A halo may be difficult to acquire in terms of virtue, but it can also be tough to calculate in terms of physics. Physicists have managed to do just that, however. A halo nucleus differs from the more traditional nuclei because it has one or more nucleons (protons or neutrons) that are only weakly bound to the nuclear core. Consequently, they drift far away from it, forming, in effect, a halo. These nuclei are difficult to study because their lives are both short (often lasting only milliseconds) and fragile.