ASTROPHYSICS REVIEW

# ASTROPHYSICS REVIEW - ASTROPHYSICS REVIEW Radiation Process...

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ASTROPHYSICS REVIEW Radiation Process o Flux and Magnitude : The flux is radiative energy moving in the form of photons at a certain distance from the source, per unit time, per unit area. Inverse square law. Apparent Magnitude is a measure of the brightness of an object as measured by someone from the earth. The brighter the object, the lower the value of the magnitude. Absolute Magnitude is a measure of the apparent magnitude of an object if it was placed at standard luminosity distance, 10 parsecs. Bolometric Magnitude is the total energy output of the astronomical object. m = -2.5log10 F + constant , where F is the flux and m is the bolometric magnitude. o Optical Depth : Optical Depth is a measure of transparency and is defined as the fraction of radiation that is scattered or absorbed on a path. τ ν ( s ) = α ν ( s ) ds d τ n =a n ds Optically Thick : Usually a medium is optically thick is it satisfies the following: τ ν ≥1. Optically Thin : Usually a medium is optically thin if it satisfies the following: τ ν < 1. o Blackbody Radiation: Radiation that is in thermal equilibrium with matter in the surroundings. Essentially no radiation passes through it and none is reflected. As a result, spectrum of blackbody radiation is directly related to and only dependent on the temperature of the body.

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Increasing temperature → Increase in B ν . Higher temperature shifts the peaks to higher frequency/shorter wavelength. Wien’s Displacement Law – Peak shifts linearly with increasing temperature to higher frequency. Rayleigh-Jean’s law and Planck’s law of black body radiation . (Equations). L = 4ρ R 2 σ 4 T for a star with radius R, and temperature T, emitting as a blackbody. F = σT 4 for emergent Flux F, T is the effective temperature. o Radiation from atoms: Atomic Processes: Bound – Bound: electron moves between two bound states (orbitals), in an atom or an ion. Photon is either emitted or absorbed. At a particular temperature, probability of some transitions higher than others. Some levels have a higher probability of being occupied, and not all transitions are allowed. (selection rules). Bound – Free: Ionization (Bound to Unbound) or Recombination (Unbound to bound). Free – Free: Free electron gains energy by absorbing a photon as it passes nearby an ion or an atom, or loses energy by emitting a photon. (Bremsstrahlung) 21 cm radiation: Ground state of hydrogen atom has two states that correspond to different orientations of the electron spin relative to the proton spin. Hyperfine Splitting – very slightly different energies. Energy difference corresponds to 21 cm wavelength. Very important for astronomy since the universe is abundant with neutral hydrogen. Spectral Lines: Two layer star model: Radiation starts from the inner layer as blackbody radiation at Temperature T in . Escapes through an atmosphere of optical depth τ and temperature T out .
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