sentences:  These fluctuations are hypothesized to be caused by flares, although the activity is much stronger relative to the size of the star.The flares on this class of stars can extend up to 20% of the circumference, and radiate much of their energy in the blue and ultraviolet portion of the spectrum.Straddling the boundary between stars that undergo nuclear fusion in their cores and non-hydrogen fusing brown dwarfs are the ultracool dwarfs.