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Stars are one of the important heavenly objects of our universe. The sun is our closest star and is responsible for the sustenance of all life on earth. Understanding the intricate chemical reactions that are happening in the star and identifying the composition of the stars has always been a fascinating study for astronomer's worldover. The sun our closest star has been the most studied object in the universe. Spectroscopy was one of the earliest available methods of studying heavenly bodies and has contributed immensely in the study and understanding of the composition of stars. Let us briefly analyze spectroscopy and touch upon the elemental analysis and chemical composition of stars.
Spectroscopy and Study of Stars
Every element in the universe emits its own light, which is characteristic of its chemical structure. Spectroscopy refers to the study of the spectral lines of the different atomic constituents. Spectrometer is an instrument, which can separate the different light frequencies. The different electromagnetic radiations can be separated using the spectrometer. Kirchoff's law, which states "The ratio between the powers of emission and the powers of absorption for rays of the same wavelength is constant for all bodies at the same temperature" is central to the study of spectroscopy. [C.R.Kitchin, 4]. By connecting a spectrometer to a telescope it is possible to segregate light of varying frequencies and match them with the atomic structure. By analyzing the spectral lines that are emitted by stars we can in effect identify the different chemical elements present in them. Spectroscopic study of sun has verified hydrogen (71%) and helium (27%) as the two major constituents along with very minor traces of other gases and heavy elements. There is a continuous process of nucleosynthesis inside the stars. Hydrogen is constantly being fused into form helium, which in turn fusses to form heavier elements. A star continues to live till it has exhausted all the available fuel. Carbon, nitrogen and oxygen are also formed in the process. Other heavy elements like iron, gold and copper are formed as a result of the supernova. (A final explosion of the star)
Spectral Lines (Elemental Abundance) spectral line is nothing but the transition of the atom from one energy state (E1) to another stateE2. The frequency of the spectrum is given by h? = E1-E2.The more number of atoms of an element in the star's atmosphere, the stronger will be the spectral lines. The nature of the spectral lines emitted by stars is different and they correspond to the surface temperature of the stars respectively. Since it is not possible to simulate certain physical and chemical conditions that exist in the universe spectroscopic study of stars and other galactic bodies offers a best scientific tool for us to study their molecular composition. The spectrograph will show us the different elements that are found in the star. To find out the abundance of an element in the star we have to study the thickness of the spectral bands. Spectroscopy can be considered at several different resolutions. Spectral energy distribution can be studied at different levels and the crudest form of spectroscopy is photometry where the spectral band is divided into relatively fewer number of regions. Using high-resolution spectroscopy the individual molecular lines can be seen clearly and it is usually used for the study of luminous objects like the sun, stars and planets. Astronomer's study stars under two different categories namely stellar atmosphere and stellar interior. The compositions of the atmosphere, its temperature and pressure all have a direct influence in controlling the emergent flux from the star's interior.
Classification according to Spectra
Astronomer Angelo Secchi first classified stars based on the appearance of their spectra. He classified Type 1 stars (white stars) as those, which only had lines of hydrogen, and type 2 stars which had a spectra resembling that of the sun. (Yellow stars)
Type three and four stars had bands, which were shaded towards the red and violet regions respectively. The modern classification however uses the temperature of the stars producing the spectra, as the basis of classification. Stars are classified as belonging to O, B, A, F, G, K and M. (classes). Even in the late nineteenth century there was considerable progress in our understanding of the theory of the physical process that leads to the production of the spectra. In 1885 Johann Balmer derived the empirical law that clearly explained the 'wavelengths of hydrogen lines' in the visible spectra.
Wavelength ? = 364.56 (n^22 / (n^2 base 2 - n^2 base 1)) [C.R Kitchin, pg 7]. The further discovery of relationship between energy and wavelength and Plank's constant (6.626*10^-34) furthered the study of Astronomical spectroscopy. E = hv=hc / ?
The overall spectrum of a star is largely governed by the ratio of O. And C. In the stellar atmosphere. CO molecule has a very high dissociation constant, which implies that all the C. And O. atoms would combine to form CO until the supply of either one of them is totally exhausted. When uncombined O. is left over the star will be of M type mainly dominated by oxygen compounds such as Sio, Tio, H2o etc. If there is plenty of uncombined C. then the star would be of C type with molecules such as c2, CN, Ch C2H2 etc. When there is more or less an equal distribution of oxygen and carbon they will combine to form CO and consequently the star's spectrum will be dominated by less abundant molecules. [I.S Glass, 76] In the case of cooler stars it is found that water vapor absorption dominates the infrared spectra. The spectra of M type stars (dwarfs) clearly reflect the dependence on the absorption coefficient of water vapor.
The different stellar classes (O, B, A, f, G, K and M) have a gradual change in the spectral lines. In class O stars for example there is a high amount of ionized silicon, nitrogen and helium. Class B is abundant in neutral helium, low stages of ionization of Silicon and Nitrogen. In class A there is complete peaking of Balmer lines and the appearance of calcium. In class G there is an observed increase on the levels of calcium and metals. In K. And M. there is a clear dominance of TIO bands. [C.R Kitchin, 190] The reason for the change in spectrum with temperature is attributed to the balance between the ionization and excitation of atoms that compose the stellar atmosphere.
Much of the work on stellar atmosphere is done based on the approximation known as Local Thermodynamic equilibrium. (Let) That is the 'properties of material are assumed to be characterized by a single temperature' while the properties of radiation are characterized by a different (single) temperature. Once we assume a value for the LET then we can use the Boltzmann's formula to find out the relative population of the different exited atoms in the stellar surface. By the Saha equation we have
Ni+1/Ni = 2Ui+1 / NeUi (2?me KT/h2)^3/2 e -xi/KT where Ne represents the free electron density while Ni represents the number density of an atom in the ith stage of ionization and Xi the ionization potential of the ith Ion. Ui and Ui+1 are the partition functions. [C.R Kitchin, 191]
The availability of spectroscopic software has allowed the possibility of quick and accurate study of quantum study of the spectral elements. GAMESS is one such software that can be used to calculate the SCF wavefunctions, perform automatic search for transition states, geometrical orientation and in analyzing the vbrational frequencies. GAMESS (General Atomic and Molecular Electronic Structure System) allows us to integrate quantum mechanics and molecular mechanics, which is essential for our understanding of the complex astronomical science. The software allows us to study chemical functions like dipole movements and to perform…[continue]
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