A pulsar is a rapidly spinning neutron star which is the small, incredibly dense remnant of much more massive star. How dense? A teaspoon of matter from a neutron star weighs as much as Mount Everest. A neutron star containing more matter than our sun would be only about 15 miles across.
Video: What is a pulsar?
Hear the word “pulsar” and wonder what it is? Take 95 seconds to watch this video from NASA Astrophysics.