Most stars in the Milky Way belong to multi-star systems, with our solitary Sun being the exception rather than the rule.

Research from institutions like the Harvard-Smithsonian Center for Astrophysics shows that star formation typically begins in dense molecular clouds. As gravity causes these clouds to collapse, they often fragment into multiple cores, giving rise to binary or multiple star systems. This process is so prevalent that single stars like our Sun actually represent the minority within the Milky Way galaxy.The most common multi-star arrangement is the binary system, where two stars orbit a common barycenter. Data from the Gaia space telescope mission, launched by the European Space Agency in 2013, reveals that the fraction of binary systems varies by stellar type. Massive O-type stars are almost always found in multiples, while smaller M-dwarf stars are more likely to be solitary.Binary systems are essential for fundamental astrophysics because they enable direct application of Newton's version of Kepler's Third Law. By measuring the orbital period and distance between two stars, scientists can calculate their precise masses with high accuracy. This method is far more reliable than estimating mass from a star's brightness or spectral characteristics alone.The existence of circumbinary planets—worlds orbiting two stars simultaneously—was famously confirmed by NASA's Kepler Mission in 2011 with the discovery of Kepler-16b. These real-world "Tatooine" planets demonstrate that stable planetary orbits can exist in complex gravitational environments. Understanding these systems helps astronomers refine their models of how solar systems evolve across billions of years.