  The Sun's atmosphere is frequently disrupted by coronal mass ejections
(CMEs), coupled with flares and energetic particles. In the standard picture,
the coupling is explained by magnetic reconnection at a vertical current sheet
connecting the flare loops and the CME, with the latter embedding a helical
magnetic structure known as flux rope. As it jumps upward due to instabilities
or loss of equilibrium, the flux rope stretches the overlying coronal loops so
that oppositely directed field is brought together underneath, creating the
current sheet. However, both the origin of flux ropes and their nascent paths
toward eruption remain elusive. Here we present an observation of how a
stellar-sized CME bubble evolves continuously from plasmoids, mini flux ropes
that are barely resolved, within half an hour. The eruption initiates when
plasmoids springing from a vertical current sheet merge into a leading plasmoid
occupying the upper tip of the current sheet. Rising at increasing speed to
stretch the overlying loops, this leading plasmoid then expands impulsively
into the CME bubble, in tandem with hard X-ray bursts. This observation
illuminates for the first time a complete CME evolutionary path that has the
capacity to accommodate a wide variety of plasma phenomena by bridging the gap
between micro-scale dynamics and macro-scale activities.
