ATMOSPHERE OF STARS.

SUBMITTED BY

RITU PARAJULI

21UMPY17

MSc PHYSICS 4^th sem

INTRODUCTION

THE EQUATION OF TRANSFER

•transport of radiation through the atmosphere of a star.

. The equation of transfer is typically written as:

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dI/ds = -kI + j

Where;

I = specific intensity of the radiation,

s = the distance traveled by the radiation through the star's atmosphere,

k = extinction coefficient (representing the absorption and scattering of the radiation by the gas and dust in the atmosphere),

j = the emissivity (representing the production of radiation by the star's interior).

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CONTD..

• The equation of transfer describes the balance between the absorption and emission of radiation within the atmosphere.
• it can be used to calculate the intensity and spectral properties of radiation emitted or absorbed by the star.
• Astronomers use this equation to observe the properties of a star's atmosphere, such as its temperature, density, and chemical composition, based on the observed radiation.

PROCESSES OF ABSORPTION IN STELLAR ATMOSPHERE.

• Photoionization: In this process, a photon with enough energy is absorbed by an atom, causing an electron to be ejected from the atom, creating an ion. This process is responsible for the ionization of atoms in the outer layers of stars, where the temperature and density are high enough to cause ionization.

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• Collisional excitation: In this process, an atom is excited to a higher energy state as a result of a collision with another atom or electron. The excited atom can then return to its ground state by emitting a photon, resulting in absorption lines in the spectrum.

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• Resonance scattering: In this process, photons are absorbed and then re-emitted by atoms in the outer layers of the star. This process results in absorption lines at specific wavelengths, corresponding to the energy levels of the atoms involved.

SPECTRAL LINE BROADENING.

• Spectral line broadening is a phenomenon in which the otherwise sharp spectral lines of stars are broadened, or widened.
• 1. Natural broadening: This is the intrinsic broadening of spectral lines due to the Heisenberg uncertainty principle, which states that the position and momentum of a particle cannot be simultaneously known. This uncertainty leads to a natural broadening of spectral lines, which is proportional to the lifetime of the energy state of the atom.

• 2. Broadening due to physical processes:

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• Doppler broadening: This occurs when the atoms in the stellar atmosphere are moving with different velocities, causing the spectral lines to be broadened. The degree of broadening is proportional to the velocity of the atoms.

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• Pressure broadening: This occurs when atoms in the stellar atmosphere are in close proximity to one another, causing collisions that result in broadening of the spectral lines. The degree of broadening is proportional to the density of the gas.

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• Stark broadening: This occurs when the atoms in the stellar atmosphere are subjected to electric fields, causing the spectral lines to be broadened. The degree of broadening is proportional to the strength of the electric field.

HYPERFINE STRUCTURE IN SPECTRAL LINES.

• Hyperfine structure is a fine splitting of spectral lines caused by the interaction between the nuclear magnetic moment and the magnetic moment of the electron in an atom. This interaction results in a splitting of the energy levels of the atom, leading to the formation of multiple closely spaced lines in the atomic spectrum.
• The hyperfine structure is most significant for atoms that have an odd number of protons or neutrons, such as hydrogen, deuterium etc.
• It is used to study the cosmic microwave background radiation, which is a remnant of the early universe.

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THE CURVE OF GROWTH.

• The curve of growth is constructed by plotting the strength of the absorption line as a function of the logarithm of the number of absorbing atoms.

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• Initially, as the number of absorbing atoms increases, the strength of the absorption line increases linearly. This is known as the linear regime of the curve of growth. However, as the number of absorbing atoms continues to increase, the absorption line begins to saturate, and the strength of the line approaches a maximum value. This is known as the saturated regime of the curve of growth.

STELLAR TEMPERATURES.

• Stellar temperatures refer to the surface temperature of stars, which can range from a few thousand Kelvin for cool stars and up to tens of thousands of Kelvin for hot stars.

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• hotter stars tend to be more massive and more luminous, while cooler stars are less massive and less luminous.

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• The temperature also determines the peak wavelength of the star's radiation, which in turn affects the color of the star as seen by an observer.

CHEMICAL COMPOSITION OF STARS

• The chemical composition of stars refers to the relative abundances of different chemical elements in their atmospheres.
• Stars are made up of mostly hydrogen and helium, but they also contain small amounts of heavier elements, which are produced through nuclear fusion.
• The chemical composition of stars can be determined by analyzing their spectra. Each element has a unique set of spectral lines that can be used to identify its presence and measure its abundance relative to other elements.

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CONCLUSION

• The study of these complex spectral lines, equation of transfer, etc leads us to the properties of stars like their chemical composition, temperature, and luminosity which ultimately helps in the study of the evolution, lifetime, fate of the stars and also provides insights in the history of a galaxy.

THANK YOU.