Stars with Photometrically Young Gaia Luminosities Around the Solar
  System (SPYGLASS) I: Mapping Young Stellar Structures and their Star
  Formation Histories
  Young stellar associations hold a star formation record that can persist for
millions of years, revealing the progression of star formation long after the
dispersal of the natal cloud. To identify nearby young stellar populations that
trace this progression, we have designed a comprehensive framework for the
identification of young stars, and use it to identify $\sim$3$\times 10^4$
candidate young stars within a distance of 333 pc using Gaia DR2. Applying the
HDBSCAN clustering algorithm to this sample, we identify 27 top-level groups,
nearly half of which have little to no presence in previous literature. Ten of
these groups have visible substructure, including notable young associations
such as Orion, Perseus, Taurus, and Sco-Cen. We provide a complete
subclustering analysis on all groups with substructure, using age estimates to
reveal each region's star formation history. The patterns we reveal include an
apparent star formation origin for Sco-Cen along a semicircular arc, as well as
clear evidence for sequential star formation moving away from that arc with a
propagation speed of $\sim$4 km s$^{-1}$ ($\sim$4 pc Myr$^{-1}$). We also
identify earlier bursts of star formation in Perseus and Taurus that predate
current, kinematically identical active star-forming events, suggesting that
the mechanisms that collect gas can spark multiple generations of star
formation, punctuated by gas dispersal and cloud regrowth. The large spatial
scales and long temporal scales on which we observe star formation offer a
bridge between the processes within individual molecular clouds and the broad
forces guiding star formation at galactic scales.
