Download Astronomical Spectroscopy: An Introduction To The Atomic And by Jonathan Tennyson PDF

By Jonathan Tennyson

Approximately the entire info we all know concerning the Universe comes from the examine of sunshine because it reaches us. the certainty of this knowledge contained in mild calls for either telescopes in a position to resolving mild into its various part colours, in addition to particular wisdom of the quantum mechanical habit of atoms and molecules. This targeted e-book, that is in line with a third-year undergraduate direction given through the writer at college university London, provides the elemental atomic and molecular physics essential to comprehend and interpret astronomical spectra. It explains what info may be extracted from those spectra and the way. vast use is made from modern astronomical spectral info to either encourage the examine of the underlying atomic physics and to demonstrate the implications.

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The vertical axis of the spectra represents the observed line-to-continuum intensity, which is equivalent to exp(−τ line). A. , Astrophys. J. ] May 17, 2005 14:40 WSPC/SPI-B267: Astronomical Spectroscopy ch03 47 Atomic Hydrogen The 21 cm line is a powerful tool because: (1) Displacement of the line gives the line-of-sight velocity; (2) Its intensity gives the number of atoms. 9 × 10−15 s−1 which corresponds to a lifetime of 10 million years, so it is always optically thin; (3) The line profile can be used to deduce the temperature of the gas.

However, molecular masers are much more common and important than those seen in atomic sources. Molecular masers will be discussed in Sec. 5. 11 Radio Recombination Lines for Other Atoms An important feature of spectroscopy is that any atom, ion or molecule has a unique spectrum by which it can be readily identified. An exception to this general rule are the radio recombination lines. When a single electron is promoted to a state with a very high principal quantum number, n, the electron experiences a potential due to the ion core which feels like that due to a single point charge such as a proton.

Indeed, in the coolest stars (T < 5000 K), molecules form and little atomic H remains. For stars significantly hotter than 10000 K, H atoms become increasing ionised and the strength of the Balmer series drops again. This discussion illustrates a general point. Atoms lose electrons to become ions. As the environment gets hotter, the degree of ionisation increases. Since this is essentially a thermal effect, for every atom, there is a May 25, 2005 11:55 WSPC/SPI-B267: Astronomical Spectroscopy Atomic Hydrogen ch03 31 particular ionisation stage which is dominant at a particular temperature.

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