PCS 181 Study Guide - Midterm Guide: H I Region, Triple-Alpha Process, Star Star
20 Oct 2017
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This is dust and gas that occupy space between stars. Is a mixture of: elongated, tiny grains of dirty ice, grains of graphite (carbon, silicate particles, complex molecules. 100x more gas mass than dust mass in universe. Visible universe contains: 75% hydrogen, 24% helium, <1% other. These are clouds of neutral (not ionized) hydrogen. Cold hydrogen gas is in its ground state (n=1 orbit) Hydrogen atom has 2 possible ground state configurations. Electron may undergo a spontaneous spin flip from aligned (parallel) state to opposed (anti- parallel) state. Cold neutral hydrogen clouds in space emit a wavelength: =21 cm (radio region, f=1420mhz. Excess energy is released as a tiny burst of radiation. =a photon of definite wavelength: =21 cm=0. 21 m. Corresponding frequency is f=1. 42x109 hz = 1. 42 ghz = 1. 42 gigahertz. This wavelength is used to map milky way galaxy, and to search for messages from extra- terrestrials. Consider a giant interstellar cold molecular h2 cloud.
