astro chapter 18

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  1. extinction
    the overall dimming of starlight by interstellar matter
  2. reddening
    • caused by interstellar medium
    • IM is more opaque to shorter wavelengths, so blocks blue light preferentially over red light
  3. interstellar dust shape
    • elongated or rod like
    • know due to the polarization of radiation that makes it to earth
    • dust tends to be aligned over large regions of space
  4. nebula
    clouds of interstellar dust and gas
  5. reflection nebula
    • caused by starlight scattered from dust particles in interstellar clouds located just off the line of sight between earth and the bright star
    • appear blue because blue light is more easily scattered
  6. photoevaporation
    • the dispersion of a cloud in the vicinity of new stars caused by heating
    • east away less dense material first
  7. ionozation state
    • I - neutral (not ionized)
    • II - singly ionized atom (missing one electron)
    • III - doubly ionized
  8. emission nebula and ionization state
    composed mainly of ionized hydriogen, they are often referred to as HII regions
  9. forbidden lines
    • HIII emission of green light
    • an ion in the higher energy stats for this transition tends to remain there for a very long time - many hours - before dropping
    • we can't observe in lab because other interactions will knock it out of orbit before it falls and emits the green light
    • can happen in emission nebula
  10. how can you study dark dust clouds
    • co emissions in radio
    • (this may be wrong) can study at optic only if they happen to block the light emitted by more distant stars or nebulae
  11. absorption spectra of dark interstellar clouds
    • absorb some of the stellar radiation in a manner that depends on the cloud's own temp, density, and elemental abundance. (much like stellar absorption lines for stars)
    • can easily distinguish from the broader absorption lines formed in stars hot lower atmospheres
  12. 21-cm (radio) Radiation
    • used to probe interstellar space more thoroughly
    • detects cold, neutral interstellar matter though its own radiation
    • in hydrogen both the electron and the proton have spin, when these go from parallel spins (slightly more energy) to antiparallel spins they emit a photon at 21cm wavelength (energy = to the energy difference between these two levels)
  13. why do we see 21cm radiation today
    energy difference between the two states is comparable to the energy of a typical atom at a temperature of 100K. Movement bumps them up to the energized state
  14. molecular clouds
    • 10^12, cold 20k, bigger than emission nebulae's
    • contain molecules
    • molecules are found only in the densest and darkest of the interstellar clouds possibly because the dust serves to protect the fragile molecules (this dust also prevents higher wavelengths from getting out). Another hypothesis is that dust acts as a catalyst that helps form the molecules
  15. molecular spectral lines
    energy states of molecules are much more complex because molecules can also undergo internal electron transitions (like atoms) as well as rotate and vibrate
  16. molecular emission
    molecule goes from a faster spinning state to a slower spinning state and emits a photon with equal energy to the difference between these two speeds (normally in radio)
  17. molecular tracers
    H2 molecules are by far the most common constituent of molecular clouds but do not emit or absorb radio radiation, only short wave UV. (21cm radiation only sensitive to molecular hydrogen)

    • molecules such as carbon monoxide (CO), hydro­gen cyanide (HCN), ammonia (NH3), water (H20), methyl alcohol (CH)OH), formaldehyde (H2CO), and about 150 others, some quite complex, are now known to exist in interstellar space. Generally 1 million to a billion times less abundant that H2
    • rotational properties of different molecules are used as suitable probes 
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astro chapter 18
2014-04-15 21:09:01

chapter 18 astro
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