When astronomers add up all the gas and dust contained in ordinary galaxies like our own Milky Way, they stumble on a puzzle: There is not nearly enough matter for stars to be born at the rates that are observed. Part of the solution might be a recycling of matter on gigantic scales – veritable galactic fountains of matter flowing out and then back into galaxies over multi-billion-year timescales.
Now, a team of astronomers led by Kate Rubin of the Max Planck Institute for Astronomy in Germany has used the W. M. Keck Observatory to find evidence of just such fountains in distant spiral galaxies.
In the Milky Way, it’s estimated that every year about one solar mass (an amount of matter equal to that of our Sun) worth of dust and gas is turned into stars. Yet a survey of the available raw materials shows that our galaxy could not keep up this rate of star formation for longer than a couple of billion years. Star ages and comparisons with other spiral galaxies show that one solar mass per year is a typical star formation rate. So the puzzle appears to be universal.