In A Nutshell
SGR 1806-20 is a neutron star and magnetar. It is different from other stars because it has a strong magnetic field and rotates slower. On December 27, 2004, SGR 1806-20 experienced a massive starquake and released a gamma-ray burst equivalent to 1036 kW in intensity. The event was so powerful that the radiation reached Earth, and it became the brightest explosion ever sighted from outside our solar system.
The Whole Bushel
In 1979, astronomers discovered a new neutron star and named it SGR 1806-20. The object was identified as a soft gamma repeater (SGR) and magnetar. It is located on the far side of our Milky Way galaxy in the constellation of Sagittarius. Generally speaking, magnetars are known to have extremely powerful magnetic fields. They give rise to strong bursts of X-rays, gamma rays, and superflares. To date, SGR 1806-20 is the most highly magnetized object ever observed, with a magnetic field that is one quadrillion (short scale) times stronger than Earth.
On December 27, 2004, SGR 1806-20 experienced a massive gamma-ray burst that reached Earth. It produced the largest explosion in history and the biggest witnessed event since the SN 1604 supernova observed by Johannes Kepler in 1604. The blast released more energy in one-tenth of a second than our sun has released in 100,000 years. It occurred 50,000 light-years away from Earth, but if SGR 1806-20 had been located within 10 light-years, it could have potentially triggered a mass extinction on Earth.
At a close range, it has been estimated that the blast would have demolished the ozone layer and produced an impact similar to a 12-kiloton (50 TJ) TNT nuclear blast at 7.5 kilometers (4.6 mi). Bryan M. Gaensler, who conducted an observation of the afterglow, noted that only the Sun and perhaps a handful of spectacular comets have even released more total energy on Earth than SGR 1806-20. The blast was 100 times more powerful than any previously observed SGR flare.
The event has baffled astronomers and forced them to confront the question of how such a tiny object, about 20 kilometers (12.5 mi) across, could unleash such a powerful burst. The explosion has been heavily investigated by NASA in hopes of identifying a similar threat in the future. Currently, the closest known magnetar to Earth is 1E 1048.1-5937, which is approximately 9,000 light-years away.