When weird things meet

Supernovae are quite amazing but there are even bigger and weirder events in the universe that occur when neutron stars and black holes get a bit too close for comfort.

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I am totally amazed by the processes that happen in stars, the life cycle of stars and the cataclysmic possibilities of when stars run out of fuel. Despite my amazement at what happens in stellar evolution, I am even more amazed by the weird objects that remain after supernovae. Who can not be mesmerised by the weird physics and seemingly unnatural characteristics of neutron stars and black holes? There is no end to my fascination of these objects and finding out more about them. Fortunately there’s been a lot going on recently in our understanding of these weird objects, caused in part by the phenomenal effects of what happens when they combine.

Last year, the Laser Interferometer Gravitational Wave Observatory (LIGO) announced on the 16th October they had observed both the light and the gravitational waves from the merger of two neutron stars. This collision gave the world a wealth of information to build an understanding of neutron stars and what happens when they collide.

Science is all about coming up with an idea and checking if it’s true based on observation and experiment.

Computer models have been used to describe what we might see if two neutron stars combine, and the collision last year gave us the opportunity to witness the real event and compare the models. Using the data from LIGO and Virgo researches have been able to calculate a new theoretical maximum size of a neutron star at a radius of 13.6km. This is a bit bigger than what was previously thought (research by Annala, Gorda, Kurkela and Vuorinen, in Physical Review Letters, 120, 25 April 2018). This small difference in size makes a big difference in understanding what happens when neutron stars collide and what they are made of. Basically neutron stars are not as squishy as once we thought as they are not quite dense enough to squish neutrons into their component quarks.

Artist impression of neutron star collision aftermath (Credit: The Begelman group and Steve Burrows/JILA)

There’s a number of options when the weird objects collide, such as neutron star vs neutron star, black hole vs black hole or neutron star vs black hole. Because neutron stars are so very dense, it takes a lot to extract bits off them and this is exactly what happens when a neutron star collides with a black hole or another neutron star.

Most people were probably quite excited when it was announced last year that gold and other heavy elements were probably formed in collisions of neutron stars rather than in supernovae, well I was excited to think that the gold coin I have was originally from a neutron star!!!! So scientists know that the heavy elements form from a process called the rapid neutron capture process also known as the r-process. This is where a nucleus of an atom attracts a heap of neutrons rather rapidly. Unfortunately you just can’t go adding neutrons because things get unstable rather quickly and the nucleus decays. Quite often some of those added neutrons decay into a proton, an electron and an anti neutrino – WOW! So for the r-process to work another neutron has to be captured before another neutron decays so the nucleus keeps building into stuff like gold! For this to happen, you need quite a few neutrons. Lucky, because neutron stars have a lot of neutrons, 1.4 times the mass of the Sun worth of neutrons! So when these come together and tear themselves apart they make a lot of gold – an alchemist’s dream come true. We know that there’s enough of these weird object collisions to make the heavy elements we observe in the universe and now we have observed one of these collision to prove it.

The moment of collision (Credit: A. Simonnet)

So what happens when neutron stars collide? Fortunately the event of 2017 gave us a very good understanding and the research conducted on the results collected by LIGO and VIRGO published by Kazen, Metzger, Barnes, Quataert and Ramirez-Ruiz in Nature give a great description on what happens. Basically as the two neutron stars get closer and closer together the tidal forces start ripping bits off the surface of the stars, these pieces are still incredibly dense and they create a bit of a tail around the spinning pair. Also the mechanism starts to compress more material towards the poles of the two stars and then as they impact each other – or more like a really fast merge, remember they’re spinning super fast – the super dense and hard surface rupture is releasing an enormous amount of energy, flinging material out at about 30% of the speed of light. The pattern that the authors above figured out from the LIGO data is that the heavy elements created around the neutrons flung out from the stars fuses together through the r-process. They are confident about it by studying the data and measuring the light curve that lasted a few days afterwards, which was consistent with radioactive decay from heavy elements created through the r-process heating the ejected material. The other thing to keep in mind is that when the two neutron stars merge the result is a black hole of a couple solar masses, so there’s a rapid end to stuff getting flung out of the collision.

But even weirder than that is what happens when black holes collide. It’s worth noting that when these combine we get bigger black holes, kind of adding the two together and subtracting a bit of mass for the energy. An interesting study was done on a 19 Solar Mass black hole merger, which was the lowest mass of a black hole merger seen by LIGO so far. From this merger they determined that about 0.85 of a Solar Mass was lost in energy – which shows just how powerful these events are.

Black hole mergers measured by LIGO (Credit: Caltech)

There obviously a lot more for us to learn about these collisions and the work of LIGO is instrumental for this research.