phenomenon, the simplest is generally to be preferred. • Or, Keep It Simple, Stupid (KISS)

  4. Gravity FG = Gm1m2/d2 The weakest of the 4 forces Controls the matter-dominated universe

  5. Mass Use Newton's Law of Gravitation: M = v2r/G (for an object in a circular orbit with velocity v at a distance r from an object of mass M) v: from Doppler shifts r: from angular separation x distance

  6. Energy • Potential Energy U • Kinetic Energy K • Virial Theorem: 2K+U=0 • E=mc2

  7. Conservation Laws • Energy • Momentum • Angular Momentum

  8. Equilibrium • Opposing forces are balanced. • Planets, stars and galaxies are in equilibrium because pressure or centripetal forces oppose gravity. • Most evolution (change) is slow

  9. Radiation • The speed of light c is constant • Photons carry energy: E = hν • Photons have wavelengths and frequencies: λν = c • Photons are emitted and absorbed by electrons

  10. Kirchoff's Laws Mechanisms for the emission and absorption of photons • continuum emission: hot opaque object • emission lines: hot transparent gas • absorption lines: cool transparent gas superposed on a hot continuum

  11. Blackbodies An opaque source in thermal equilibrium emits a blackbody spectrum. • The shape of the spectrum depends only upon the temperature T • The wavelength of peak brightness ~ T • Luminosity ~ area x T4 All opaque sources, from people to planets to stars, approximate blackbodies.

  12. Telescopes • Collect light • Provide spatial resolution Sloan SDSS 2.5m

  13. Techniques Imaging • Brightness • Spatial size Spectroscopy (Brightness as a function of wavelength) • velocities (redshifts or blueshifts) • temperatures • compositions

  14. Distances The distance ladder and standard candles • Trigonometric Parallax: direct determination of distance to a few hundred light years. • Spectroscopic Parallax: main sequence stars. • Main sequence fitting: clusters. • Giant branch/brightest giants: old clusters. • Cepheid period-luminosity relation: Pop I distances to a few tens of millions of ly. • Tully-Fisher relation: luminosities of spiral galaxies. • Brightest cluster galaxy: clusters of galaxies. • Type Ia supernovae peak brightnesses: distances to billions of light years • Hubble Law: v = H0d

  15. Hertzsprung-Russell Diagram

  16. Heirarchy of Structure • Planetary Systems • Star Clusters • Galaxies • Clusters of Galaxies • Superclusters The universe is dominated by dense structures in low density voids

  17. The Hubble Law V = H0d Expansion of the universe

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