Starquakes

In 1979, some Russian satellites were heading off to Venus and beyond when their sensors 

became overwhelmed and shut down by a unknown burst of energy, more than 200,000 times stronger than the usual reading in open space.  Within a few seconds, this energy burst reached Earth and disrupted many other satellites.  Given that the energy was detected from several different locations, it was possible to determine that the source was a then unknown type of object in the distant Large Magellanic cloud over 165,000 light years away.  Since this event, it has been found that the source of the energy was most likely a rarely occurring and newly discovered celestial object known as a magnetar.

The Crab Nebula has a neutron star at its centre after a supernova

Magnetars are a type of neutron star formed occasionally as a result more regular stars 10-50 times larger than our own sun reaching the end of their life and going supernova. Much of the stars remnants falls back under gravity to form a neutron star (or even a black hole) which are roughly 10 kilometers in diameter and a spoonful of its material could weigh 5 billion tonnes on Earth.  Neutron stars usually spin extremely quickly, multiple times each second, and of which there are several other varieties with other unusual effects.  The magnetar variety has the difference where the spin is much slower by comparison, maybe once every 2-10 seconds.  This results in the creation of a magnetic force that is unmatched by anything else in the known universe.

Artists Impression of a Magnetars appearance

This force is so strong that by example, standing within 1000km of its centre would strip the electrons from the atoms in your body and render life impossible.
To maybe make a different illustration, the magnetic field is 100 billion times stronger than the magnet you may be familiar with from school.  Every now and then, as a result of the spinning forces and energy released, the surface of the star has to adjust itself to remain round.
This adjustment results in vast amounts of energy being kicked into the cosmos in under a millionth of a second, a starquake.  The energy, in the force of x-rays and gamma rays travels across the cosmos for eternity, until some of it hits a Russian satellite and knocks its sensors offline.  In 2004, the most powerful starquake yet was detected some 50,000 light years away, on the other side of the galaxy, and released enough energy that if it had occurred just 10 light years from Earth, it would have burnt through the atmosphere and triggered a mass extinction.

There are currently 21 magnetars that have been observed and measured in the universe and the nearest being at a distance of 9,000 light years away. So it's fortunate that our corner of the galaxy just happens to be relatively safe!