Like other neutron stars, magnetars are around 20 kilometres (12 mi) in diameter and have a mass 2–3 times that of the Sun. The density of the interior of a magnetar is such that a thimble full of its substance would have a mass of over 100 million tons.
The magnetic field of a magnetar would be lethal even at a distance of 1000 km due to the strong magnetic field distorting the electron clouds of the subject’s constituent atoms, rendering the chemistry of life impossible. [The “How the Universe Works” episode about supernovas that I watched today on TV said that “it would suck the iron out of your blood.”] At a distance of halfway from earth to the moon, a magnetar could strip information from the magnetic stripes of all credit cards on Earth. As of 2010, they are the most powerful magnetic objects detected throughout the universe.
It is estimated that about one in ten supernova explosions results in a magnetar rather than a more standard neutron star or pulsar.