A magnestar is thought to be a very small, very dense star with an enormous magnetic field. But this is all a theory so far...there are none close to us. They are thought to be only about 12 miles in diameter, but more massive than our Sun. They have magnetic field that are hundreds of thousands of times stronger than any man-made magnets. They result when a star explodes in its supernova stage and then collapses into a neutron star

Paulett

Magnestars are indeed very exotic objects! Scientific America has published a great article about magnetars if you want to read more:

http://solomon.as.utexas.edu/~duncan/sciam.pdf