The space between stars is emptier than any lab vacuum, yet behaves like weather
On 25 August 2012 Voyager 1 became the first human-made object to reach interstellar space. What it entered is thinner than the best vacuum chambers on Earth, sometimes just 100 ions in a cubic metre. Yet across light-years this wispy material acts like a turbulent gas, full of shocks, bubbles and magnetic storms.
By mass, the matter between the stars of our galaxy is 99 percent gas and 1 percent dust. Counting atoms, about 91 percent are hydrogen and nearly 9 percent helium, mostly left over from the Big Bang, with a sprinkle of heavier elements that astronomers loosely call metals, forged later inside stars. Densities swing enormously, from about a trillion molecules per cubic metre in cold clouds down to the hot, ionised extremes, compared with around ten trillion trillion in sea-level air.
The gas sorts itself into phases that roughly balance each other's pressure. A 1969 model proposed cold clouds below 300 kelvin mixed with warm gas near 10,000; in 1977 Christopher McKee and Jeremiah Ostriker added a third, million-degree phase heated by exploding stars and filling most of the volume. Hot gas is so sparse that its particles rarely collide, so it can stay hot for hundreds of millions of years, while cooler gas can suddenly radiate its energy away in runaway cooling.
Stars and this medium constantly remake each other. Stars condense in the densest molecular clouds, and when big hot ones ignite they heat their birth gas, which bursts outward in what astronomers call a Champagne flow. After a few million years those stars explode, sending blast waves through their surroundings, and winds and supernovae inflate superbubbles that X-ray telescopes can see. The rate of this cycle helps decide how long a galaxy keeps forming stars.
Galaxy shape matters. In spirals like the Milky Way, the gas lies in a thin disk about 300 light-years thick, and spiral arms squeeze it and trigger star birth. Elliptical galaxies hold almost only the hot, thin phase, which fits their lack of new stars. Voyager 2 followed its twin into interstellar space on 5 November 2018.
Source: Interstellar medium
