THE INTERSTELLAR MEDIUM

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THE INTERSTELLAR MEDIUM

• The Interstellar Medium (ISM) is the material
  which fills the space between the stars.
• The ISM consists of gas (mostly hydrogen and
  helium), plus solid particles, or dust (composed
  mostly of heavier elements, such as carbon).
• The elemental abundances of the ISM (gas and dust
  components) are similar to those of the Sun and
  most stars.
• The ISM is the material from which new stars and
  planets are formed (a process continuing at
  present).
• The ISM is highly variable in its density
  distribution in space. However, the ratio of gas
  to dust percentage is roughly constant.
• Temperature, composition, and other properties of
  the interstellar material are highly variable
  throughout space.
• The ISM requires a broad range of techniques,
  equipment, and wavelength ranges of observation
  for its full and complete characterization.

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THE INTERSTELLAR MEDIUM

• The interstellar medium is detectable and
  measurable by a variety of methods, involving
  remote sensing of electromagnetic radiation over
  a very wide range of wavelengths (extending from
  radio waves to gamma rays).
• The distribution of interstellar material in our
  Galaxy is very non-uniform but tends to be more
  concentrated toward the plane of the Galaxy than
  are the visible stars.
• Interstellar gas is most evident, at visible
  wavelengths, in the regions near very hot stars,
  whose extreme-ultraviolet radiation excites and
  ionizes the interstellar gas.
• Interstellar atomic hydrogen is evident by its
  emission or absorption of radio waves (21 cm
  wavelength) in all regions of our Galaxy (not
  just in the local vicinity of stars).
• Interstellar dust is evident by its obscuration
  of starlight, from stars lying behind
  concentrations of dust, and by its reflection
  and/or scattering of starlight from stars in or
  near concentrations of interstellar dust.

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The Milky Way in Sagittarius (Including Galactic
Center Direction)
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THE INTERSTELLAR MEDIUM

• When the Universe was first created, all material
  was in the form of interstellar material, which
  consisted of only hydrogen, helium, and small
  amounts of deuterium (heavy hydrogen) and
  lithium.
• All heavier elements have been created by
  thermonuclear reactions in the cores of stars
  formed from this primordial gas.
• As a result of succeeding generations of star
  formation, evolution, and death, the interstellar
  medium has been enriched in heavier elements and
  depleted of H and especially D and Li (which are
  the most easily burned in thermonuclear
  fusion).
• However, even now, the heavy elements constitute
  less than 1 (by number of atoms) of the
  interstellar medium in our galaxy .

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COSMIC ABUNDANCES OF THE ELEMENTS(By Number
of Atoms)
Hydrogen H 106 Helium He 105 Oxygen O 890 Neon
Ne 500 Carbon C 400 Nitrogen N 110 Silicon Si 32

Iron Fe 20-30 Magnesium Mg 25 Sulfur S 22 Argon
Ar 7.8 Sodium Na 2 Aluminum Al 1.7 Calcium Ca 1.
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THE INTERSTELLAR MEDIUM

• The ratio of dust to gas density is nearly the
  same in most regions of interstellar space.
• However, the density and temperature of the
  interstellar medium can vary over a wide range.
• For static conditions to prevail, the gas
  pressure p nkT must be constant.
• Therefore, regions of high gas density are
  regions of low gas temperature, and vice versa.
• A typical atomic hydrogen density in interstellar
  space is one H atom per cubic centimeter.
• Very hot, low density regions of interstellar
  space can have less than 0.01 H atom /cm3 and
  temperatures in excess of 105 K.
• Dark molecular clouds, on the other hand, can
  have very low temperatures (lt30 K) and very high
  densities (gt106 /cm3).

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THE INTERSTELLAR MEDIUM

• In most regions of interstellar space, hydrogen
  exists in the neutral, atomic form, as do most
  other common elements.
• Atomic hydrogen is detectable in space by its
  emission or absorption of radio waves at a
  wavelength of 21 cm.
• Some elements, such as carbon, can exist in
  ionized form in the general interstellar medium.
  This is due to stellar ultraviolet radiation not
  sufficiently energetic to ionize hydrogen.
• In dense regions of the interstellar medium,
  molecules can be formed. These include H2, CO,
  CH, CN, OH, and many others.
• In dense molecular clouds, a major fraction of
  the hydrogen can be in the form of H2.
• The dust component of the interstellar medium is
  made up of carbon (largely in the form of
  graphite), silicon, and lesser amounts of heavier
  elements and their compounds.
• In the very darkest and densest clouds, complex
  organic molecules are detected using microwave
  and infrared spectroscopy.

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THE INTERSTELLAR MEDIUM

• These dense molecular clouds are also the sites
  of new star (and planetary system) formation.
• The relationship p nkT, as mentioned
  previously, establishes pressure uniformity
  between cold, dense regions and hot, low-density
  regions of the interstellar medium.
• However, an additional factor tends to compress
  the cold, high-density regions to an even greater
  degree stellar radiation pressure on the dust
  component of the interstellar material.
• Once a cold, dense cloud has reached the point at
  which its opacity to starlight is a significant
  factor, it will tend to contract as a result.
• In turn, as the cloud contracts, its
  self-gravitational force also comes into play, to
  further expedite contraction.
• Since the clouds usually have some rotational
  velocity, due in part to the general rotation of
  the Galaxy about its center, the law of
  conservation of angular momentum results in
  greater contraction parallel to, vs.
  perpendicular to, the axis of rotation.
• The result is, potentially, a disk-shaped
  condensation from which planets, as well as
  central stars, can form, of which our solar
  system is the best-known example.

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INTERSTELLAR NEBULAE

• Regions of diffuse glow, or obscuration of
  background stars, due to interstellar material
  are called nebulae (Latin for clouds).
• Regions of diffuse glow due to reflection of
  starlight by the dust particle component of the
  interstellar medium, are known as reflection
  nebulae.
• Regions of emission of light by atoms or
  molecules, excited by radiation from nearby
  stars, are called emission nebulae.
• Regions of the sky in which background stars
  appear to be obscured by material in front of
  these stars, are known as dark nebulae.
• Many diffuse nebulae have both emission and
  reflection components.
• Supernova remnants are a special case of emission
  nebulae, in which interstellar gas is excited by
  the shock wave produced by the explosion of a
  star, a supernova event.

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Clouds of Interstellar Gas and Dust in
Scorpius/Ophiuchus
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Interstellar Gas and Dust in Scorpius/Ophiuchus
? Ophiuchi
? Scorpii
? Scorpii (Antares)
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Pleiades Star Cluster and Reflection Nebulosity
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EMISSION NEBULAE

• The most common type of emission nebula is the
  result of the photoionization and excitation of
  surrounding interstellar gas, by the far- and
  extreme-ultraviolet radiation of very hot early
  type stars (spectral types O and early B).
• These stars emit a major part of their radiation
  at wavelengths shortward of that which
  corresponds to the photoionization energy of
  atomic hydrogen (91.2 nanometers wavelength, or
  13.6 electron volts energy).
• Since the lifetimes of these very hot stars are
  much shorter than those of cooler and less
  luminous stars, such as our Sun, most emission
  nebulae of this type (such as the Orion Nebula)
  are associated with regions of dense interstellar
  material in which new stars are still in the
  process of formation.
• Another type of emission nebula, the so-called
  planetary nebula, is associated with the late
  stages of stellar evolution, in which the star
  casts off its outer envelope on its way to its
  becoming a white dwarf star (which can be even
  hotter, though much less luminous, than the very
  young early type stars).

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The Rosette (Emission) Nebula in Monoceros
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Very Wide Field, Deep Exposure View of the Orion
Region
N
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Interstellar Hydrogen Fluorescence in Orion
Interstellar Hydrogen Fluorescence in Orion
North
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Flame Nebula and Horsehead Nebula in Orion
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The Horsehead Nebula in Orion (Ground-Based and
HST)
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The Orion Nebula Region
The Orion Nebula Region
N
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The Orion Nebula
The Orion Nebula - Closeup
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The Orion Nebula
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Orion Nebula Hubble Space Telescope View
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Cone Nebula in Monoceros - Ground-Based Visible
Light Image
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The Cone Nebula in Monoceros As Observed with the
HSTs Advanced Camera for Surveys
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Trifid Nebula (M20)- Ground-Based Telescope
(NOAO) View
HST View
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Trifid Nebula as Viewed by Hubble Space Telescope
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The Eagle Nebula (M16), Viewed by National
Optical Astronomy Observatory (NOAO)
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Ground-Based and HST Images of M16
HST - ACS
HST WFPC2
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HST Advanced Camera for Surveys (ACS) View of
Eagle Nebula (M16) Northeast Region
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M16 HST-ACS Image Closeups
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Ground-Based Telescopic Image of the ? Carina
Nebula
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? Carina, Keyhole Nebula Ground-Based
Telescopic Image, Closeup
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IC 2944 Thackerays Globules
HST View