The cosmos is littered with clouds of star-forming gas, but few are as well studied as the Smith Cloud, set to crash into our galaxy in 30 million years. God-fearing humans might ask: Where did this unholy dust ball come from, and why is it heading straight for us? Now, science has the answer.
It's a giant galactic fart, expelled from the Milky Way 70 million years ago and back to bite us in the arse. First discovered in the 1960s, Smith Cloud is a starless ball of dust roughly 30 times the angular diameter of our Moon as seen from Earth — making at approximately 11,000 light years long and 2500 light years across. It's speeding toward us at a rip-roaring 1.13 million kilometres per hour, meaning it will crash into the disk of the Milky Way in the not-too-distant future. For decades, astronomers suspected that Smith Cloud hailed from a faraway locale beyond the edge of our galaxy.
Illustration of how astronomers decoded Smith Cloud's composition, by measuring ultraviolet light as it passes through the cloud against the background of bright, distant galaxies. Via NASA, ESA, A. Fox and A. Feild (STScI)
But new measurements taken with the Hubble Space Telescope tell a different story. By observing Smith Cloud in the ultraviolet spectrum, University of Notre Dame astrophysicist Nicolas Lehner and his colleagues were able to decode its composition for the first time. If the gas ball came from the void of intergalactic space, it'd be composed of hydrogen and helium. Instead, Smith Cloud contains a number of heavier elements, indicating that it was cut from a star-forming region of space.
In fact, it looks exactly like the outer disk of our galaxy. The most logical conclusion? The Milky Way ripped a brown cloud with enough fuel to ignite two million suns.
And that's exactly what's going to happen. If you're alive to witness Smith Cloud's homecoming, it will smack into our galaxy and produce a spectacular burst of star formation. Whoever said flatulence can't be beautiful clearly wasn't thinking big enough.
Top image: Illustration of Smith cloud, with the size of the Moon added for scale, via B. Saxton and F. Lockman (NRAO/AUI/NSF), and A. Mellinger