giant molecular cloud

Stars are not scattered through empty space at random; they are born in cold, dark, lumpy clouds of gas, and the biggest of these are giant molecular clouds. Imagine a vast bank of frigid fog drifting between the stars, so cold and dense that ordinary hydrogen atoms have paired up into hydrogen molecules. These clouds are the wombs of stars: nearly every star you can see was assembled out of one.

A giant molecular cloud is a region of the interstellar medium, typically tens to a couple of hundred light-years across, holding anywhere from ten thousand to a few million times the mass of the Sun in gas and dust. It is bitterly cold, only about 10 to 20 degrees above absolute zero (roughly minus 260 degrees Celsius), and that cold lets hydrogen exist as the molecule H2 rather than as separate atoms. The clouds are mostly hydrogen, with about a quarter helium and a sprinkle of heavier atoms; tiny solid dust grains make them dark, blotting out the stars behind. They are not smooth: turbulence stirs them into filaments, sheets, and knots, and the densest knots are where stars actually form.

Giant molecular clouds matter because they set the pace of star formation across a galaxy. Although they hold most of a galaxy's star-making fuel, they turn only a small percentage of it into stars before the newborn stars themselves blow the rest away, which is why galaxies make stars slowly rather than all at once. A subtle point: H2, the dominant molecule, is almost invisible at the clouds' frigid temperatures, so astronomers usually trace these clouds with a minor companion molecule, carbon monoxide (CO), which glows brightly in radio waves and reveals where the hidden hydrogen lies.