Our Sun is a bit of an outlier in the general stellar population. We typically think of stars as being solitary wanderers throughout the galaxy. But roughly half of Sun-like stars are locked in with more than one companion star. If there are two, it’s known as a “binary” system, but in many cases there are even more stars all collectively tied together by gravity. Astronomers have long debated why this happens, and a new paper, available in pre-print on arXiv from Ryan Sponzilli, a graduate student at the University of Illinois, makes an argument for a mechanism known as disk fragmentation. Disk fragmentation is one of two competing theories for why close-companion formation happens. In this scenario, a single, massive disk of gas and dust surrounding a newborn star becomes unstable and breaks apart. Eventually, it coalesces into another star right next door. Critically for the study, since these stars were formed from the same spinning disk, their rotational axes should be aligned.…