Heggie-Hills law

The Heggie-Hills law (i.e., Heggie's law or Heggie's theorem) in stellar dynamics is: hard binary stars tend to harden, and soft binaries soften. Given some distinction between small (termed hard) and large (termed soft) orbits, binaries' encounters with third objects that affect its dynamics tend, on average, to make such small orbits smaller and such large orbits larger.