Lyman and Balmer series Light Speed of light is wavelength times frequency Photon energy is proportional to frequency Black body flux is proportional to temperature4 c lf E hf F sT 4 lmaxa1 T Spectra of the hydrogen atom Maximum black body wavelength is proportional to 1 over temperature a Absorption of a UV photon left by a hydrogen atom causes the momentary excitation of the atom into its first excited state center Eventually the atom returns to its ground state right in the process emitting a photon having exactly the same energy as the original photon b Absorption of a higher energy UV photon may boost the atom into a higher excited state from which there are several possible paths back to the ground state At the top the electron falls immediately back to the ground state emitting a photon identical to the one it absorbed At the bottom the electron initially falls into the first excited state producing visible radiation of wavelength 656 3nm the characteristic red flow of excited hydrogen Absorption of still more energy can boost the electron to even higher orbitals within the atom As the excited electron cascades back down to the ground state the atom may emit many photons each with a different energy and hence a different color In this case the resulting spectrum shows many distinct spectral lines For hydrogen all transitions ending at the ground state produce ultraviolet photons However downward transitions ending at the first excited stated give rise to spectral lines in or near the visible portion of the electromagnetic spectrum Because they form the most easily observable part of the hydrogen spectrum and were the first to be discovered these lines also known as the Balmer lines are often referred to simply as the hydrogen series and are denoted by the letter H Individual transitions are labeled with Greek letters in order of increasing energy decreasing wavelength The H line corresponds to the transition from the second to the first excited state and has a wavelength of 656 3nm red H third to first has wavelength 486 1nm green H fourth to first has wavelength 434 1nm blue and so on FORMATION OF SPECTRAL LINES CONTINUUM SOURCE CLOUD ONLY ABSORBS AT DISCRETE WAVELENGTHS RE EMITS PHOTONS RANDOMLY IN ALL DIRECTIONS ISOTROPIC DECREASE IN INTENSITY AT ABSORBED S DARK OR ABSORPTION LINE ONLY PHOTONS EMITTED BY ATOMS IN THE CLOUD ARE SEEN EMISSION LINES ATOMS ABSORB AND EMIT LIGHT ONLY AT CERTAIN WAVELENGTHS OR FREQUENCIES THAT ARE CHARACTERISTIC OF EACH ELEMENT Ionization and recombination Doppler effect Ionization Electron absorbs photon gains energy and becomes unbound ionized HII Recombination Electron radiates photon loses energy and becomes bound neutral HI The Doppler effect is any motion induced change in the observed wavelength or frequency of a wave The greater the relative speed of source and observer the greater the observed shift In terms of the net velocity of recession between source and observer the apparent wavelength and frequency measured by the observer are related to the true quantities emitted by the source by true f apparent f 1 recessionv wavev apparentl truel A positive recession velocity means that the source and the observer are moving apart a negative value means that they are approaching The wave speed is the speed of light c in the case of electromagnetic radiation Note that only motion along the line joining the source and observer known as radial motion appears in the above equation Motion transverse perpendicular to the line of sight has no significant effect Overall structure of stars Core the site of powerful nuclear fusion reactions that generate the Sun s enormous energy output Radiation zone where solar energy is transported toward the surface by radiation rather than by convection Convection zone a region where the material of the Sun is in constant convective motion Photosphere the part of the Sun that emits the radiation we see Chromosphere the Sun s lower atmosphere Transition zone where the temperature rises dramatically Corona a thin hot upper atmosphere Solar wind flows away from the Sun and permeates the entire solar system Hydrostatic equilibrium Hydrostatic equilibrium is the condition in a star or other fluid body in which gravity s inward pull is exactly balanced by internal forces due to pressure This stable balance between opposing forces is the basic reason that the Sun neither collapses under its own weight nor explodes into interstellar space It also has an important consequence for the solar interior Because the Sun is very massive its gravitational pull is very strong so very high internal pressure is needed to maintain hydrostatic equilibrium This high pressure in turn requires a very high central temperature a fact crucial to our understanding of solar energy generation In this way the assumption of hydrostatic equilibrium lets us determine the density and the temperature inside the Sun This information in turn allows the model to make predictions about other observable properties luminosity radius spectrum and so on Scientists fine tune the internal details of the model until the predictions agree with observations This is the scientific method at work the standard solar model is result Convective and radiative heat transport The very hot solar interior ensures violent and frequent collisions among gas particles In and near the core the extremely high temperatures guarantee that the gas is completely ionized With no electrons left on atoms to capture photons and move into more excited states the deep solar interior is quite transparent to radiation Only occasionally does a photon interact with a free electron The energy produced by nuclear reactions in the core travels outward toward the surface with relative ease in the form of radiation As we move outward from the core the temperature falls and eventually some electrons remain bound to nuclei With more and more atoms retaining electrons that can absorb the outgoing radiation the gas in the interior changes from being relatively transparent to almost totally opaque By the outer edge of the radiation zone all of the photons produced in the Sun s core have been absorbed Not one of them reaches the surface The photon s energy must travel beyond the solar interior the fact that we see sunlight proves that the energy escapes The energy is carried to the solar surface by convection the same basic physical process we saw in our study of Earth s atmosphere although