Decoding the Morphological Evolution of Open Clusters
  Base on Gaia Second Data Release and the combination of nonparametric
bivariate density estimation with the least square ellipse fitting, we derive
the shape parameters of the sample clusters. By analyzing the dislocation of
the sample clusters, the dislocation $d$ is related to the X-axis pointing
toward the Galactic center, Y-axis pointing in the direction of Galactic
rotation, and the Z-axis (log(|H|/pc)) that is positive toward the Galactic
north pole. This finding underlines the important role of the dislocation of
clusters in tracking the external environment of the Milky Way. The orientation
($q_{pm}$) of the clusters with $e_{pm}$ $\geq$ 0.4 presents an aggregate
distribution in the range of -45$\degr$ to 45$\degr$, about 74\% of them. This
probably suggests that these clusters tend to deform heavily in the direction
of the Galactic plane. NGC 752 is in a slight stage of expansion in the
two-dimensional space and will deform itself morphology along the direction
perpendicular to the original stretching direction in the future if no other
events occur. The relative degree of deformation of the sample clusters in the
short-axis direction decreases as their ages increase. On average, the severely
distorted sample clusters in each group account for about 26\% $\pm$ 9\%. This
possibly implies a uniform external environment in the range of $|$H$|$ $\leq$
300 pc if the sample completeness of each group is not taken into account.
