The spiral galaxy NGC 4631 shines so brightly because it has much higher rate of star formation than most galaxies. The many dark knots are patches of dust; in most spiral galaxies dust is arranged into lanes, but not so here. Note that there is no visible nucleus here. The galaxy appears flat because we are seeing it edge-on.
The bright spot immediately above NGC 4631 is a dwarf elliptical galaxy (NGC 4627). The two galaxies are physical companions, gravitationally linked.
