Stellar spectra

Atoms and molecules have available to them a number of energy levels associated with the allowed values of the quantum numbers for the energy levels of the atom. As atoms are heated, some will gain sufficient energy either from the absorption of photons or by collisions to populate the levels above the ground state. The partitioning of energy between the levels depends on temperature and the atom is then said to be in local thermal equilibrium with the populations of the excited states and so the local temperature can be measured with this atomic thermometer. 

The Balmer series is seen in many but not all stars because the first energy level in the series is an excited state with quantum number n = 2. There has to be a mechanism by which the excited stated is populated and this is the local temperature of the star. Hence only if the star is sufficiently hot will the spectrum contain the Balmer series. The strength of the Balmer series within the stellar spectrum can be used to derive a temperature for the surface of the star to compare with black body temperature and the B/V ratio. 

When the atom is in thermal equilibrium with its surroundings in the photosphere of the star, the population of the n = 2 level is given by the Boltzmann law  

For the Sun the ratio 2/ i = 5.15 x 1(T9 and the Balmer series lines are not very strong. The difference in the energy levels for the H atom is given by the expression  

The degeneracies of each of the levels can be calculated from the value of n, the allowed values of l and the total degeneracy of the /-orbitals  

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Figure 9-2. Decision tree for classifying stellar spectra. ...

Qassifying Stellar Spectra http /fshim.uwp.edu/astronomyfstars/ spectclass.html... 

We are therefore developing a set of routines for an automatic or semiautomatic abundance analysis of stellar spectra based on equivalent widths (EW). The first product is DAOSPEC, a code developed by P. B. Stetson for automatic EW measurement (http //cadcwww.hia.nrc. ca/stetson/daospec/). The preliminary abundance analysis presented here is the first step of an iterative and automatic procedure under development at the Bologna Observatory. 

A library of stellar spectra or absorption-line strengths, taking into account differences in a-element iron and possibly other element abundance ratios. The spectra may be either observational or synthetic, i.e. theoretically computed. 

A full account of the appearance of different elements in stellar spectra is given in... 

Stellar evolution has consequences in the development of luminosity and colours of stellar populations, as well as chemical enrichment. Boissier and Prantzos (1999) have produced a more-or-less classical model of the evolution of the Milky Way, making a detailed study of this aspect, known as chemo-photometric evolution , using an IMF similar to the Kroupa-Scalo function in Chapter 7 this detail is significant because the Salpeter(O.l) function often used has a smaller contribution from stars of around solar mass which dominate the light at late times. The chemical evolution results are combined with metallicity-dependent stellar isochrones, synthetic stellar spectra by Lejeune et al. (1997) and a detailed treatment of extinction by dust. Some of their results are shown in Fig. 8.39. 

The existence of outsized hydrogen atoms was inferred early on from the observation that 33 terms of the Balmer series could be observed in stellar spectra, compared to only 12 in the laboratory [58]. More recently [59] Rydberg atoms have been produced by exciting an atomic beam with laser light. When the outer electron of an atom is excited into a very high energy level, it enters a spatially extended orbital which is far outside the orbitals... 

The isotopic and elemental abundance table shows that, in the Solar System, iron is more abundant than its neighbours. Analysis of stellar spectra conhrms this result, giving it a universal character. 

When Payne began her work in the 1920s, stellar spectroscopy was a very active area of research. Numerous elemental and molecular lines had been identified in stellar spectra. The lines observed in each star varied with the inferred temperature of the star, which was understood to mean that the elemental abundances varied with temperature. This body of data was the basis for the spectral typing of stars ( , B, A, F, G, , M, L). However, the power source for stars was not understood and it was not clear why the composition of a star should be related to its temperature. In the 1920s, it was also widely believed that the Sun had the same composition as the Earth models considered the Earth to have formed from the outer layers of the Sun. Payne used the new guantum mechanical understanding of atomic structure to show how and why the spectral lines of the different elements varied as a function of stellar spectral type. She demonstrated how the temperature of the stellar surface controls the spectral lines that are observed. Her analysis led to the conclusion that the chemical... 

Hill, V. (2001) From stellar spectra to abundances. Astrophysics and Space Science, 277 (Supplement), 137-146. 

The solar abundances of all of the chemical elements are shown in Figure 12.2. These abundances are derived primarily from knowledge of the elemental abundances in Cl carbonaceous chrondritic meteorites and stellar spectra. Note that 99% of the mass is in the form of hydrogen and helium. Notice that there is a general logarithmic decline in the elemental abundance with atomic number with... 

A rational deduction of elemental abundance from solar and stellar spectra had to be based on quantum theory, and the necessary foundation was laid with the Indian physicist Meghnad Saha s theory of 1920. Saha, who as part of his postdoctoral work had stayed with Nernst in Berlin, combined Bohr s quantum theory of atoms with statistical thermodynamics and chemical equilibrium theory. Making an analogy between the thermal dissociation of molecules and the ionization of atoms, he carried the van t Hoff-Nernst theory of reaction-isochores over from the laboratory to the stars. Although his work clearly belonged to astrophysics, and not chemistry, it relied heavily on theoretical methods introduced by and associated with physical chemistry. This influence from physical chemistry, and probably from his stay with Nernst, is clear from his 1920 paper where he described ionization as a sort of chemical reaction, in which we have to substitute ionization for chemical decomposition. [81] The influence was even more evident in a second paper of 1922 where he extended his analysis. [82]... 

R. C. Kenat and D. H. DeVorkin, Quantum physics and the stars III Towards a rational theory of stellar spectra, Journal for the History of Astronomy 21 (1990) ... 

Bob pauses and looks at Miss Muxdroozol, Let me list some facts. Scientists have observed around 74 different elements in stars. We re sure other natural elements exist in stars, but their quantities may be too low to make their lines visible. The majority of stars have similar compositions, with hydrogen making up about 90 percent of the number of atoms, followed by helium at 9.9 percent. All the other chemical elements are in the remaining 0.1 percent. Generally, the more complex the atom, the less you ll find of it in stars. Incidentally, the stellar spectra help scientists measure the radial velocities of stars because the wavelengths of the lines are shifted slightly by the Doppler effect. We ll talk about that later. ... 

Bob has lofty dreams of making the Sun shine in strange ways. The odd stellar spectra will say to the Universe, Humans are here The technetium seeding will require a few days to prepare, and Bob wants Miss Muxdroozol and Mr. Plex to fully appreciate the stars before he embarks on his audacious experiment. 

Annie Jump Cannon (1863-1941) American astronomer who classified stellar spectra. Discovered hundreds of variable stars and five novae. 

For the first time a universe composed of H and He has a scientific explanation. Only 1H, 2H, 3He, 4He and much 7Li seem to have inherited their abundances as ashes of that Big Bang. The stars manufactured the remainder of the elements, except for small abundances created by cosmic-ray collisions. Stars in fact slowly destroy 2H by fusing it into He. Cecelia Payne s discovery in stellar spectra that H dominates the abundances in stars became aprime factof the cosmology of the universe. The origin of both isotopes of hydrogen, the first and seventh most abundant nuclei in the universe, as well as ofhelium, in the initial fireball is one of the great achievements of thattheory. 

The neutral V atom has a large number of observable lines in stellar spectra. The abundance ratio V/Fe remains essentially constant in observed galactic stars ranging in Fe/H from io-3 of solar to solar. This is unsurprising considering the close link between the nucleosynthesis of V and that of Fe. 

Ozone occurs in small quantities in the atmosphere as is evidenced by certain absorption bands in solar and stellar spectra.1 It is also present in certain natural waters in sufficient quantity to be recognisable by the smell.2... 

Me Kellar, A. The unidentified blue-green bands in certain N-type stellar spectra. 

Wickramasinghe, N. C. 1970, in Ultraviolet Stellar Spectra and Related Ground-Based Observations, ed. L. Houziaux H. E. Butler, Reidel, 42 19. 

About 1940, three interstellar radicals were detected through their UV absorption lines in stellar spectra. This made it clear that interstellar matter had another component Interstellar molecules. 

X-ray spectroscopy has also been applied to the interpretation of solar spectra, which are emitted by solar flares. Now stellar objects are under investigation by X-ray satellites such as Chandra and XMM. Whereas the present X-ray telescopes are medium resolution devices, the next generation (Constellation-X, XEUS) will provide sufficient spectral resolution for detailed analysis. The spectra from distant object usually suffer from low statistics solar flares have low emission time and the observation time of stellar objects is limited. In addition, the electron distribution is not Maxwellian, in general, and some of the spectral lines may be polarized. Therefore, verified theoretical data are of great importance to interpret solar and stellar spectra, where they provide the only source of information on the plasma state. 

It often takes time for the implications of experimental data to be understood and to be acted upon. Fraunhofer s earlier observation that the solar D-lines coincided with the spectral lines of a sodium lamp eventually prompted further important experiments. In 1849, Jean Bernard Leon Foucault (1819-1868), a Parisian physicist, made an unexpected discovery. He passed sunlight through a vapor of sodium and he found that the solar D-lines were darker. His conclusion was that the sodium vapor presents us with a medium which emits the rays D on its own account, and which absorbs them when they come from another quarter. The consequences of Foucault s experiment, however, were expressed more cogently by Sir William Thomson (later Lord Kelvin). He drew the following explicit conclusion That the double line D, whether bright or dark, is due to the vapor of sodium. . . That Fraunhofer s double dark line D, of solar and stellar spectra, is due to the presence of vapor of sodium in atmospheres surrounding the Sun and those stars in whose spectra it has been observed. ... 

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