But the seeming void contains a barely perceptible fog of hydrogen and other neutral atoms. In 2020, NASA’s New Horizons spacecraft, having passed the orbit of Pluto, measured a proxy for the amount of hydrogen around it. In the area between stars in our galaxy — the “interstellar medium,” or ISM — New Horizons found around 130 hydrogen atoms per liter of space.
This hydrogen, along with some helium and other molecules, is mostly left over from the Big Bang, and it’s constantly supplemented by infusions from cosmic phenomena such as stars ejecting jets of gas and dust as they are born and shedding their outer layers as they die. Because the ISM is so diffuse and changes so slowly, astronomers have largely ignored its details for the last half-century, treating it as a stationary backdrop to the flashier happenings of the universe. But mostly empty space is less simple than it seems.
Over the past few years, astrophysicists have transformed our understanding of the interstellar medium. Using newly developed maps stretching thousands of light-years across our galaxy, they’ve discovered that it’s not the quiet, steady backdrop that researchers thought. Instead, much of the ISM appears to be unstable — a finding that directly contradicts the leading theoretical models taught in textbooks.
The surprise finding could help resolve a long-unanswered question about how quickly stars form. In 1969, the researchers George Field, Donald Goldsmith, and Harm Habing came up with a model of how the ISM behaves. They predicted that it exists in two stable states: cold and warm.
As gas and dust float through space, they pull more gas and dust together through gravity, eventually forming a cloud. These clouds tend to cool down, because fast-moving molecules will collide with other molecules and lose energy. Their temperature stays at a bitter 100 kelvins, or about minus 173 degrees Celsius.
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