
The diffuse gas in the space between stars, which can interact with cosmic rays.Interstellar gas is the diffuse matter that fills the space between stars in a galaxy. It consists primarily of hydrogen (atomic and molecular) and helium, with trace amounts of heavier elements, dust, and cosmic rays. This gas is the raw material for star formation and is constantly recycled through stellar birth and death. It also serves as a medium through which cosmic rays propagate, and its interactions with radiation reveal the structure and chemistry of galaxies.
The existence of interstellar gas was first inferred in the early 20th century from the absorption lines of calcium and sodium in starlight, and later confirmed by the detection of interstellar hydrogen via the 21-cm line in 1951.
Interstellar gas exists in multiple phases: cold molecular clouds (10–20 K), warm neutral and ionized gas (10⁴ K), and hot ionized gas (10⁶ K) from supernova remnants. Cosmic rays, high-energy charged particles, travel through this gas, ionizing atoms and exciting molecular emissions. The gas is also influenced by magnetic fields, which shape its structure and affect star formation rates.
Interstellar gas is studied using radio, infrared, and X-ray observations to map the structure of the Milky Way, identify regions of star formation, and understand the life cycle of matter in galaxies. It is also relevant to the search for complex organic molecules in space.
Interstellar gas is so sparse that a cubic centimeter of space contains only about one atom on average, yet it is dense enough to block visible light in some regions, creating dark nebulae.






















