  Our uncertainties about binary star systems (and triples and so on) limit our
capabilities in literally every single one of the Thematic Areas identified for
Astro2020. We need to understand the population statistics of stellar
multiplicity and their variations with stellar type, chemistry, and dynamical
environment: Correct interpretation of any exoplanet experiment depends on
proper treatment of resolved and unresolved binaries; stellar multiplicity is a
direct outcome of star and companion formation; the most precise constraints on
stellar structure come from well-characterized binary systems; stellar
populations heavily rely on stellar and binary evolution modeling;
high-redshift galaxy radiation and reionization is controlled by
binary-dependent stellar physics; compact objects are the outcomes of binary
evolution; the interpretation of multi-messenger astronomy from gravitational
waves, light, and neutrinos relies on understanding the products of binary star
evolution; near-Universe constraints on the Hubble constant with Type Ia
supernovae and gravitational-wave mergers are subject to systematics related to
their binary star progenitors; local measures of dark-matter substructure
masses are distorted by binary populations. In order to realize the scientific
goals in each of these themes over the next decade, we therefore need to
understand how binary stars and stellar multiplets are formed and distributed
in the space of masses, composition, age, and orbital properties, and how the
distribution evolves with time. This white paper emphasizes the
interdisciplinary importance of binary-star science and advocates that
coordinated investment from all astrophysical communities will benefit almost
all branches of astrophysics.
