This animation shows a neutron star -- the core of a star that exploded in a massive supernova. This particular neutron star is known as a pulsar because it sends out rotating beams of X-rays that sweep past Earth like lighthouse beacons. X-ray telescopes like NASA's Nuclear Spectroscopic Telescope Array, or NuSTAR, pick up these beams, registering them as pulses of X-ray light.
What causes a pulsar to pulse? In the case of "accreting pulsars," the process is set in motion when matter from a companion star falls onto the pulsar. The gravity of the pulsar pulls this material from a surrounding disk, as shown in the animation. The strong magnetic fields surrounding the pulsar funnel the infalling material onto two spots above and below the stellar core. This causes the material to heat up to extreme temperatures and release X-rays. As the star rotates, the two X-ray hot spots behave like a lamp in a lighthouse, sweeping around. Only when the "lamps" are facing Earth will NuSTAR pick up the signal -- a pulsing of X-rays.