Galaxies have different angular momentum. Some rotate faster, some slower. Some rotate in the direction north, some in south, some in any other given direction. When they pass by each other, they precess like gyroscopes and exchange by their angular momentums.
Many have black hole(s) in their center.
The smaller the star the faster it rotates (conservation of angular momentum L=Iw=mr^2w = const as a consequence of anisotropy of space).
Hydrogen burns in helium slow (billions of years). Helium in Be-C - faster (millions of years). C in Si - much faster (thousands and less). Si in big stars may burn into Fe in a few seconds only which is called supernova II explosion. Compact core left behind may be a white dwarf, or (if massive enough to collapse futher) can become a neutron star or black hole and due to small size (few km only) rotates "like crazy" (tens and hundreds rev/sec).
Similar effect but in a binary where one (regular)star pours matter on close compact white dwarf companion until it suddenly burns all accumulated and greatly compressed matter in a less than 1 second is called supernova I explosion (imagine amount of energy about all Sun's energy but released not in 10 billion years but rather in 1 second or less).
Pulsars have strong magnetic field due to extremely high angular velocity (B up to 10^15 gauss - see "magnetars"). Thus a falling hydrogen(which becomes plasma accelerated to high velocities by strong gravity) can not move across magnetic field, thus can only fall on star's surface (and crash in it producing radiation) only in narrow spots near star magnetic poles (where magnetic lines ener the star), and what we see is a radiation from these "hor spots" near magnetic poles. Because magnetic poles do not neseccarily coinside with geografic poles (=axis of rotation), then we see "hot spots" at different angles as the star rotates, thus radiation intensity appears for us modulated with the period equal to the rotational period of the magnetar - tens and hundreds hertz.