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loganus
2008-Dec-16, 12:17 AM
just want to comment first that this website is a godsend


hi can someone give me a simple explanation of how to date the universe by nuclear cosmochronology- and
ii) how you can use spectra lines of elements like Th OR Uranium to calculate abundances at present - as i dont understand how you can use instruments in the laboratory to measure the amount in stars at present compared with the amount billions of years ago
iii) how do we know which stars or meteorites to look at


all in all i understand the principle behind spectroscopy and absorbtion/emmission lines etc but i just do not know how they measure the amount of an element


i hope my questions make some kind of sense lol
thanks logan

Spaceman Spiff
2008-Dec-17, 04:09 AM
Kind of a tall order there, eh?

I suggest first going out to the web, read up on what you can, and then come back with a more focused question. Wikipedia has a nice, brief, introduction (http://en.wikipedia.org/wiki/Cosmochronology) to nuclear cosmochronology with links to articles for more information. A quick google search led me to several nice resources: 1 (http://nedwww.ipac.caltech.edu/level5/ESSAYS/Clayton/clayton.html), 2 (http://arxiv.org/abs/astro-ph/0109526), 3 (http://arxiv.org/abs/astro-ph/0105384), 4 (http://arxiv.org/abs/astro-ph/0207596), 5 (http://arxiv.org/abs/astro-ph/0202429), 6 (http://arxiv.org/abs/astro-ph/0209308), 7 (http://www.lpi.usra.edu/meetings/lpsc2005/pdf/1126.pdf). This (http://adsabs.harvard.edu/abstract_service.html) is a nice facility for tracking down peer reviewed articles and reviews in astronomy and astrophysics.

Deriving elemental abundances from stellar spectra is an enormous endeavor of astrophysics (mainly contained within the field of stellar atmospheres), and not something that can be easily described on a bulletin board. To get any meat at all, you have to dive in at the level of at least an upper level undergraduate course, if not higher. Try here (http://www.astro.uvic.ca/%7Etatum/stellatm.html) for an on-line course, and here is a link (http://www.amazon.com/Observation-Analysis-Stellar-Photospheres/dp/0521066816/) to a book (upper undergrad level) you can purchase.

In brief, a star's spectrum allows one to measure the strengths of absorption lines, itself an involved process. Knowledge of this, how absorption line profiles grow as a function of strength, and some atomic and other physical parameters allows for the determination of the column density (number of absorbers per unit area in a column passing through star's atmosphere) of a particular absorber due to a bound-bound transition forming a particular absorption line. Certain equations which are exponential functions of temperature allow one to correct for the states of excitation (Boltzmann) of the ion in question and for the relative abundance of that ion (Saha) compared to the total abundance of that element. Knowledge of quantum mechanics and especially radiation transfer are essential. And it gets much more complicated when the approximation of Local Thermodynamic Equilibrium (http://en.wikipedia.org/wiki/Thermodynamic_equilibrium) breaks down, as it inevitably does in stellar atmospheres.

Let us know if you need any further assistance getting started.

loganus
2008-Dec-18, 10:28 AM
thanks postmaster that helps alot ,im background reading on wiki now its real good thanks