Spiral galaxy NGC 2841.

For decades astronomers have wanted to know the secret to how galaxies get so smooth. After all, the stars that make up these cosmic neighborhoods start out as dense, cloudy clumps, yet mature galaxies are sleek disks of evenly distributed material.

NASA's Spitzer Space Telescope found the answer in "rivers" of young stars, as in this new Spitzer image of spiral galaxy NGC 2841. These stellar streams are created as a budding galaxy rotates, generating a shearing motion, which pulls the stars out of their dusty cocoons and spreads the stars across the galactic disk.