A giant sunspot -- a magnetically strong and complex region on the sun's surface -- is making a third trip across the face of the sun, NASA reported Friday.

Sunspots like the one being tracked by NASA scientists are part of active regions of the sun's surface that often produce large explosions such as solar flares and coronal mass ejections, the space agency said.

Each time an active region appears it is assigned a number, and active regions that have survived their trip around the back of the sun and reappear as a result of the sun's 27-day rotation are assigned a new number.

This numbering convention is a holdover from a time when there were no space telescopes capable of observing the far side of the sun, so it could not be certain a new sunspot was indeed the same as the old one, astronomers said.

This active region is currently labeled AR11990; on its previous appearance it was labeled AR11967 and on its first journey across the sun's face it was AR11944.

During its three trips across the sun's face so far, this sunspot region has produced two significant solar flares, labeled as the strongest kind of flare, an X-class, they said.

While most sunspots do not last more than a couple of weeks some have been known to be stable for many months at a time, they said.