Short answers, pulled from the story.
The interstellar medium is composed of gas and dust, with gas making up 99% of its mass and dust 1%. By number, 91% of gas atoms are hydrogen and 8.9% are helium, with 0.1% being heavier elements. The gas exists in ionic, atomic, and molecular forms across a wide range of temperatures and densities.
Voyager 1 entered the interstellar medium on the 25th of August 2012, crossing the heliopause and becoming the first human-made object to do so. Its twin, Voyager 2, entered the interstellar medium on the 5th of November 2018.
The interstellar medium has several distinct phases: molecular clouds, the cold neutral medium, the warm neutral medium, the warm ionized medium, H II regions, and the hot ionized medium (coronal gas). These phases differ in temperature, density, and the state of hydrogen, ranging from molecular clouds at 10-20 kelvin to coronal gas at one million to ten million kelvin.
Johannes Hartmann made the first direct detection of cold diffuse matter in interstellar space in 1904. Observing the binary star Mintaka (Delta Orionis) with the Potsdam Great Refractor, he found that calcium absorption did not move with the star's orbital motion, proving the gas was a separate cloud along the line of sight.
Stars form within the densest regions of the interstellar medium, specifically inside molecular clouds. After stars form and eventually die, they return matter and energy to the medium through stellar winds, planetary nebulae, and supernovae, replenishing the raw material for future generations of stars. This cycle determines the rate at which a galaxy depletes its gas and how long it can sustain active star formation.
Polycyclic aromatic hydrocarbons, or PAHs, are carbon-rich molecules found throughout the interstellar medium. More than 20% of all carbon in the universe may be associated with them. Under interstellar conditions, PAHs can be transformed into more complex organic molecules through hydrogenation, oxygenation, and hydroxylation, representing steps toward amino acids and nucleotides.