A wet sponge sits in a bucket of water. Something massive drifts by, and the sponge stretches slightly toward it because the water pressure is uneven. A neutron star behaves similarly when another star passes nearby: tidal forces stretch it, and it subtly changes shape.
The interior of a neutron star is the densest matter in the universe, aside from black holes. A teaspoon of this stuff would weigh as much as a mountain. When the star deforms, its interior responds in complex ways. Physicists describe this with the Love number, which indicates how easily the star can be 'squashed' by tidal forces.
The authors of the new study tackled two subtleties in the calculations. First, they found an error in the standard formulas for the very center of the star, where density is highest. The error lurked in a lower-order term of the expansion. Fortunately, it did not affect the final Love number. You could compare it to a shoe laced incorrectly: the lacing is off, but it still fits comfortably. Second, the scientists extended the calculations to cases where the space around the star is not empty but has an event horizon, like our universe with its dark energy.
The practical benefit of these refinements lies in analyzing gravitational waves. When two neutron stars spiral together and merge, their tidal deformations leave a trace in the signal detected by LIGO and Virgo. The more precisely we know the Love number, the better we understand what these stars are made of. The corrected formula for the star's center now reliably fixes the initial conditions. And the extension to the case with a horizon could prove useful in cosmological models where dark energy pushes space apart.
Now, when astrophysicists model the dance of two neutron stars, they can be confident that the 'core' of the calculations contains no hidden inaccuracies. And the sponge in the bucket remains a great analogy. Only now we know that even a tiny error at its center does not change how much it stretches.
A neutron star spins so fast that its equator can make up to 700 rotations per second — faster than the blades of a kitchen blender.
In Robert Forward's novel 'Dragon's Egg,' life is depicted on the surface of a neutron star, where tidal forces are so monstrous that they would tear a human apart into atoms before they even got close.