Weird Wednesday – Magnetars

The universe is full of amazing objects that have some impressive characteristics. I’ve always been fascinated by how stars work and the scale of what is happening. It’s amazing to feel the Sun’s warmth on your face knowing that some of those photons, that are contributing to that warm feeling, have spent tens of thousands of years making their way to the Sun’s surface before the blistering fast 8 minute trip direct to Earth. It’s also amazing knowing, that as the Sun ages, it will slowly consume its supply of hydrogen in the core before fusing helium and growing dramatically bigger as it becomes a red giant and starts engulfing the inner planets, including, possibly, the Earth (fortunately that’s a few billion years off). If our Star was a bit bigger it wouldn’t just fizzle down to a white dwarf slowly cooling to an ember in billions of years time, it would explode in a bright cataclysmic burst of energy that creates many heavy elements and spreads them, and the remains of the star, all over this part of the galaxy – possibly recycling those elements into new stars and even planets (and maybe the building blocks of life). That’s not the end of the story though, as what remains following the supernova explosion is an amazing and somewhat frightening remnant of the original star.

This remnant is called a neutron star and they are normally about 20km in diameter with a mass of 2-3 times that of the Sun. The normal neutron star has an enormous magnetic field of about 10^8 teslas, the Earth’s magnetic field is 30-60 micro teslas by comparison. This massive magnetic field makes neutron stars particularly lethal to be near – not to mention the huge amount of gravity they have. They also spin incredibly fast at about one revolution every 2-10 seconds. The amazing thing is, that in about 1 in 10 supernovas, the remaining neutron star has some properties that cause it to have a more massive magnetic field and spin even faster at one revolution in less than a second. These objects are called magnetars and, as they decay and the internals of the star settle down, they emit very powerful x-rays and gamma rays until finally wearing out and becoming like a regular neutron star.

If it was possible to get within 1000km of a magnetar, the magnetic field is so strong that it would deform the electron clouds of a person’s atoms. Even at a distance of 150,000 km it would ruin your cassette tape collection of 1970s hits. Fortunately the relativistic effects of such a powerful gravitational source would probably give you more to worry about than your cassette tape collection. Basically what happens is that when the core of a star collapses, it gets very small very quickly. Essentially all of the matter causing the original star’s magnetic field gets squashed into a 20km diameter sphere, massively increasing the magnetic field. Whereas before the collapse the magnetic field was spread out over a red giant star, with a size similar to the orbit of the Earth or even Mars, now its compressed into an area the size of a city.

One of the really nasty things that can occur on a magnetar is called a starquake. This is when the surface of the object cracks and releases a massive amount of energy. On 27 December 2004 the magnetar SGR 1802-20 had one of these starquakes, it released an astonishing amount of energy as gamma rays at around 10^37 KW. Had that happened within 10 light years of the Earth we would have all died out. These starquakes are massive physical disruptions to the remnant star.

It is absolutely mind blowing to think that the whole thing is only about 20km in diameter, a star the size of a small city with a crust of about 1km thick. Underneath the crust is a liquid soup of highly compressed neutrons, protons and electrons, but nothing like we’d be familiar with as these particles are whizzing around very close to each other, unlike in normal matter which is mostly space. As the magnetic field of the object slowly twists and turns it puts enormous stress on the surface and eventually causes it to rupture – like a big crack, which is what releases the energy. You can read all about it in this article by Kaspi and Beloborodov which is fascinating and a lot more technical than my interpretation.

Magnetars are a class of object that is deeply interesting and one which we will no doubt continue to learn more and more about. It’s incredible to think that these violent neutron stars can have a solid crust and it’s mind boggling to think what it would be like on the surface with their massive gravity and ferocious magnetic fields, not to mention the occasional outburst of x-rays and gamma rays and the incredibly fast spin rates. The universe is an amazing place, and somewhat dangerous!