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.
The relationship between starlight and gas density in galaxies is so stable that it is called the "Kennicutt-Schmidt law"—the main rule of the stellar kitchen in the universe.
In Isaac Asimov's novel "The Gods Themselves," inhabitants of a parallel universe desperately seek a way to pump matter from our world — much like astronomers search for hidden gas reservoirs feeding stars.