7.1 Composition and phases of the interstellar medium

The interstellar medium (ISM) is a cosmic soup of gas and dust that fills the space between stars. It's mostly hydrogen and helium, with a sprinkle of heavier elements and tiny dust grains. This mixture plays a crucial role in the life cycle of stars and galaxies.

The ISM exists in different phases, from cold molecular clouds where stars are born to hot ionized gas heated by supernovae. These phases interact and evolve, shaping galactic structure and driving the ongoing cycle of star formation and death.

Composition of the Interstellar Medium

Composition of interstellar medium

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• Gas components dominate ISM
  • Hydrogen most abundant element comprises ~90% of atoms
    • Atomic hydrogen (H I) found in diffuse clouds emits 21 cm line
    • Molecular hydrogen (H2) concentrated in dense clouds detected via CO emission
  • Helium second most abundant ~10% of atoms important in stellar evolution
  • Trace amounts of heavier elements enrich ISM over time (carbon, oxygen, nitrogen)
• Dust components make up ~1% of ISM mass
  • Silicate grains composed of silicon and oxygen absorb UV light
  • Carbonaceous particles include graphite and PAHs emit in infrared
  • Ice mantles on dust grains host complex organic molecules (methanol, formaldehyde)
• Cosmic rays accelerate chemical reactions in ISM
  • High-energy particles mostly protons originate from supernova remnants
• Magnetic fields influence gas dynamics and star formation
• Radiation fields heat and ionize gas UV from hot stars, X-rays from compact objects

Element distribution in space

• Hydrogen and helium dominate ISM composition
  • Primordial origin from Big Bang nucleosynthesis sets initial abundances
  • Hydrogen ~70% by mass primary fuel for star formation
  • Helium ~28% by mass affects stellar structure and evolution
• Metals (elements heavier than helium) enrich ISM over time
  • Produced by stellar nucleosynthesis released through stellar winds and supernovae
  • ~2% by mass overall abundance increases with cosmic time
• Abundance variations across galactic structures
  • Higher metal content in galactic disks due to ongoing star formation
  • Lower metal content in galactic halos older stellar populations
• Depletion of elements from gas phase
  • Some elements (iron, silicon) less abundant in gas incorporated into dust grains
  • Depletion factors vary with ISM density and environment

Phases and Role of the Interstellar Medium

Phases of interstellar medium

• Molecular clouds form stars
  • Cold (10-20 K) and dense ($> 10^2$ cm$^{-3}$) regions shield from UV radiation
  • Composed mainly of H2 traced by CO emission
• Cold neutral medium (CNM) or H I regions transition between phases
  • Atomic hydrogen regions emit 21 cm line
  • Temperature ~100 K density ~10-100 cm$^{-3}$
• Warm neutral medium (WNM) fills much of ISM volume
  • Diffuse atomic hydrogen heated by cosmic rays
  • Temperature ~6000-10000 K density ~0.1-1 cm$^{-3}$
• Warm ionized medium (WIM) or H II regions surround hot stars
  • Ionized hydrogen regions emit H-alpha radiation
  • Temperature ~8000 K density ~0.1-1 cm$^{-3}$
• Hot ionized medium (HIM) or coronal gas fills superbubbles
  • Very hot, low-density plasma heated by supernovae
  • Temperature $> 10^5$ K density $< 10^{-2}$ cm$^{-3}$ emits in X-rays

Role in star formation

• Star formation occurs in molecular clouds
  • Dense regions collapse under gravity fragment into protostellar cores
  • Accretion disks form around protostars eventually forming planets
• Galaxy evolution driven by gas cycle
  • ISM provides gas reservoir for ongoing star formation
  • Metal enrichment through stellar feedback increases over time
• Interstellar medium dynamics shape galactic structure
  • Supernova shock waves compress gas trigger new star formation
  • Galactic fountains circulate gas between disk and halo
• Dust grains catalyze chemical reactions
  • Act as sites for H2 formation shield molecular clouds from UV radiation
  • Provide surface for complex organic molecule synthesis
• Regulation of star formation maintains equilibrium
  • Feedback processes (stellar winds, supernovae) heat and disperse gas
  • Heating and cooling balance determines gas phase structure