Binary neutron stars: new ground for LIGO

Image credit: LIGO Laboratory

As covered in our previous article, the Laser Interferometer Gravitational-wave Observatory (LIGO) has made several successful detections of gravitational waves. The latest detection differs in the previous made to date in that it comes from two neutron stars colliding with one and other. For the first time, scientists have directly detected gravitational waves in addition to light emitted from a cosmic event.

But why is this such a remarkable detection? To begin with, gravitational waves are very different than electromagnetic waves. They are the distortions or ripples in spacetime created by accelerating massive objects, while electromagnetic waves are associated with electromagnetic energy. It’s very easy to observe the latter, as everything our  eyes can see is part of the electromagnetic spectrum! In contrast gravitational waves are very weak, so much so that to detect them the typical extension of the interferometer is just 1/10,000th of the width of a proton.

Neutron stars are very small, with their radius about the size of a city, but very dense. So much so that one teaspoon of the material that they are comprised of would weigh one billion tons on Earth! Sometimes stars can form binary systems, in which the two stars orbit about the centre of mass of the system dancing beautifully in the cold space. They are however heading towards a violent death as they draw closer and closer together. When neutron stars do this, they emit gravitational waves due to their strong gravitational fields distorting space. On the 17th of August this summer, two neutron stars about 130 million light years away, started spiraling together emitting a 100-second gravitational signal, detected by three of LIGO’s detectors. The aftermath of this cosmic event is an intense beam of gamma ray radiation which was detected as being emitted about two seconds after they merged.

In the days that followed this cosmic event, its detection was made in all regions of the electromagnetic spectrum by more than 70 observatories. Additionally, scientists detected a kilonova event, an outburst of heavy elements like gold and platinum, which was predicted to occur following a neutron star merger.

Observing both electromagnetic and gravitational waves from the same event has been a long term goal for scientists. The importance of the detection is huge as it has given astronomers an unprecedented opportunity to probe a collision of two neutron stars, providing a vast amount of data that can help solve puzzles that have confused scientists for as long as gravitational waves have been theorised, like the confirmation that both types of waves travel at the same speed. The LIGO collaboration has once again opened the doors to a new way of doing astrophysics, giving the science community an event that will undoubtedly be remembered as one of the most studied astrophysical events in history!


This article was written by Christos Kourris and edited by James Hitchen.

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