In a lake, the top layer of water is warm and light, while deeper down it’s cold and dense. A swimmer moving through such water creates unusual waves. A similar scene unfolds inside bloated, giant stars — red giants. Only instead of water, it’s scorching hot gas (plasma — a state where electrons are stripped from atoms), and the swimmer is a superdense object: a neutron star or a massive planet.

Back in the 1930s, [scientist:Fritz Zwicky] realized that a similar kind of gravitational dynamical friction (drag created by an object’s own wake in gas or stars) governs the motion of entire galaxies. Later, [scientist:Subrahmanyan Chandrasekhar] derived a formula for the deceleration of a star in a uniform medium. Then [scientist:Bohdan Paczyński] applied the idea to common envelopes — when one companion is already swallowed by the giant’s atmosphere. But all these calculations assumed the gas was the same everywhere. In reality, a stellar envelope is more like a layered pastry: density and temperature vary from layer to layer. The new work is the first to account for this stratification (layered density variation). It turns out that sound waves from the companion form a complex wake that depends on the entire envelope, not just the orbit’s immediate surroundings.

For planets falling into a star, this means that planetary migration (movement through gas) proceeds faster. In close binary systems, the merger also speeds up. To calculate everything precisely, we need three-dimensional hydrodynamic simulations — computer models of gas motion. Then it will become clear how the waves exchange angular momentum with the orbit. And the mystery of the layered ocean inside stars will be solved.