The Golden Merger Of Neutron Stars

Yesterday ESO announced the discovery of gravitational waves and electromagnetic radiation, that originated from a neutron star merger in a galaxy far, far away. Media all over the world covered the story as it was revealed that some 3,000 scientists had worked for six weeks to decipher the signal, which was received in multiple telescopes and observatories on August 17 this year.

Composite of images of NGC 4993 and kilonova
Photo: ESO/N.R. Tanvir, A.J. Levan and the VIN-ROUGE collaboration.

Events like this one are important for not only science itself, but also public science outreach. This discovery has a very good storytelling and it is crucial to tell the right story. By succeeding to do so, the public attention towards science can only increase, which is what astronomers like myself care much about.

Neutron Stars In A Galaxy Far, Far Away

In a galaxy 130 million light years away from us, two neutron stars merged. Neutron stars are dead stars, that used to have a high mass (more than 8 times the mass of our Sun) before they died in a violent supernova explosion. This explosion throws away much of the star’s mass into the interstellar medium leaving a neutron star, i.e. a star consisting only of neutrons, with a radius of 10 km (6 miles) and a mass roughly of our Sun. This star is very dense! A spoon of material from the star will have the same weight as Mount Everest.

When two relatively close stars die and leave two neutron stars, they can start to orbit each other. This happens because of the gravitational force between them, much the same way the Earth and the Sun “orbit” each other – only here the Sun is so much more massive than the Earth, so effectively the Sun barely moves. But the principle is the same.

Artist’s impression of merging neutron stars
Photo: ESO/L. Calçada/M. Kornmesser

As the two neutron stars approach each other in their spiral dance, they send out ripples in spacetime, which become more and more pronounced. Spacetime is the fabric, that we all live in – the same way a floor carpet gets rippled, when you slide across it, spacetime gets rippled when dense objects moves around. These ripples are called gravitational waves and travel across the Universe. Every now and then, they will reach Earth, and this is what happened on August 17. LIGO and Virgo measured gravitational waves from a source that could be seen from the Southern hemisphere.

The gravitational waves LIGO and Virgo picked up came from an event, where two neutron stars spiraled into each other to become one object. This happens in a big explosion called a kilonova.

Exactly 1,7 seconds after LIGO and Virgo measured the gravitational waves, the Fermi space telescope picked up a burst of gamma rays, i.e. electromagnetic radiation, from the same region on the sky. And this is where things got (even more) interesting! It turned out that the origin of the gravitational waves and the origin of the gamme ray burst (GRB) were the same.

This means that two independent forces were measured from the same event! It is like being able to hear AND see, in stead of just seeing. The two forces measured were the gravitational and the electromagnetic forces.

Gold Comes From Neutron Stars

The strength of the gravitational waves tells us something about the mass of the merger. The spectrum from the electromagnetic radiation tells us something about the composition of merger – and which elements were formed in the process. It had long been speculated, that heavier elements like gold were created in the process of two neutron stars merging.

Photo: Cmglee (Own work) via Wiki Commons.

Unfortunately, this had not been confirmed by observation. By looking at the electromagnetic spectrum from the merger, especially in the short window after the kilonova explosion, then astronomers can see, which elements are created in the explosion, and hence how gold came to Earth.

When the kilonova explosion happens, material is violently thrown in all directions away from the star and hence distributed in the interstellar medium, i.e. in the space between stars in the galaxy. Essentially, this is a large cloud of gas and dust, and at some point stars can be born out of this cloud.

The process on how supernova is linked to star formation and hence planet formation. For illustrative purpose only. The Henize 206 source is not related to the GW170817 discovery. Photo: NASA/JPL-Caltech/R. Hurt (SSC-Caltech).

Planets are born out of the remaining gas and dust around their star (in our case: the Sun). This is where e.g. gold on Earth is believed to come from.

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