U Geminorum is the class prototype for the dwarf nova subclass of cataclysmic variables. Discovered in 1855 by British astronomer J.R. Hind, U Gem has intrigued generations of astronomers, and remains a fascinating (and scientifically useful) target to this day. U Gem, like all of the dwarf novae, are binary stars composed of a white dwarf primary star and a main sequence secondary star locked in a close orbit. In fact, they orbit so closely that matter spills off of the secondary star and onto the white dwarf, forming a disk of material around it. Dwarf nova outbursts occur when this disk of material suddenly heats up becoming as much as 100 times brighter in the process. The disk remains bright -- in "outburst" -- for days or a few weeks, and then fades suddenly away.
This particular segment of data shows two kinds of outbursts -- a rather rare long outburst where it remained bright for more than a month, and two short outbursts lasting only a few days each. U Gem has irregularly spaced outbursts, but they typically occur at least once every 100 days or so.