A magnetar is a type of neutron star with an extremely powerful magnetic field. The magnetic field decay powers the emission of high-energy electromagnetic radiation, particularly X-rays and gamma rays.
Like other neutron stars, magnetars are around 20 kilometers (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. Magnetars are differentiated from other neutron stars by having even stronger magnetic fields, and rotating comparatively slowly, with most magnetars completing a rotation once every one to ten seconds, compared to less than one second for a typical neutron star. This magnetic field gives rise to very strong and characteristic bursts of X-rays and gamma rays.
Magnetars are characterized by their extremely powerful magnetic fields of 100 million to 100 billion tesla. These magnetic fields are hundreds of millions of times stronger than any man-made magnet, and quadrillions of times more powerful than the field surrounding Earth. Earth has a geomagnetic field of 30–60 microteslas. 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.
In a field of about 100000 teslas atomic orbitals deform into rod shapes. At 10 billion teslas, a hydrogen atom becomes a spindle 200 times narrower than its normal diameter.
When in a supernova, a star collapses to a neutron star, its magnetic field increasing dramatically in strength. Halving a linear dimension increases the magnetic field fourfold. When the spin, temperature and magnetic field of a newly formed neutron star falls into the right ranges, a dynamo mechanism could act, converting heat and rotational energy into magnetic energy and increasing the magnetic field, normally an already enormous 100 million teslas, to more than 100 billion tesla. The result is a magnetar. It is estimated that about one in ten supernova explosions results in a magnetar rather than a more standard neutron star or pulsar.