The Interstellar Medium

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The Interstellar Medium

• Gas and dust, between the stars
• Region where stars form
• Milky Way Galaxy disk shaped, 3.0 x 104 parsecs
  in diameter. ISM is confined to within a few
  hundred parsecs of the plane
• ISM - studied in Radio, IR, visible, UV and
  X-ray. Indicates gas is in various physical states

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• Nebulae clouds of illuminated gas/dust
• Emission Nebulae excited ions and atoms
  (Kirchoffs 2nd Law)
• Emission nebulae are low density (100 to 1000
  atoms/cc) T 10,000K
• Emission lines of Hydrogen (Balmer series) have
  a pinkish red color
• Also detect emission from He, N, O and other
  elements
• Source of excitation/ionization? UV light from
  nearby hot stars

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• To ionize H, need photons with ? less than 91.2
  nanometers require stars with T 25,000 K
  (type B1 or hotter)
• Why ionized? Balmer lines are formed (in this
  case) by recombination
• Proton captures free electron
• Free electron has some energy of motion, so H
  atom is in an excited state
• Excited state decays atom emits photons
• Emission-line nebula H II regions (ionized H H
  I is neutral hydrogen

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• Forbidden Lines are also emitted, for example O
  III 495.9, 500.7 nm
• These are excited by collisions
• Why forbidden? Rules of Quantum Mechanics
  electron stays in excited state for a long time
• Permitted lines (like Balmer) decay in 10-7 sec,
  Forbidden lines decay in hour
• Best vacuums on Earth are too dense to these
  lines collisional de-excitation

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• Reflection Nebulae shine by reflected
  starlight light is reflected off dust grains
• These nebulae appear blue, because dust grains
  are small, reflect shorter wavelengths more
  efficiently
• Grain sizes? 0.003 to 1 nm

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• Dark Nebulae block light of more distant stars
• Structure shows evidence of twisting, distortion
• Bok globules - less than parsec in diameter, 10
  100 Msun may be contracting to form stars
• Interstellar Extinction, due to dust grains
  makes stars appear fainter, redder (blue light is
  scattered)
• To study extinction/reddening, plot the ratio of
  the brightness of two stars, of same spectral
  type, as a function of wavelength
• Dust grains made of Carbon, also Silicate metals
  (often with water or ammonia ice coatings) they
  are formed in the atmospheres of cool stars.

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• Absorption Lines already discussed lines formed
  in the atmosphere of stars
• Absorption lines can be formed in the ISM
• They are very narrow low density, no
  collisional broadening
• Ionization states higher than stars can produce
  (typically)
• Often detect multiple components clouds of gas
  moving at different radial velocities
• From these lines can determine the abundances
  of elements in the ISM also, this is a way to
  confirm what dust grains are made from

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• H I clouds --- cold, neutral hydrogen
• 10 a few 100 atoms/cc
• 50 150 parsecs in diameter
• T 100 K
• Tend to be twisted, long flat filaments
• Between H I clouds, there is a warm intercloud
  medium T few 1000 K, density 0.1 atom/cc
• The H I clouds and the warm intercloud medium are
  in pressure equilibrium low density, higher
  temperature balances higher density, lower
  temperature

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• Intercloud medium stays ionized, due to low
  density (ions have few electrons nearby to
  capture)
• Elements in intercloud medium? H, He, C, N, O
  less Fe, Ca, Ti than in Suns atmosphere
  (actually, less C too) Why?
• Can map H I gas via 21cm radiation (detected with
  radio telescopes)

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• 21 cm radiation is produced by cold H atoms
• Inside an H atom, proton and electron these
  charged particles spin, which produces magnetic
  fields
• Since charges are opposite (positive for proton,
  negative for electron), when spin is the same,
  the magnetic fields are reversed.
• So, opposite spin, lower energy state same spin,
  higher energy state

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• Note the lowest energy level of H atom (ground
  state) is actually made of these two states
• Collisions put H atom in higher state the spin
  of the electron eventually will flip, putting
  atom in lower state a photon with ? 21 cm is
  emitted
• Takes 1.1 x 107 years! Why do we see any of this
  radiation

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• Molecules found in molecular clouds
• Vibration and rotation states of molecules
  excited by collisions hence photons emitted
• H2 most common, but does not produce much radio
  emission
• Molecular clouds are detected by radiation from
  other molecules (about 100 types detected)
• Example, CO emits line at ? 2.6 mm
• UV radiation breaks up molecules, so only survive
  deep inside dense clouds

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• Molecular radiation cools the cores of these
  dense clouds excitation by collisions, energy
  carried away by photons
• Giant Molecular Clouds 15 60 parsecs across,
  T 10K (very cold), masses of 102 to 106 Msun
• These are where stars form

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Other forms of Radiation from ISM

• IR radiation from dust grains
• Dust makes up 1 of mass of ISM
• T lt 100 K
• Lots of surface area, so grains can radiate lots
  of energy
• X-rays from ISM
• Coronal Gas T 106 K, like the solar corona
• 0.0004 0.003 atoms/cc
• In equilibrium with H I clouds
• Heated by Supernova explosions create bubbles
  of coronal gas (Sun is inside one of these)

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4 Basic Components of ISM

• H I clouds 25 of mass
• Warm intercloud medium 50 of mass
• Molecular clouds 25 of mass
• Coronal Gas 5 of mass

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Star Formation

• For most of the lives, stars exist in state of
  Hydrostatic Equilibrium balance between gravity
  and gas pressure
• In Sun, energy is generated by nuclear fusion
  4H ? 1He ( T 107 K)
• A thermostat if star contracts, T increases,
  gas pressure increases, rate of fusion reactions
  increases if star expands, T, pressure, and
  rate of reactions all decrease

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• So, in balance as long as there is an internal
  energy source
• Stars still being formed in Milky Way Galaxy
• Age of galaxy 1010 years
• Sun is 5 x 109 years old
• Hot, blue stars (O, B type) can live only 107
  years, so must have formed recently
• Young stars found near clouds of gas clue to
  how they form

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• Molecular Clouds, 50 parsec across, 105 Msun
  so, lots of mass, but 1020 times less dense than
  a star (and they are cold)
• So, How does it form into stars?
• There are dense cores, 0.1 parsec, 1 Msun are
  these stars forming?
• How does the gas contract? Must overcome
• Thermal motion
• Interstellar magnetic fields
• Rotation
• Needs a push shock waves (fast moving gas hits
  slower moving medium)

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• Source of Shock Waves?
• Supernovae
• ignition of hot stars
• Cloud collisions
• Spiral pattern of the Milky Way Galaxy
• Once Cloud begins to contract
• Atoms gain speed as they fall towards center of
  gravity gravitational energy converted into
  energy of motion
• During free-fall, atoms collide free-fall
  velocities converted into thermal motion
  (randomization)

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• So, Gravitational Energy converted into Thermal
  Energy the contracting gas is heated
• As cloud contracts, a proto-star forms
• Dense center, surrounded by low density cocoon
• Rotation rate will increase (Conservation of
  Angular Momentum) a proto-stellar disk forms
• Proto-star hot enough to emit radiation, but
  fusion reactions have not started
• Proto-stars luminous, cool red stars, but
  cocoon absorbs all visible light, reradiates in IR

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• Eventually, proto-star is hot enough to drive
  away the cocoon star becomes visible, can place
  it on H-R diagram (birth line)
• Will continue to contract until it reaches point
  on Main Sequence position, and time it takes
  to get there, depend on Mass
• Examples T Tauri stars, 0.75 3 Msun, 105 to
  108 years old found in clusters of hot, young
  stars less massive stars have not yet reached
  Main Sequence

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• In Orion T associations, plus associations of O
  stars
• Herbig-Haro objects jets of gas, flowing away
  from young stars, smashing into interstellar gas
  (example of a bipolar flow).
• Some H-H objects associated with T Tauri stars,
  but many still hidden by cocoons.

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• Back inside Stars
• Fusion in addition to proton-proton chain,
  there is the CNO cycle
• 12C 1H ? 13N ?
• 13N ? 13C e ? (decay)
• 13C 1H ? 14N ?
• 14N 1H ? 15O ?
• 15O? 15N e ? (decay)
• 15N 1H ? 4He 12C

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• So, 4 H atoms converted into 1 He, with C as
  catalyst
• Note 13C has a high column barrier so need
  very high temperatures (1.6 x 107 K
• Cool stars? Low mass, low temperature p-p chain
• Hot stars? High mass, high temperature CNO
  cycle

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Stellar Structure

• Energy Transport how does the energy created in
  the core of a star escape?
• Conduction need close contact between
  particles not important in normal stars.
• Radiation important in interiors absorption
  and re-emission of photons (remember the lab??)
• High energy photons are converted into low energy
  ones. 1 ?-ray photon ?2500 visible light photons
  (in 106 years)

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• (radiation, continued)
• Opacity resistance to the flow of radiation in
  a gas highly temperature dependent
• For example hot gas low opacity
• cool gas high opacity
• Cooler gas will block the radiative energy flow
  which will, in turn, heat up this opaque layer.
• 3. Convection hot gas rises, cooler (denser) gas
  sinks. Gas Layers mix, get a uniform composition

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• Sun has a radiative core and a convective
  envelope
• Massive stars (M gt few times Msun) have
  convective cores. Why? Higher energy production,
  so core cant lose energy fast enough
  radiatively.
• Also have radiative envelopes, extending to their
  photospheres
• Low Mass stars (lt 0.4 times Msun) completely
  convective, Why? Opacity is high throughout.

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• The Orion Nebula
• Part of a huge molecular cloud
• Gas illuminated, ionized by hot young stars (O
  stars, T 40,000 K) in a group called the
  Trapezium lt 2 x 106 yrs old
• West shoulder of Orion stars 1.2 x 107 yrs
• The Belt stars 8 x 106 yrs old