6.2 Molecular Clouds and Star-Forming Regions

Molecular clouds are cosmic nurseries where stars are born. These vast structures of gas and dust, spanning hundreds of light-years, contain the raw materials for stellar formation. Magnetic fields and turbulence play crucial roles in shaping these clouds.

Star formation begins when parts of a molecular cloud become unstable and collapse under gravity. This process is influenced by factors like the cloud's mass, temperature, and density. Newly formed stars provide feedback, shaping their environment and triggering further star formation.

Molecular Cloud Structure

Characteristics and Types of Molecular Clouds

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• Giant Molecular Clouds (GMCs) form vast structures in the interstellar medium spanning hundreds of light-years
• GMCs contain primarily molecular hydrogen (H2) and helium with traces of heavier elements and dust
• Dark Nebulae appear as opaque regions against bright background stars or nebulae
  • Consist of dense concentrations of gas and dust that block visible light
  • Often indicate potential sites of future star formation
• Bok Globules represent small, dense molecular clouds
  • Typically measure less than a light-year across
  • Appear as isolated dark patches in bright emission nebulae
  • Serve as nurseries for low-mass star formation

Magnetic Fields and Cloud Dynamics

• Magnetic fields permeate molecular clouds influencing their structure and evolution
• Field strengths typically range from a few microgauss to several milligauss
• Magnetic pressure provides support against gravitational collapse in molecular clouds
• Magnetic fields can channel material along field lines affecting cloud morphology
• Magnetic flux freezing occurs as clouds contract preserving the magnetic field strength
• Ambipolar diffusion allows neutral particles to drift relative to charged particles gradually weakening magnetic support

Star Formation Processes

Cloud Fragmentation and Collapse

• Cloud fragmentation initiates when portions of a molecular cloud become gravitationally unstable
• Jeans Mass determines the minimum mass required for a cloud fragment to collapse under its own gravity
  • Depends on the cloud's temperature and density
  • Typical values range from 1 to 100 solar masses for different cloud conditions
• Gravitational collapse begins when a cloud fragment exceeds the Jeans Mass
  • Collapse proceeds faster in the cloud's central regions leading to a density profile ρ ∝ r^(-2)
  • Free-fall timescale for collapse given by $t_{ff} = \sqrt{\frac{3\pi}{32G\rho}}$
• Isothermal collapse occurs initially as the cloud remains optically thin
  • Transitions to adiabatic collapse when the core becomes opaque to its own radiation

Turbulence and Feedback in Star Formation

• Turbulence in molecular clouds plays a crucial role in regulating star formation
  • Generates density fluctuations that can seed gravitational collapse
  • Provides support against global cloud collapse on large scales
• Supersonic turbulence observed in molecular clouds with velocities exceeding the sound speed
• Energy cascade transfers turbulent energy from large to small scales
• Turbulence decay occurs on timescales comparable to the cloud crossing time
• Feedback mechanisms from newly formed stars can drive turbulence
  • Stellar winds, outflows, and supernovae inject energy into the surrounding medium
  • Photoionization from massive stars creates expanding HII regions
• Competitive accretion model suggests protostars grow by accreting gas from a shared reservoir influenced by turbulent motions