The interstellar medium (ISM) is the cosmic soup between stars. It's a mix of gas and dust that clumps into clouds, ranging from diffuse to dense. This stuff isn't just floating around—it's the raw material for new stars and planets.

Observing the ISM is tricky due to its low density, but scientists use clever techniques. The 21-cm line helps map hydrogen gas, while cosmic rays reveal magnetic fields. Understanding the ISM is key to grasping how galaxies evolve and new stars form.

The Interstellar Medium in the Milky Way

Composition of interstellar matter

• ISM composed of gas and dust
  • Gas primarily hydrogen and helium exists in atomic, molecular, and ionized forms
  • Dust microscopic solid particles made of silicates (sand), graphite (pencil lead), and ices (water ice)
    • Also known as cosmic dust, which plays a crucial role in star formation
• ISM not uniformly distributed
  • Clumps together due to gravity to form clouds
    • Diffuse clouds low density, mostly atomic hydrogen
    • Molecular clouds higher density, mostly molecular hydrogen, contain dust which blocks visible light
    • Dark nebulae densest and coldest clouds, appear as dark patches in the sky due to dust obscuration (Horsehead Nebula)
  • HII regions: areas of ionized hydrogen surrounding hot, young stars

Evolution of interstellar clouds

• Gravity pulls ISM together, denser regions attract more matter, clouds become more massive and denser over time
• Dense cores within molecular clouds collapse under gravity
  • Protostars form within these collapsing cores
  • Accretion of surrounding material onto the protostar
  • Nuclear fusion begins, marking the birth of a new star (Sun)
• Stellar winds and supernova remnants contribute to the recycling of material in the ISM

Density of interstellar vs terrestrial matter

• ISM density extremely low compared to Earth's atmosphere
  • Typical ISM density $10^{-24}$ to $10^{-20}$ g/cm³
  • Earth's atmosphere at sea level $\sim10^{-3}$ g/cm³ (1,000 times denser than ISM)
• Best vacuum created on Earth still denser than most ISM
  • Ultra-high vacuum $\sim10^{-13}$ g/cm³ (10 billion times less dense than air)
• Interstellar clouds denser than average ISM but still much lower density than Earth's atmosphere
  • Diffuse clouds $\sim10^{-22}$ g/cm³ (Orion Nebula)
  • Molecular clouds $\sim10^{-19}$ g/cm³ (Taurus Molecular Cloud)

Observing the Interstellar Medium

• Interstellar extinction: the absorption and scattering of light by interstellar dust, affecting our observations of distant objects
• 21-cm line: radio emission from neutral hydrogen, used to map the distribution of gas in the galaxy
• Cosmic rays: high-energy particles that travel through the ISM, providing information about galactic magnetic fields and energy processes