  We summarize some of the compelling new scientific opportunities for
understanding stars and stellar systems that can be enabled by sub-mas angular
resolution, UV/Optical spectral imaging observations, which can reveal the
details of the many dynamic processes (e.g., variable magnetic fields,
accretion, convection, shocks, pulsations, winds, and jets) that affect their
formation, structure, and evolution. These observations can only be provided by
long-baseline interferometers or sparse aperture telescopes in space, since the
aperture diameters required are in excess of 500 m - a regime in which
monolithic or segmented designs are not and will not be feasible - and since
they require observations at wavelengths (UV) not accessible from the ground.
Two mission concepts which could provide these invaluable observations are
NASA's Stellar Imager (SI; http://hires.gsfc.nasa.gov/si/) interferometer and
ESA's Luciola sparse aperture hypertelescope, which each could resolve hundreds
of stars and stellar systems. These observatories will also open an immense new
discovery space for astrophysical research in general and, in particular, for
Active Galactic Nuclei (Kraemer et al. Decadal Survey Science Whitepaper). The
technology developments needed for these missions are challenging, but
eminently feasible (Carpenter et al. Decadal Survey Technology Whitepaper) with
a reasonable investment over the next decade to enable flight in the 2025+
timeframe. That investment would enable tremendous gains in our understanding
of the individual stars and stellar systems that are the building blocks of our
Universe and which serve as the hosts for life throughout the Cosmos.

Solar and Stellar Astrophysics