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ASTR 151 Chapter 4

by: Wesley Fowler

ASTR 151 Chapter 4 ASTR 151 001

Marketplace > Astronomy > ASTR 151 001 > ASTR 151 Chapter 4
Wesley Fowler

GPA 3.97

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These notes cover the entirety of chapter 4, explaining topics such as Spectroscopy, emission and absorption spectrums, molecular composition, the Lyman and Balmer series, and ionization. I've trie...
Journey Thr Solar Sys Lecture
Dr. Sean Lindsay
Spectroscopy, atoms, Molecules, emission, absorption, spectrum, Lyman, Balmer, ion, ionization
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This 4 page Bundle was uploaded by Wesley Fowler on Saturday February 20, 2016. The Bundle belongs to ASTR 151 001 at a university taught by Dr. Sean Lindsay in Spring 2016. Since its upload, it has received 37 views.


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Date Created: 02/20/16
Wesley Fowler ASTR Chapter 4 Spectroscopy Spectroscopy is the study and analysis of how matter emits and absorbs radiation. It is the study of how light interacts with matter. - Radiation is analyzed by a spectrometer Three types of Spectra Continuous spectra: Full emission. From a blackbody source that is dense enough. Emission Spectra: Distinct lines of color in “emission” Absorption: Distinct lines of color removed by absorption - Every element has a unique set of absorption lines The radiation given off by certain gases, as observed by a spectroscope, is seen as a few narrow emission lines. These are all the colors/wavelengths not emitted. - The frequency or wavelength cannot be altered! Absorption Line: Black lines/gaps represent wavelengths that have been absorbed. Kirchhoff’s Three Laws of Spectroscopy 1. A luminous solid, liquid or gas with high-density emits a continuous spectrum of radiation (Blackbody) 2. Hot gases with low-density emit an emission spectrum with a series of bright emission lines, unique to each gas 3. Cool thin gases (along line of sight of the emitting object) emit the absorption spectrum, leaving dark absorption lines in place of certain wavelengths Atoms and Radiation Light is also a particle Quantum mechanics: The branch of physics governing the balancing of atoms and subatomic particles. Protons: Positive charge Electrons: Negative charge Neutrons: Neutral charge *The Bohr model is outdated; the electron cloud model is current. • Excited state: When an electron orbiting an atom occupies a farther than normal orbital. Contains a greater amount of energy than normal. • Ground State (of an atom): State of lowest energy. “Normal” • When an electron achieves its maximum energy it breaks free from the nucleus’ pull, and the atom is ionized. An ion is an atom missing one or more of its electrons. When an electron drops closer to an energy level closer to the nucleus it emits energy. If it jumps away from the nucleus it absorbs energy. Atoms can only absorb or emit specific energies; each electron has an associated energy level. - The amount of energy absorbed or emitted is directly proportional to the energy difference between two electron orbitals Photons A particle of “electromagnetic energy”, the packet of energy equal to the difference between two electron orbitals. Photon energy is directly proportionate to radiation frequency, and thus inversely proportional to wavelength. Intensity is not a factor in electron energy. Shorter wavelength (blue), more energy. Longer wavelength (red) less energy E = hf = (hc)/λ -34 - Planck’s constant: h= 6.63 x 10 Joule seconds (JS) - E: Photon energy (in electron volts, eV) E(eV) = 1240/(λ(nm)) eV: The energy gained by an electron accelerated through an electric potential of one volt. - 1 eV = 1.60 x 10 J 19 Wesley Fowler ASTR Chapter4 Spectral Lines and Ionization The frequencies of gases are not continuously spread out, rather they vary depending on their atomic structure. Emission lines correspond to the energy differences in an atom’s electron orbital levels. This helps us understand the composition of the universe. The energy of any state (n): ⎛ ⎞ 1 E n13.6 1 --⎜ ⎟ eV. ⎝ ⎠ n 2 (Don’t need to memorize this) - E: Energy - eV: Electron Volt (unit) The minimum amount of energy needed to ionize hydrogen from its ground state is 13.6eV (Ionize is to free an electron from its local nucleus) The Lyman Series: Transitions starting or ending at the ground state. Ultraviolet Lines - Lα = 121.6nm (“Lye” on the ground) 10.2eV st The Balmer Series: Transitions starting or ending at the 1 excited state. Visible Lines. - Hα = 656.3nm 1.9eV There are two ways that atoms/molecules can become excited st - Direct excitation: Electron goes from ground state, to 1 excited state, back to ground. This releases a UV photon. - Cascade De-excitation: Electron goes from ground state, to 2 excited state, then either drops directly back to ground state emitting a UV photon, or it cascades down the 1 st excited state to the ground state, releasing a visible and then UV photon. These lines make up the emission and absorption lines that can be seen in a spectrum! - If you view a gas cloud along sight with continuous spectrum (light bulb) we see Absorption Spectrum. - If viewed from a side angle without the continuous spectrum, we see Emission Spectrum. Hydrogen is the simplest element to teach this concept. The more electrons, neutrons, and protons in an element, the more complicated it becomes due to there being more spectral lines. Molecular Motion Electron transitions -> Visible and ultraviolet spectral lines (largest energy changes) Changes in vibration -> Infrared spectral lines Changes in rotation -> Radio and microwaves lines The strength of a spectral line (how bright or dark it is) is dependent on the amount of atoms, or concentration. It is also dependent on the temperature of the gas containing the atoms because temperature determines how atoms transition between orbitals. - At higher temperatures, more atoms are in an excited state. The radial velocity of an object can be determined by comparing the pre-determined wavelength of the element to the presently-observed color. By seeing how “reder” or “bluer” it is, astronomers can determine the radiations’ radial motion. The environment in which absorption or emission occurs causes spectral lines to broaden. Measuring the width of expansion can help determine the speed of particles. Rotation and turbulence can also be used to measure speed. Additionally, these factors also broaden spectral lines: 1) Thermal motions of particles 2) Rotation of a macroscopic object 3) Collisions between atoms 4) Magnetic Fields – Zeeman Effect Electrons and particles are moving randomly like crazy, so their observational frequency is also affected by the Doppler Effect too. The amount of collisions also alters the apparent energy, collisions dependent on the density and pressure of the gas. The more rapid a star’s motion, the broader its spectral line appears. The half rotating away is “reder”, rotation towards is “bluer”. The Zeeman Effect: The effect of splitting spectral lines into multiple components in the presence of a static magnetic field.


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