The Cygnus Loop's Distance, Properties, and Environment Driven
  Morphology
  The Cygnus Loop is among the brightest and best studied evolved Galactic
supernova remnants. However, its distance has remained uncertain thus
undermining quantitative understanding about many of its fundamental
properties. Here we present moderate-dispersion spectra of stars with projected
locations toward the remnant. Spectra of three stars revealed Na I 5890,5896 A
and Ca II 3934 A absorption features associated with the remnant's expanding
shell, with velocities ranging from -160 to +240 km/s. Combining Gaia DR2
parallax measurements for these stars with other recent observations, we find
the distance to the Cygnus Loop's centre is 735 +/-25 pc, only a bit less than
the 770 pc value proposed by Minkowski some 60 years ago. Using this new
distance, we discuss the remnant's physical properties including size, SN
explosion energy, and shock velocities. We also present multi-wavelength
emission maps which reveal that, instead of being located in a progenitor
wind-driven cavity as has long been assumed, the Cygnus Loop lies in an
extended, low density region. Rather than wind-driven cavity walls, these
images reveal in unprecedented clarity the sizes and locations of local
interstellar clouds with which the remnant is interacting, giving rise to its
large-scale morphology.
