Notes C: Difference between revisions

Revision as of 00:49, 23 January 2009

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 [[1st Week: Introduction to Nuclear Physics & Abundance Determination in Astrophysics C]]
-==Abundancies==
-Several related quantities for measuring the abundances of the various elements exist in astrophysics.  The first, and most obvious, is simply the number of particles of a given species <math>i</math> divided by the total number of particles in a given volume, that is, the <b>particle abundance:</b>
-:<math>X_i = \frac{n_{i}}{\sum_j{n_j}}</math>
-The '''relative abundance''' <math>\epsilon_i</math> of a species <math>i</math> is also sometimes given.  It's the log of the number of particles divided by the number of hydrogen particles in the volume, with a constant added to make negative values unlikely.
-:<math>\epsilon_i =\log_{10}({\frac{n_i}{n_H}})+12</math>
-Finally if we're interested in the portion of the mass made up by a given element, we can calculate the '''mass fraction''':
-:<math>X_i = \frac{n_im_i}{\rho}</math>
-We might also be interested in the number of electrons (or protons) in the sample relative to all atomic particles, so we also define the '''electron abundance''':
-:<math>Y_e = \frac{\sum{Z_in_i}}{\sum{A_in_i}}</math>