🪐Intro to Astronomy Unit 20 – Interstellar Gas and Dust

Key Concepts and Definitions

• Interstellar medium (ISM) refers to the matter and radiation that exists in the space between the stars within a galaxy
• Consists of gas (99%) and dust (1%) spread throughout the Milky Way and other galaxies
• Serves as the repository for material ejected by stars through stellar winds or supernovae explosions
• Provides the raw material for the formation of new stars and planets
• Plays a crucial role in the cycle of stellar birth and death
• Interstellar extinction is the absorption and scattering of electromagnetic radiation by dust and gas between an emitting astronomical object and the observer
• Interstellar reddening occurs when blue light is scattered and absorbed more than red light, making distant stars appear redder than they actually are

Composition of Interstellar Medium

• Primarily composed of hydrogen and helium, with trace amounts of heavier elements
• Exists in various forms, including neutral atomic gas, molecular gas, and ionized gas
• Neutral atomic hydrogen (HI) is the most abundant component, detectable through the 21-cm radio line
• Molecular hydrogen (H2) is difficult to detect directly but can be traced by carbon monoxide (CO) emissions
• Ionized hydrogen (HII) regions surround hot, young stars and emit light through recombination
• Interstellar dust consists of silicates, graphites, and ices, ranging in size from a few molecules to 0.1 microns
  • Dust grains play a vital role in the formation of molecules by providing a surface for chemical reactions
  • Dust contributes to interstellar extinction and reddening by absorbing and scattering light

Physical Properties of Gas and Dust

• Gas in the ISM can be classified by its temperature, density, and ionization state
  • Cold neutral medium (CNM) has temperatures around 100 K and densities of 10-100 cm^-3
  • Warm neutral medium (WNM) has temperatures of 6000-10,000 K and densities of 0.1-1 cm^-3
  • Warm ionized medium (WIM) has temperatures of ~8000 K and densities of 0.1 cm^-3
  • Hot ionized medium (HIM) has temperatures of 10^5-10^6 K and densities of 0.001-0.01 cm^-3
• Dust grains have temperatures ranging from 10-100 K, depending on their size and composition
• Interstellar gas is often turbulent, with velocities of a few km/s to hundreds of km/s
• Magnetic fields thread through the ISM, influencing gas dynamics and star formation
  • Typical magnetic field strengths range from a few microgauss to a few milligauss
• Cosmic rays, high-energy charged particles, permeate the ISM and contribute to its ionization and heating

Distribution in the Galaxy

• The ISM is not uniformly distributed throughout the Milky Way
• Concentrated along the galactic plane, with a scale height of ~100 parsecs for neutral hydrogen and ~50 parsecs for molecular gas
• Organized into distinct structures such as spiral arms, molecular clouds, and superbubbles
  • Spiral arms contain higher densities of gas and dust, as well as increased star formation activity
  • Molecular clouds are the densest regions (10^2-10^6 cm^-3), often associated with star-forming regions like the Orion Nebula
  • Superbubbles are large, shell-like structures created by multiple supernovae and stellar winds, spanning hundreds of parsecs
• The Milky Way's central region contains higher gas densities and more complex structures compared to the outer regions
• Interstellar gas also exists in the galactic halo, with lower densities and higher temperatures than in the disk

Observational Techniques

• Radio observations, particularly the 21-cm hydrogen line, are used to map the distribution and velocity of neutral atomic gas
• Millimeter and submillimeter observations of molecular line emissions (CO, CS, HCN) trace the distribution and properties of molecular clouds
• Infrared observations reveal the presence and composition of interstellar dust through its thermal emission and absorption features
  • Polycyclic aromatic hydrocarbons (PAHs) have distinct emission features in the mid-infrared
  • Silicate grains show absorption features at 9.7 and 18 microns
• Optical and UV observations are used to study the properties of interstellar gas through absorption lines (Ca II, Na I) and emission lines (H-alpha, [O III])
• X-ray observations probe hot, ionized gas in supernova remnants and the galactic halo
• Polarization measurements of starlight and dust emission provide information on the orientation and strength of interstellar magnetic fields

Interactions with Stellar Evolution

• The ISM plays a crucial role in the life cycle of stars
• Stars form from the gravitational collapse of dense regions within molecular clouds
  • Protostellar jets and outflows from young stellar objects interact with the surrounding ISM, creating shocks and influencing the star formation process
• Throughout their lives, stars continually lose mass through stellar winds, enriching the ISM with heavy elements
  • Low-mass stars contribute to the ISM through planetary nebulae
  • High-mass stars have more powerful winds and end their lives as supernovae, ejecting a significant fraction of their mass into the ISM
• Supernova explosions can trigger new star formation by compressing nearby gas clouds
• The ISM's composition evolves over time as successive generations of stars enrich it with metals

Importance in Star and Planet Formation

• The ISM serves as the reservoir of material for star and planet formation
• Molecular clouds, the densest regions of the ISM, are the birthplaces of stars
  • Gravitational instabilities within these clouds lead to the formation of protostars and protoplanetary disks
• The initial mass function (IMF) of stars is influenced by the properties of the parent molecular cloud, such as its density, turbulence, and magnetic field strength
• Dust grains in the ISM are the building blocks of planets
  • Dust settles into the midplane of protoplanetary disks, facilitating the growth of planetesimals through collisions
  • The composition of dust grains determines the initial chemical makeup of planets and their atmospheres
• The ISM's metallicity (abundance of elements heavier than helium) affects the efficiency of planet formation and the likelihood of forming terrestrial planets

Current Research and Open Questions

• Understanding the detailed physical processes that govern the ISM, such as turbulence, magnetic fields, and cosmic ray propagation
• Investigating the role of feedback from stars and supernovae in regulating star formation and shaping the ISM
• Characterizing the properties and distribution of interstellar dust, particularly in the context of planet formation and the search for prebiotic molecules
• Studying the ISM in different galactic environments (e.g., dwarf galaxies, galaxy clusters) to understand its evolution and the impact on star formation
• Exploring the connection between the ISM and the intergalactic medium (IGM) through galactic outflows and accretion
• Developing advanced observational techniques and instruments to probe the ISM in greater detail, such as the Atacama Large Millimeter/submillimeter Array (ALMA) and the James Webb Space Telescope (JWST)
• Combining multi-wavelength observations with numerical simulations to create a comprehensive model of the ISM and its role in galaxy evolution