Common-envelope ejection

A star can expand until its companion is orbiting inside the same huge cloud of gas. The two stellar cores then move through that shared envelope. Will they push the gas away and survive as a close pair, or keep spiraling toward a merger?

The open problem is to predict which outcome occurs and, if the cores survive, how far apart they finish. Energy released during the encounter can help remove the gas, but energy that escapes as radiation may not do that work.[1]

When one star's expanding envelope engulfs its companion, the cores orbit inside shared gas. The task is to predict whether the envelope becomes unbound—able to escape—and the final separation of any surviving pair, from the binary's initial conditions.

As the orbit contracts, it can transfer energy to the gas. Recombination, when ions regain electrons, can release additional energy. Whether that energy helps eject the envelope depends on where it is released and how effectively it remains in the gas instead of escaping as radiation. A useful model must follow energy transport as well as the total budget.[1]

Models succeed in some cases, but how heat escapes and helps eject the gas remains uncertain.[1]

Recombination helps in particular simulations, but its thermalization versus photon loss remains important. The accepted 2026 three-binary reconstruction challenges simple efficiency models under its evolutionary assumptions, not energy conservation itself.[1] [2]

This episode can decide whether two stars merge or survive close enough to interact again later. The surviving separation helps determine what kind of binary system they can become.

That makes the problem consequential beyond a single dramatic encounter: predictions of populations of close binaries depend on how often the gas is successfully expelled and where the surviving cores end up.[1] [2]

Common-envelope evolution connects the earlier lives of two stars to the survival and separation of a close binary. Uncertain ejection and final-separation predictions therefore affect estimates of how many close systems with compact remnants can form.

Reconstructing observed binaries provides one way to test the proposed histories. Those constraints depend on assumptions about the stars' other evolutionary stages, so a tension with a simple ejection-efficiency model can identify missing physics or inadequate bookkeeping without implying a failure of energy conservation.[1] [2]