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Just What Are Starquakes?

Just What Are Starquakes?

On Boxing Day 2004, the Earth experienced one of its largest ever earthquakes. The infamous Indian Ocean earthquake and its associated tsunami was a hugely energetic seismological event, releasing the the same amount of energy as a 100 gigaton bomb and causing the entire planet to vibrate by a centimeter. You might thinkthat quakes like these are some of the most powerful around, but you’d be wrong. Very wrong.

On that same day in 2004, the largest quake ever detected on Earth was witnessed. However, it didn’t occur on Earth – or any other planet for that matter. It occurred on a star.

This is where we met SGR 1806-20. A neutron star located 50,000 light years from Earth. A neutron star is what remains after a particularly massive star has used up its nuclear fuel and exploded – in a process known as a supernova. These stars are composed only of neutrons and are among the densest things in the universe, packing the mass off our Sun many times over into a space no larger than a city. Specifically, SGR 1806-20 has a mass 12 times larger than our Sun, but its radius is just over 1 km. Indeed, a thimble full of the star would weigh well over 100 million tons.

SGR 1806-20 falls into a category known as magnetars, which release bursts of gamma rays and x rays as their immensely strong magnetic fields degrade. This effect is key when you look at these immense quakes of the kind that rumbled through space on Boxing Day 2004 – creatively known as starquakes.

As magnetars radiate out the energy contained within their magnetic fields, they begin to slow down. This makes them form a more spherical shape and disrupts the neutron ‘crust’ that has developed on the surface of the star. This sudden adjustment of the crust, which can be as small as 1 micrometer, is analogous to the adjustment of Earth’s crust that occurs during an Earthquake.

An artist’s impression of the huge burst of gamma rays produced by a starquake. Credit: NASA.
An artist’s impression of the huge burst of gamma rays produced by a starquake.
Credit: NASA.

The amount of energy released by the rupturing of SGR-1806-20 was phenominal – the star produced a flash of gamma rays equivalent to what the Sun produced in 150,000 years, in just one tenth of a second. Even though the star is 50,000 light years away – that’s 300,000 trillion miles – it still had a minor effect on our planet. The released gamma rays struck the ionosphere – one of the upper regions of our atmosphere – and temporarily expanded it. This effect was simply a transient brush on the shoulder from the star – if a blast like this occurred within 10 light years of Earth, it would undoubtedly lead to a mass extinction event, stripping away our atmosphere and destroying life as we know it. It has been theorised that something like this has happened before in Earth’s early history.

This event wasn’t entirely doom and gloom though. It allowed researchers to determine just how magnetars are formed and what they are made up of. Starquakes such as this produce x ray oscillations, analogous to the oscillations produced by earthquakes, measured by the Richter scale. The frequency of the oscillations can tell scientists exactly what is contained within the neutron crust of such stars.

‘Til next time…

Joe

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