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# Per13_over - Preview of Period 13 Astrophysics 13.1 What is...

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Preview of Period 13: Astrophysics 13.1 What is the origin of matter? How did matter form following the Big Bang? 13.2 How did galaxies form? Why did matter clump together into galaxies? 13.3 What is composition of stars? How can the chemical composition of stars be determined? 13.4 What is the age of the Universe? What is the Hubble constant? How can it be used to estimate the age of the Universe? 13-1

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Energy in the Early Universe Energy in the early Universe was in the form of radiation, rather than matter. As in the case of gases, the energy content of the radiation is proportional to its temperature. The temperature of the early Universe is estimated to have been 10 32 kelvin The energy per photon can be found from E = 3 k T with E = energy (in joules or electron volts eV) k = Boltzman’s constant =1.38 x 10 – 23 J/K (or 8.62 x 10 – 5 eV/K) T = temperature (in kelvin) 13-2
Temperature of Stable Particles Over time, the Universe expanded and cooled. To calculate the temperature at the time of formation of a particular type of particle, 1) Convert the particle’s restmass from megaelectron volts (MeV) into electron volts (eV) (1 MeV = 1 x 10 6 eV) 2) Solve E = 3 kT for T. Since the restmass is given in electron volts, use k = 8.62 x 10 – 5 eV/K. 13-3

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Photon Emission Emission spectra are bright lines light from the photons emitted by nuclei. Each chemical element has a characteristic pattern of spectral emission lines. Photon 1 Photon 2 An electron drops one energy level and emits a photon of visible light. Another electron drops two energy levels and emits a more energetic photon of visible light. 13-4
What Holds a Star in a Spherical Shape? The gravitational force exerts an inward pressure on the core of the star. If the pressure from the gravitational force is great enough to raise the temperature of the matter in the core to at least 15,000,000 K, nuclear fusion begins. Thermal energy from the core is transferred out to the star surface, which produces radiation pressure. In a stable star, the outward radiation pressure of the hot gas is balanced by the inward force of gravity. Inward force of gravity Outward force of radiation pressure 13-5

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A stationery light source emits waves of light uniformly in all directions. If the same light source moves to the right, the motion of the source means that the wavelengths are no longer evenly spaced. Spread of waves over time The space between waves is reduced on the right side (the wavelengths are shorter) and the space between waves on the left side is increased (the wavelengths are longer). The shorter wavelengths on the right side of the light source shift the light waves to the blue end of the visible light spectrum. The longer wavelengths on the left side of
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## This note was uploaded on 06/11/2011 for the course PHYSICS 104 taught by Professor Staff during the Winter '11 term at Ohio State.

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Per13_over - Preview of Period 13 Astrophysics 13.1 What is...

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