02/02/2016: Giant Cloud Came from the Milky Way

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The Milky Way enjoys a light drizzle throughout its galactic year. These cosmic raindrops are speedy clouds of mostly hydrogen gas that rain down onto our galaxy’s spiral disk. They fly through space at hundreds of kilometers per second (millions of miles per hour) and don’t rotate with the Milky Way. They’re appropriately named high-velocity clouds.

This shower of gas is important for fueling star formation. The Milky Way assembles stars at a respectable rate of about one solar mass per year. But it takes gas to keep that starbirth going. Much as the manna from heaven fed the Israelites, these clouds feed star formation in the Milky Way by replenishing the galaxy’s gas at the same rate as the galaxy burns through it.

Despite their importance, we know a paltry amount about these star-fueling clouds. What they’re made of, where they come from, or even how far away they are — astronomers generally scratch their heads about all of these.