Lecture Summary 12

Lecture Summary 12 - Lecture Summary (11/30 & 12/2)...

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With early telescopes, astronomers deduced a disk arrangement to our galaxy. Early this century, the arrangement of globular clusters was used by Harlow Shapley to estimate the size of the Milky Way Galaxy, the location to galactic center, and the Sun's location. The globular clusters define a sphere centered on galactic center. From Earth we observe more globular clusters in some directions than others indicating that Earth is not at galactic center but approx. 28,000 LY away from center. Most of the galaxy is comprised of hydrogen. If this hydrogen is near hot stars, it is ionized and visible as an emission nebula (electrons recombine and cascade emitting visible light). But if the hydrogen is away from stars, and most of it is, it is too cool to be emitting visible light. This cool gas would be comprised of mostly atoms of hydrogen with electrons in the ground state. The ground state actually has 2 sub-states depending on electron and proton spins. If the spins are in the same direction, the atom is in a slightly greater energy state than if the spins are opposite. In order to make this hyperfine transition in the ground state, energy must be given up by the atom. A long wavelength, low energy radio photon is emitted with wavelength 21 cm. Where there is a concentration of cool hydrogen atoms, there is a steady source of these 21 cm radio waves. For the nearby spiral arms that we can see visually, we also
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This note was uploaded on 02/01/2011 for the course ASTR 101 taught by Professor Deming during the Spring '07 term at Maryland.

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Lecture Summary 12 - Lecture Summary (11/30 & 12/2)...

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