Inside a neutron star, matter is squeezed so tightly that a single teaspoon weighs as much as a mountain. Under such pressure, ordinary particles, protons and neutrons, can transform into exotic particles known as hyperons. These particles resemble neutrons but contain strange quarks.

For a long time, it was believed that the appearance of hyperons makes a star softer, preventing it from being heavier than one and a half Suns. But astronomers observe neutron stars with masses of two Suns. A contradiction! New calculations show that the interaction between hyperons can be not only attractive but also repulsive. At short distances they attract, and at long distances they push apart, like a spring.

The scientists used data on nuclei with hyperons and observations of neutron stars to refine the interaction forces. It turned out that repulsion between hyperons makes the star stiffer. The maximum mass grows by up to 22 percent, as if a person could lift a fifth more just by changing posture.

This is a step toward a complete description of matter in the densest corners of the universe — from atomic nuclei to neutron stars.