Interstellar medium

The interstellar medium is not a vacuum, although its matter is extremely diffuse compared with air on Earth. It consists chiefly of hydrogen in atomic, molecular and ionized forms, with helium, trace heavier elements and small solid dust grains. Cosmic rays and magnetic fields are also important components. The material is distributed unevenly: cold, dense molecular clouds and dusty regions sit within more diffuse gas, while stellar activity creates hot, low-density cavities and expanding shells. These structures overlap and evolve; the ISM is better understood as a dynamic environment than as a single uniform substance.

Interstellar clouds provide raw material for star formation. Where gas becomes sufficiently dense, gravity can cause regions of a cloud to contract; dust helps shield molecular gas from starlight and provides surfaces for chemical reactions. Newly formed stars draw matter from the clouds. Later, stellar winds and, for some stars, supernova explosions return gas and newly formed elements to the surrounding medium. Mixing and repeated cycles help shape the chemical composition of later generations of stars and planetary systems. The processes are complex, and the paths from diffuse gas to star-forming clouds vary across a galaxy.

Dust grains, though a small fraction of the ISM's mass, affect what astronomers observe and how clouds evolve. They absorb and scatter visible light, making some regions appear dim or obscured, and re-emit absorbed energy at infrared wavelengths. Grain surfaces also assist the formation of molecules, including molecular hydrogen, and dust helps protect molecules inside clouds from ultraviolet radiation. Astronomers therefore study the ISM across different wavelengths: radio observations can trace atomic hydrogen through its characteristic 21-centimetre emission, while infrared measurements reveal cool dust and regions hidden at visible wavelengths.

The Solar System moves through nearby interstellar material. The Sun's outward-flowing solar wind creates the heliosphere, a region that interacts with the surrounding interstellar gas and dust. Spacecraft such as Voyager have sampled the boundary region, and measurements of neutral atoms and dust, together with observations of nearby stars, help researchers characterize the local environment. The properties of this nearby material are not a complete description of the entire Milky Way: density, temperature, ionization and structure vary from place to place.