What is a magnetar?

A magnetar is an extreme kind of neutron star, the ultra-dense collapsed core left behind by a massive star that exploded as a supernova. What sets magnetars apart is their magnetic field, which can be a thousand trillion times stronger than Earth field and by far the most intense known anywhere in the cosmos. This immense magnetism stresses the star solid crust, and when it suddenly shifts or cracks, the magnetar can unleash colossal bursts of X-rays and gamma rays. Magnetars are rare and short-lived in cosmic terms, with their strong fields decaying over thousands of years. A single giant flare from a magnetar can briefly outshine the entire galaxy in gamma rays.