Pre-Exploration Study and Information-Astronomy, Science

Pre-Exploration Study and Information-Astronomy, Science

Pre-Exploraton S±udy and Informaton1. In±roductonAt least 95% of the celesTal informaTon that we receive on Earth is in the form of light (elec±romagnetc radiaton). Most of what astronomers know about stars, galaxies, nebulae, and planetary atmospheres comes from spectroscopy, the study of the wavelengths of light emi±ed by such objects. Spec±roscopyis used to idenTfy chemical composiTons, temperatures, velociTes, gas densiTes, pressures, and magneTc Felds.An a±omconsists of a nucleusand elec±ronsthat “orbit” around the nucleus. An atom emits energy when an electron jumps from a high-energy orbit around the nucleus to a smaller low-energy orbit. ²he energy appears as a pho±onof light having energy exactly equal to the di³erence in the energies of the two electron levels. A pho±onis a wave of electromagneTc radiaTon the wavelength (distance from one wave crest to the next) of which is inversely proporTonal to its energy: high-energy photons have short wavelengths, low-energy photons have long wavelengths (´ig. 1). Because each element has a di³erent electron structure — and, therefore, di³erent electron orbits — each element emits a unique set of spectral lines.Figure 1.— Emission of photons of light from an atom. See your text for explanaTon.²he light produced when electrons jump from a high-energy orbit to a smaller low-energy orbit is known as an emission line.²herefore an emission spectrum is characterized by radiaTon only at speciFc wavelengths and appears as several bright lines with dark gaps in between. An absorpton lineis produced by the opposite phenomenon — e.g., the passing photon of light is absorbed by an atom and provides the energy for an electron to jump from a low-energy orbit to a high-energy orbit. ²he energy of the photon must be exactly equal to the di³erence in the energies of the two levels. If the energy is too much (wavelength too short) or too li±le (wavelength too long), the atom cannot absorb it and the photon will pass by the atom una³ected. ²hus, the atom can selecTvely absorb light from a conTnuum source but only at the very same wavelengths (energies) as it is capable of emiµng light. An absorpTon spectrum appears as dark lines superimposed against a bright conTnuum of light, indicaTng that at certain wavelengths the photons are being absorbed by overlying atoms and do not survive passage through the rareFed gas.

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