A handful of modeling clay with one ball stuck far off to the side becomes loose and stretched out. That's roughly how the beryllium-11 nucleus is built: a single neutron dangles at the periphery, forming a "halo" around the dense core.

Physicists used a new method to bypass mathematical hurdles and precisely calculate the shape of this nucleus. They found that both beryllium-10 and beryllium-11 consist of two clusters of particles, but in beryllium-11 the lone neutron takes a special position, as if perched on a seesaw between the two groups. This makes the nucleus stretch more along one axis, and its outer edge becomes fuzzy, like mist.

Understanding these shapes matters for astrophysics: similar structures may influence how elements are born in stars.