The giant explosions that rock the universe

We take a look at stellar quakes, the massive eruptions of energy that are released when a neutron star’s surface cracks open.


Star quake

Throughout the universe there are all sorts of explosions that send radiation crashing outwards at an alarming rate. One type of explosion that is largely unheard of, however, is a stellar quake. As the name suggests, these are quakes on the surface of a star that quite literally rip it apart, sending out a huge amount of energy in a short space of time. But what are they, and how do they come about?

Stellar quakes are caused by neutron stars, highly dense masses of protons and electrons that have been forced together to form a solid mass of neutrons. Neutron stars have up to five times the mass of the Sun but are only about 20 kilometres (12 miles) in diameter. They form as the result of a giant star going supernova and leaving behind its dense core, and the resultant angular momentum from becoming such a small object sees them spin at an average of 400 rotations per second. A day on these stars lasts just a split second, but their strong magnetic fields cause them to slow down over time.

As they rotate, the incredibly strong gravitational force of the star counteracts the spin of the star. The former attempts to draw in the equator, while the centrifugal forces resulting from the spin of the star try to push the equator out. This changes its shape from a slight oblong to a sphere, cracking the rigid iron crust. Mountains only a few centimetres tall begin to appear across the surface as the tension builds.

Star quake 2

Eventually, the tension in the surface reaches such a level that the crust ‘snaps’ and a huge number of gamma rays and x-rays are released as a stellar quake. As the geometry of the star readjusts, the strong magnetic fields temporarily drop to a lower energy level and the star momentarily lets down its guard. The energy released from inside the star is now free to escape, creating one of the largest known flashes of x-rays in the universe.

The biggest stellar quake ever detected, and the largest explosion ever observed by humans since a supernova in 1604, was a massive eruption 50,000 light years from Earth on 27 December 2004. Scientists said that if it had been 10 light years closer, we would have been facing a mass extinction.

Recent evidence suggests that the primary causes of the largest stellar quakes are magnetars, large neutron stars with an incredibly powerful magnetic field. At twice the size of a regular neutron star, a magnetar can have up to 30 times more mass than the Sun despite being over 40,000 times smaller. In fact, a tablespoon of mass from a magnetar would weigh almost the same as 300 Empire State Buildings. The magnetic field of a magnetar is several trillion times stronger than that of Earth, while its rigid crust is 10 billion times stronger than steel and 1.5km (1 mile) thick.

Images courtesy of NASA.

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