A galaxy is not a static picture but a bubbling cauldron where new stars are constantly born from tenuous gas. To understand how long this boiling will last, astronomers weigh the key ingredient — cold molecular hydrogen. It is invisible itself, but it is betrayed by carbon monoxide (CO), which shines in the radio range.

A team of scientists pointed the James Clerk Maxwell telescope at 42 small galaxies — similar to our neighbor in the constellation Triangulum. In half of them, they managed to catch a signal from carbon monoxide. A curious detail emerged: the thermal glow of tiny dust grains at a wavelength of 12 micrometers rises and falls in unison with the amount of molecular gas. It is like gauging the height of a flame by the heat from the stove without looking inside.

The most interesting part is how the fuel depot itself changes. As a galaxy gains mass, its gas cloud, initially light and fluffy atomic gas, increasingly compresses into dense molecular clumps — future stellar nurseries. In dwarf systems, almost all fuel sloshes around as diffuse atoms, while in more massive ones it is already packed into tight, cold clouds ready for action.