Interstellar medium

The space between a galaxy’s star systems is not empty. It is filled with the interstellar medium (ISM): a mix of matter and radiation. This matter takes the form of gas—in ionic, atomic, or molecular states—along with dust and cosmic rays. The ISM gradually merges into the intergalactic medium beyond the galaxy. The radiation within the same volume is called the interstellar radiation field. Though the ISM’s atomic density is far lower than that of the best laboratory vacuums, the average distance a particle travels before colliding with another is short compared to typical interstellar distances. On those scales, the ISM behaves as a gas—or more precisely, a slightly ionized plasma—rather than a collection of non-interacting particles.

The ISM has multiple phases, defined by whether the matter is ionic, atomic, or molecular, and by its temperature and density. Hydrogen is the dominant element, followed by helium, with trace amounts of carbon, oxygen, and nitrogen. The thermal pressures of these phases are roughly balanced. Magnetic fields and turbulent motions also exert pressure, and these forces are usually more important dynamically than thermal pressure. In dense molecular regions, the number density can reach 10¹² molecules per cubic meter (1 trillion per m³). In hot, diffuse regions, the gas is highly ionized, with densities as low as 100 ions per m³. For comparison, air at sea level has about 10²⁵ molecules per m³, and a high-vacuum chamber has roughly 10¹⁶ molecules per m³. By mass, 99% of the ISM in our galaxy is gas, and 1% is dust.

Of the gas, 91% of atoms (by number) are hydrogen, 8.9% are helium, and 0.1% are heavier elements (called “metals” in astronomy). By mass, this works out to 70% hydrogen, 28% helium, and 1.5% heavier elements. The hydrogen and helium come mainly from primordial nucleosynthesis, while the heavier elements were added later through stellar nucleosynthesis and stellar evolution.

The ISM is central to astrophysics because it connects stars to the galaxy as a whole. Stars form in its densest regions—molecular clouds—and later return matter and energy to the ISM via planetary nebulae, stellar winds, and supernovae. This cycle determines how quickly a galaxy uses up its gas and thus how long it can sustain star formation. Voyager 1 entered the ISM on August 25, 2012, becoming the first human-made object to do so; it will study interstellar plasma and dust until its estimated mission end in 2025. Its twin, Voyager 2, followed on November 5, 2018.

The ISM’s components are often described by a three-phase model. In 1969, Field, Goldsmith, and Habing proposed a static two-phase equilibrium: a cold, dense phase (below 300 K) of neutral and molecular hydrogen clouds, and a warm intercloud phase (around 10⁴ K) of rarefied neutral and ionized gas. In 1977, McKee and Ostriker added a dynamic third phase: very hot gas (around 10⁶ K), shock-heated by supernovae, which occupies most of the ISM’s volume. These phases exist at temperatures where heating and cooling can reach a stable equilibrium. That paper guided research for the next thirty years, though the exact proportions and subdivisions of the phases remain unclear.

The behavior of hydrogen—the ISM’s main constituent—explains the basic physics. Most of the Galactic disk has roughly balanced pressure between phases: high-pressure regions expand and cool, while low-pressure regions compress and heat up. Since pressure equals number density times temperature times a constant (P = n k T), hot regions have low density. Coronal gas is so diffuse that collisions are rare, producing little radiation and allowing temperatures to stay high for hundreds of millions of years. Once the temperature drops to about 10⁵ K, density rises enough that protons and electrons recombine into hydrogen atoms, emitting photons that carry energy away and cause runaway cooling. Left alone, this would produce the warm neutral medium. But OB stars are so hot that some of their photons have energies above 13.6 eV (the Lyman limit), enough to ionize hydrogen. These photons ionize any neutral hydrogen they encounter, creating a dynamic equilibrium between ionization and recombination. Gas close to OB stars becomes almost entirely ionized at around 8000 K (unless already in the coronal phase), until the ionizing photons are exhausted. This ionization front marks the boundary between the warm ionized and warm neutral media.

OB stars, and cooler ones, also emit many photons with energies below the Lyman limit. These pass through the ionized region almost unabsorbed. Some have energies above 11.3 eV, enough to ionize carbon atoms, creating a C II (ionized carbon) region outside the hydrogen ionization front. In dense areas, the size of this region may be limited by the available photons, but often such photons can penetrate throughout the neutral phase.