When galaxies merge, it is expected that their central supermassive black holes merge too.[21] These are potentially the loudest gravitational-wave signals. The most massive binaries are a source for PTAs.[22] Less massive binaries (about a million solar masses) are a source for space-borne detectors like LISA.[23] Extreme-mass-ratio systems of a stellar-mass compact object orbiting a supermassive black hole. These are sources for detectors like LISA.[23] Systems with highly eccentric orbits produce a burst of gravitational radiation as they pass through the point of closest approach;[24] systems with near-circular orbits, which are expected towards the end of the inspiral, emit continuously within LISA's frequency band.[25] Extreme-mass-ratio inspirals can be observed over many orbits.