interstellar medium

Look up at a clear night sky and the space between the stars seems perfectly empty — a black, silent vacuum. It is not. Sprinkled through that darkness is a thin haze of gas and dust, so tenuous that on Earth we would call it a near-perfect vacuum, yet so vast that, added up across the galaxy, it weighs as much as billions of Suns. This is the interstellar medium, or ISM: the stuff that fills the gaps between the stars.

By volume the ISM is mostly hydrogen (about 70 percent by mass) and helium (about 28 percent), with a sprinkle of heavier elements and tiny solid dust grains making up the rest. Its density is staggeringly low — typically about one atom per cubic centimeter on average, where the air you breathe holds roughly 25 billion billion atoms in the same volume. But it is not uniform: it ranges from cold, dense molecular clouds at a few atoms per cubic centimeter compressed to a million, down to a searing hot, near-empty gas at a hundredth of an atom per cubic centimeter and a million degrees. In the Milky Way the ISM makes up only about 10 to 15 percent of the mass of the disk, the rest being stars.

The ISM matters because it is the womb and the graveyard of stars at once. Stars condense out of its densest clouds, and when they die they blow their enriched ashes back into it, seeding the next generation. It is also why distant starlight looks dimmer and redder than it should — the dust absorbs and scatters light along the way. A common misconception is that interstellar space is truly empty; in reality it is a turbulent, magnetized, chemically active medium, just spread unimaginably thin.