20.2 Interstellar Gas

The interstellar medium is a cosmic soup of gas and dust between stars. It's mostly hydrogen in three forms: atomic, molecular, and ionized. Each type has unique properties and detection methods, shaping our understanding of galactic structure and star formation.

Interstellar gas plays a crucial role in the life cycle of stars and galaxies. From the cold, dense molecular clouds where stars are born to the hot, ionized regions surrounding young stars, this gas is constantly recycled and transformed throughout the cosmos.

Interstellar Gas

Categories of interstellar gas

• Atomic hydrogen (HI)
  • Consists of neutral hydrogen atoms not bound to other atoms
  • Most abundant element in the interstellar medium (ISM) makes up majority of gas between stars
• Molecular hydrogen (H2)
  • Formed when two hydrogen atoms bond together through shared electrons
  • Forms in the densest, coldest regions of the ISM where gas clouds are shielded from radiation (molecular clouds)
• Ionized hydrogen (HII)
  • Hydrogen atoms stripped of their electrons by high-energy radiation
  • Found in regions surrounding hot, young stars that emit intense ultraviolet light (HII regions)

Detection methods for interstellar gas

• Atomic hydrogen (HI)
  • Detected through the 21-cm radio line emission
    • Caused by a hyperfine transition between two energy levels in the hydrogen atom's electron
    • Allows mapping of HI distribution throughout the galaxy (radio telescopes)
• Molecular hydrogen (H2)
  • Cannot be directly observed in cold interstellar clouds as H2 lacks a permanent dipole moment
  • Presence inferred through the detection of other molecules, such as carbon monoxide (CO)
    • CO emits radio waves when its rotational state changes, acting as a tracer for H2
• Ionized hydrogen (HII)
  • Detected through the observation of characteristic emission lines in the visible spectrum
    • Balmer series lines like Hα (656.3 nm) and Hβ (486.1 nm)
    • Caused by electrons transitioning between energy levels in the hydrogen atom
    • Appear as bright nebulae in visible light images (Orion Nebula)

Characteristics of interstellar gas types

• Atomic hydrogen (HI)
  • Temperature ranges from 50-100 K, too warm for atoms to bind together
  • Density varies from 1-100 atoms per cm³, diffuse and spread out
  • Distributed throughout the Milky Way galaxy, fills much of the space between stars
• Molecular hydrogen (H2)
  • Temperature remains low at 10-20 K, allowing molecules to form and survive
  • Density much higher at 10³-10⁶ molecules per cm³, gas is concentrated
  • Found in giant molecular clouds, the birthplaces of new stars and planets (Orion Molecular Cloud)
• Ionized hydrogen (HII)
  • Temperature reaches 10,000-20,000 K due to heating by UV radiation from hot stars
  • Density ranges from 1-10⁴ ions per cm³, varies with distance from ionizing stars
  • Found in HII regions surrounding hot, young stars (O and B-type)
    • Intense ultraviolet radiation from these stars ionizes the surrounding hydrogen gas (Eagle Nebula)

Other components of the interstellar medium

• Interstellar dust: Tiny solid particles mixed with the gas, affecting light transmission and absorption
• Stellar winds: Streams of particles ejected from stars, contributing to the interstellar medium
• Cosmic rays: High-energy particles traveling through space, interacting with interstellar gas
• Supernova remnants: Expanding shells of gas and dust from exploded stars, enriching the interstellar medium with heavy elements