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Longest Running Thread EVER


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We've been looking madly for a new apartment (amazing how a deadline will motivate you!), and we found a place we really like. It's a large (1600 square foot) side of a duplex with three bedrooms on a

Re: Longest Running Thread EVER   Edit: Okay, I managed to fill every line, but goodness knows I'm open to suggestions for improvement!     Welcome to the Forum NGD   Welcome to the Forum NG

Long preamble here.   The administrative unit in which I have the Associate Director title got moved out of one building last spring (that one was subsequently demolished and the spot is bei

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That's a term (mostly) from quantum mechanics.  An electron in an atom can jump from one level to a different level (by absorbing energy by consuming a photon if the new level is higher in energy , or releasing energy in the form of a photon if the new level is lower energy than the initial one) by a given mechanism (of which there are several) only if the "selection rules" are obeyed.  These mostly involve the angular momentum values of the two states involved. 

 

"Permitted" lines come from electric dipole transitions, which couple most strongly to electromagnetic waves (and therefore photons).  Different mechanisms (these have names like magnetic dipole, electric quadrupole, ...) have different selection rules, and there are electron state changes that can happen according to those other mechanisms, but they have lower probability of happening, by factors of ten-to-the-several.  Now, lower probability means it will still happen (if nothing else interferes with the set-up before the jump happens), but you have to wait longer to see it happen. 

 

Under terrestrial conditions, you more or less never see anything but the "permitted" transition spectral lines, because collisions between atoms are usually frequent enough that no atom goes unmolested long enough for a detectable number of those "forbidden" transitions to occur.  But in conditions that humans call vacuum, that need not be the case, and "vacuums" are pretty common in astrophysical situations, and included in those are the Sun's atmosphere.

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