Observing previously unseen binary companions to very young protostars is helping in understanding how such multiple-star systems form, U.S. astronomers say. Astronomers have long known about half of all sun-like stars are members of double- or multiple-star systems, but there have been competing theories of how such systems are formed. Observations by radio telescopes in New Mexico have provided strong support for one of the competing explanations for how double-star systems form, the National Radio Astronomy Observatory reported Friday. "The only way to resolve the debate is to observe very young stellar systems and catch them in the act of formation," NRAO astronomer John Tobin said. "That's what we've done with the stars we observed, and we got valuable new clues from them." Those clues support the idea double-star systems form when a disk of gas and dust whirling around one young star fragments, forming another new star in orbit with the first, he said. Tobin and an international team of astronomers studied gas-enshrouded young stars around 1,000 light-years from Earth, finding two previously unseen companions in the plane where their disks would be expected. One of the systems also clearly had a disk surrounding both young stars. "This fits the theoretical model of companions forming from fragmentation in the disk," Tobin said. "This configuration would not be required by alternative explanations." Previous studies have shown disks of dust and gas are commonly present early in the star formation process. "Our new findings, combined with the earlier data, make disk fragmentation the strongest explanation for how close multiple star systems are formed," Leslie Looney of NRAO and the University of Illinois said.