Interstellar medium

The interstellar medium (ISM) is the matter and fields that fill the space between stars in a galaxy, dominated by hydrogen and helium gas with a smaller fraction of heavier elements and solid dust grains by mass of order one percent. Physics of the Interstellar and Intergalactic Medium (astro.princeton.edu) The average mass density is roughly equivalent to about one hydrogen atom per cubic centimetre when averaged over the disk of the Milky Way, though local conditions vary by many orders of magnitude. Molecular Astrophysics (Cambridge), ch. “Molecular clouds in the Milky Way” (resolve.cambridge.org)

Structure and phases

–The ISM is commonly described as a multiphase medium: cold neutral medium (CNM, T ≲ 100 K), warm neutral medium (WNM, T ≈ 6000–8000 K), warm ionized medium (WIM, T ≈ 8000 K), and hot ionized medium (HIM, T ≈ 10⁶ K), alongside dense molecular gas (T ≈ 10–50 K). The interstellar environment of our galaxy (arxiv.org) Wolfire and collaborators showed that thermal balance in diffuse atomic gas admits two stable phases (CNM and WNM) over a narrow pressure range set by heating and cooling. The Neutral Atomic Phases of the ISM in the Galaxy (arxiv.org) The classical three-phase picture emphasizes the volume-filling role of supernova-driven hot gas with embedded clouds. Theory of the interstellar medium: three components regulated by supernova explosions (osti.gov)

Composition and energetics

–Hydrogen and helium dominate the mass; heavier elements reside partly in the gas phase and partly locked into solid grains that account for about 1% of the mass and control much of the opacity from the ultraviolet to the infrared. Physics of the Interstellar and Intergalactic Medium (astro.princeton.edu) The wavelength dependence of extinction through dust is well represented by the Cardelli–Clayton–Mathis (CCM) law with one parameter, R(V) = A(V)/E(B−V), widely used to deredden observations. The Relationship between Infrared, Optical, and Ultraviolet Extinction (ui.adsabs.harvard.edu) Energy densities in magnetic fields, starlight, thermal gas, and Cosmic rays are broadly comparable in the solar neighborhood, implying strong dynamical coupling among ISM constituents. The interstellar environment of our galaxy (journals.aps.org) A review of cosmic-ray transport places their typical energy density near ∼1 eV cm⁻³ and documents their feedback on the gas and magnetic field. The Nine Lives of Cosmic Rays in Galaxies (annualreviews.org)

Heating and cooling processes

–In diffuse neutral gas, photoelectric emission from very small grains and PAHs is the dominant heating mechanism, while fine-structure line emission (notably C II 158 μm and O I 63 μm) and Lyα provide the principal cooling channels. The photoelectric heating mechanism for very small graphitic grains and PAHs; The Neutral Atomic Phases of the ISM in the Galaxy (cita.utoronto.ca) In the warm-ionized regime, balance between photoionization by massive stars and radiative cooling maintains T ≈ 8000 K. The warm ionized medium in spiral galaxies (arxiv.org) Hot (T ∼ 10⁶ K) gas created by supernova shocks cools primarily via metal-line emission and bremsstrahlung. The interstellar environment of our galaxy (arxiv.org)

Magnetic fields and turbulence

–Interstellar magnetic fields of a few microgauss are measured across a wide range of scales via Faraday rotation, Zeeman splitting, and polarized synchrotron/dust emission; such fields guide cosmic rays, influence cloud support, and shape filamentary structure. The spatial energy spectrum of magnetic fields in our Galaxy; Magnetic Fields in Spiral Galaxies (arxiv.org) In molecular clouds, typical field strengths and scaling relations indicate dynamically important magnetization, probed by Zeeman detections and dust polarization. Magnetic Fields in Molecular Clouds (annualreviews.org) Turbulence pervades all phases and, in combination with Magnetohydrodynamics, mediates pressure balance and density structure from parsec to kiloparsec scales. The interstellar environment of our galaxy (arxiv.org)

Dust, extinction, and polarization

–Interstellar dust reddens and attenuates starlight, with sightline-to-sightline variations captured by the CCM law and later refinements, and emits thermally in the infrared–submillimetre. The Relationship between Infrared, Optical, and Ultraviolet Extinction (ui.adsabs.harvard.edu) All-sky maps from the [Planck (spacecraft)] revealed the polarization of dust emission and its alignment with the magnetized filamentary ISM. Planck intermediate results XXXVIII: dust polarization and filaments (aanda.org) Stellar surveys now map three-dimensional extinction and dust properties across the Galaxy; ESA’s Gaia mission has produced global dust maps via stellar photometry and spectroscopy. Gaia's view of dust in the Milky Way (sci.esa.int)

Observational tracers

–Neutral atomic hydrogen is traced by its hyperfine 21‑cm line; the HI4PI survey provides a full-sky data set of unprecedented resolution and sensitivity for Galactic H I. HI4PI: a full-sky H I survey based on EBHIS and GASS (aanda.org) Ionized gas is mapped via the Hα line; the Wisconsin H‑Alpha Mapper delivered a kinematically resolved, absolutely calibrated Hα map of the northern sky that traces the diffuse WIM. The Wisconsin Hα Mapper Northern Sky Survey (arxiv.org) Molecular hydrogen (H₂) is observed indirectly using the CO J=1→0 line as a surrogate; a composite CO survey of the Galactic plane charts the distribution of giant Molecular cloud complexes. The Milky Way in Molecular Clouds: A New Complete CO Survey (lweb.cfa.harvard.edu) Dust is traced by extinction in stellar photometry and by far‑IR/submm thermal emission; Planck and other missions provide sky maps of dust column and temperature. Planck 2015 results. I. Overview of products and scientific results (aanda.org)

Chemistry and molecules

–ISM chemistry proceeds in the gas phase and on grain surfaces; shielding in dense regions allows H₂ and a rich molecular inventory to form, while dust and PAHs regulate photoelectric heating and charge balance. The Physics and Chemistry of the Interstellar Medium (cambridge.org) Photodissociation region theory and observations connect PAH emission and fine‑structure cooling to the local radiation field. Probing the role of PAHs in photoelectric heating (aanda.org)

Star formation and feedback

–Stars form in the densest molecular structures; turbulence, magnetic fields, and gravity set the star‑formation efficiency and the distribution of stellar masses. Theory of Star Formation (annualreviews.org) Young massive stars and Supernova explosions heat, ionize, and stir the surrounding medium, drive winds and superbubbles, and help maintain the multiphase structure, linking stellar life cycles to galactic‑scale gas flows. Theory of the interstellar medium: three components regulated by supernova explosions (osti.gov)

Distribution in the Milky Way

–Atomic, molecular, and ionized gas have distinct vertical and radial distributions set by gravity, pressure, and feedback; H I extends to large radii and heights, H₂ concentrates along spiral arms, and ionized gas forms a thick disk traced by Hα. The interstellar environment of our galaxy; The Milky Way in Molecular Clouds: A New Complete CO Survey; The Wisconsin Hα Mapper Northern Sky Survey (arxiv.org)

Key diagnostics and methods

–Combining multiwavelength tracers—21‑cm H I, CO, recombination lines, UV/optical absorption, and far‑IR/submm dust emission—with stellar surveys enables three‑dimensional reconstructions of ISM density, composition, and kinematics across the Galaxy. HI4PI full‑sky H I survey; Gaia dust maps; Planck dust polarization (aanda.org)