6.1 Phases of the Interstellar Medium

The interstellar medium (ISM) is a cosmic soup of gas and dust between stars. It's divided into different phases based on temperature and ionization state. These phases shape galaxies and star formation.

From hot ionized plasma to cold molecular clouds, the ISM's diverse components interact in complex ways. Understanding these phases is key to grasping how galaxies evolve and new stars are born.

Ionized Interstellar Medium

Hot and Warm Ionized Media

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• Hot Ionized Medium (HIM) comprises ~50% of ISM volume
  • Characterized by temperatures exceeding 10^6 K
  • Consists of highly ionized plasma created by supernovae explosions
  • Occupies vast regions of space (galactic bubbles, superbubbles)
  • Emits primarily in X-ray wavelengths due to its extreme temperature
• Warm Ionized Medium (WIM) accounts for ~20-30% of ISM volume
  • Maintains temperatures around 8000 K
  • Contains partially ionized hydrogen gas
  • Primarily heated by ultraviolet radiation from hot stars
  • Detected through optical emission lines (Hα emission)
• Both media play crucial roles in galactic structure and evolution
  • Influence star formation processes
  • Contribute to galactic magnetic field dynamics
  • Facilitate matter and energy transfer within galaxies

HII Regions and Supernova Remnants

• HII Regions form around hot, young stars
  • Consist of ionized hydrogen gas (primarily protons and electrons)
  • Temperatures range from 7000 to 10000 K
  • Appear as bright, glowing nebulae in visible light (Orion Nebula)
  • Serve as indicators of recent star formation activity
• Supernova Remnants result from massive star explosions
  • Expand rapidly, reaching speeds up to 10,000 km/s
  • Temperatures can exceed 10^7 K in the initial stages
  • Emit across a wide range of wavelengths (radio, optical, X-ray)
  • Enrich the ISM with heavy elements produced during stellar evolution
• Both phenomena significantly impact their surrounding environment
  • Trigger star formation through shock compression of nearby gas clouds
  • Contribute to the chemical enrichment of the interstellar medium
  • Shape the morphology and dynamics of the galactic ISM

Neutral Interstellar Medium

Warm and Cold Neutral Media

• Warm Neutral Medium (WNM) constitutes ~30% of ISM volume
  • Maintains temperatures between 6000 and 10000 K
  • Consists primarily of neutral atomic hydrogen (HI)
  • Detected through 21 cm radio emission
  • Distributed throughout the galactic disk in a diffuse manner
• Cold Neutral Medium (CNM) accounts for ~1-5% of ISM volume
  • Exhibits temperatures around 50-100 K
  • Composed of dense, neutral atomic hydrogen
  • Observed through HI absorption lines
  • Typically found in compact clouds or filaments
• Both media play crucial roles in galactic processes
  • Serve as reservoirs for future star formation
  • Contribute to the overall mass budget of galaxies
  • Influence the propagation of cosmic rays and magnetic fields

Molecular Gas and Interstellar Dust

• Molecular Gas comprises less than 1% of ISM volume but ~20% of its mass
  • Consists primarily of molecular hydrogen (H2)
  • Found in dense, cold regions with temperatures around 10-20 K
  • Detected indirectly through CO emission lines
  • Serves as the primary fuel for star formation (molecular clouds)
• Interstellar Dust accounts for ~1% of the total ISM mass
  • Composed of tiny solid particles (0.1-1 μm in size)
  • Materials include silicates, carbonaceous compounds, and ices
  • Absorbs and scatters starlight, causing interstellar extinction
  • Plays a crucial role in molecular hydrogen formation
• Both components significantly influence galactic evolution
  • Molecular gas acts as the birthplace of stars and planetary systems
  • Dust grains catalyze chemical reactions and shield molecules from radiation
  • Together, they shape the observed properties of galaxies and star-forming regions