Spectroscopy - Spectroscopy Spectroscopy Spectroscopy is...

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Unformatted text preview: Spectroscopy Spectroscopy Spectroscopy is the technique that allows astronomers to decode the messages contained in starlight. Stellar spectra can tell us the intrinsic luminosity of a star, how hot it is, how fast it rotates, and its composition. First Types of Classifications In 1823, Joseph Fraunhofer observed that stars have spectra that are characterized by dark lines crossing a continuous band of color. William Huggins, in 1864, first identified some of the lines in stellar spectra with those of known terrestrial elements. First Types of Classifications In 1863, Angelo Secchi classified stars into four groups according to the general arrangement of their spectra. Pickering and Fleming (both at Harvard) in the 1890s labeled spectra with capital letters according to the strength of their hydrogen absorption lines, beginning with the letter A for the broadest lines. Hydrogen Line Strength Order ABCDEFGHIJKLMNOPQRST Temperature Effects The dark lines are due to the presence of a particular chemical element in the atmosphere of the star observed. Detailed analysis has shown that most stars have very similar compositions. The primary cause of the differences in stellar spectra is that stars have very different temperatures in their outer layers. Temperature Effects Cool gas; electrons in ground state; no lines. Hot gas; electrons have been ionized; no lines. Warm gas, electrons in various energy levels; many lines. Temperature Effects For example, hydrogen. In the hottest stars, it is completely ionized, so it cannot make absorption lines. In the coolest stars, it is neutral and can absorb photons. In a star of 10,000 K, many hydrogen atoms are excited. Absorption lines due to hydrogen are strongest in the spectra of stars whose atmospheres have temperatures near 10,000 K. We need only to observe what patterns are present in the spectrum of the star of choice to learn its temperature. Spectra of Different Elements Henry Draper Classification In 1909, Annie Jump Cannon rearranged and consolidated the sequence of spectra and added decimal subdivisions. A0, A1, A2, …, A8, A9 With these changes, the Harvard classification scheme became a temperature sequence. Temperature Order ABCDEFGHIJKLMNOPQRST O B A F G K M OBAFGKM Oh Be A Fine Girl/Guy Kiss Me Oh Big And Fierce Gorilla Kill Me Often Buzz Attacks Ferociously Georgia’s K-9 Mascot Occasionally Barney And Fred Go Kill Mastodons Spectroscopy PRS Question 1. Which spectral type has the coolest stars? a. A d. M b. B e. O c. G Ionization via Temperature Spectral Sequence Spectral Sequence Diameters of Stars L = 4 R2 T4 L R2 T4 How do the spectra look different if you have two stars of the same Temperature but with different Radii? Calculating Line Profiles The simplest model used for calculating a line profile assumes that the star’s photosphere acts as a source of blackbody radiation, and that the atoms above the photosphere remove photons from this continuous spectrum to form absorption lines. Values for the temperature, density, and composition must be adopted for the region above the photosphere where the line is formed. The density determines the broadening. PRS Question 2. What is the spectral type and luminosity class of a star whose spectrum contains very strong and broad He I lines, some H lines, and no He II lines? a. A V c. A Ia b. B V d. B Ia Table of Colors What We Learn Via Spectroscopy A. Temperature B. Pressure C. Radius D. Luminosity E. Chemical Composition F. Radial Velocity G. Turbulence H. Magnetic Fields I. Shells and Ejected Gases ...
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This note was uploaded on 03/04/2012 for the course PHYS 2022 taught by Professor Jarrio during the Spring '12 term at Central GA Tech.

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