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The deafening sound of neutron stars crashing together When large objects in space orbit around each other, they make big waves, and a big sound.

Large stars and planets in space have intense gravity. The gravity of these large celestial bodies is so strong, that they actually warp the empty space around them. Also, in space, time progresses differently when near celestial bodies. Because of these effects on space and time, space is regarded as a space-time fabric. Large planets, stars, or other celestial bodies distort this fabric. This can be envisioned as a bowling ball sitting in the middle of a trampoline. The fabric will stretch close to the bowling ball, but the farther away from the bowling ball, the trampoline material will look unaffected.

Gravity of Earth distorting the space-time fabric. Source: Wikimedia Commons

Gravity waves occur when objects in this space-time fabric accelerate or decelerate. If you throw a loose planet into the empty space between stars, there would be no forces on it. This lack of forces means there is nothing to cause it to slow down or speed up. To get celestial bodies to accelerate, decelerate, or change direction, they have to interact with each other.

Here is a helpful analogy. When you drive a boat on a lake, it disrupts the water and creates a wake. This same idea happens when planets and stars rush through space. The faster they spin or change direction, just like the boat in the water, the bigger the resulting wave.

For example, the Earth undergoes acceleration as it rotates around the Sun. The Earth is not large compared to the sun, so the gravity waves that result from this interaction are quite small. However, if there were more massive objects, the waves in the fabric would be larger, meaning larger gravity waves. It is the difference between rushing your hands quickly through the water, versus driving a boat through the water. The boat is bigger and faster and going to cause a bigger wave. When two larger bodies, like stars, orbit around one another, they create even larger gravity waves. When even larger bodies, like black holes, orbit around one another, they create very large gravity waves.

Most stable matter, like people, water, and rocks, are made of atoms, which are made of neutrons, electrons, and protons. Neutron stars are stars made entirely of neutrons.

Model of an atom, blue are electrons, red are protons, and black is neutrons. Source: Wikimedia Commons

A neutron star is a star that was once made up of regular atoms, but at some point in its life it was hit with an enormous amount of pressure. The pressure was so large that the electrons and protons were smashed together and became neutrons. Neutron stars are so dense that they will be about 6 or 7 miles across, smaller than the average farm, but heavier than our entire sun, which is 1.3 million times the size of Earth. That is one dense star!

Throughout the galaxy, stars sometimes orbit each other. If this would happen in our solar system, the Earth would have two suns. These stars will typically orbit each other, and slowly their orbits will get smaller and smaller. They will orbit closer and closer to one another, gradually speeding up until they fall into each other and form one big star. These disruptions of the space-time fabric cause big waves and big sounds. These sounds may be detected by modern astronomy instruments such as LIGO.

Supermassive binary stars orbiting one another. Source: Wikimedia Commons

This process of binary stars falling into each other is called a star merger. This is one of the loudest contributions of gravity waves in the universe. When scientists measure gravity waves, the easiest gravity waves to detect are caused by these star mergers. The loudest of them all, the only one humanity has ever measured, are black hole mergers: when two black holes orbit one another until they collapse into one giant black hole.

That brings us to the work by the scientists of today’s paper. These scientists studied the size of the gravity waves of neutron star mergers. These neutron stars are not as massive as black holes, but they are very dense and orbit very close together. Neutron stars orbiting close together at high speeds result in strong gravity waves. But just how strong are they?

To break this problem down, the scientists identified three unique phases of the merging process for neutron stars. These three phases occur right after the neutron stars begin touching and right before they collapse into each other, forming a black hole.

Temperatures and densities of the distinctions for phase 1, phase 2, and phase 3 of neutron star mergers. Source: https://arxiv.org/pdf/1512.06397.pdf 

To determine the size of the gravity waves that result from these neutron star mergers, the scientists had to break down which phase of the neutron star merger is the loudest. For the average neutron star, they found that Phase 3 releases the loudest gravitational waves. Phase 3 is so loud that its gravity waves are louder than the entire merging process.

When compared to black holes, which are more massive than neutron stars, neutron stars are still relatively loud. The average black hole merger was about 100 times louder for gravity waves than the average neutron star merger. Nevertheless, the waves of both types of star mergers are so loud that any human would be deaf instantly, and the resulting waves would likely destroy a human all together.

The scientists’ calculations were limited, however. They were only able to model the size of the gravity waves for most neutron star mergers, the average size ones. For neutron stars that are larger than average, they suggest the gravity waves would be even more loud than their findings suggest, likely louder than some black holes! These scientists admit that more work is needed to truly understand the maximum strength gravity waves of the big neutron stars.

In conclusion, neutron star binary mergers produce high amplitude gravity waves. It’d be difficult to measure them with our current equipment because the mergers occur on such short time scales. Nonetheless, these simulations give great direction to scientists building the next generation of gravity wave detectors!

Article Information

Paper Authors: Sebastiano Bernuzzi, David Radice, Christian D. Ott, Luke F. Roberts, Philipp M¨osta, and Filippo Galeazzi
Edited By: Gina Riggio
Source: How loud are neutron star mergers?
Publication Date: July 2016

Featured Image Source:
NASA. https://svs.gsfc.nasa.gov/vis/a010000/a010500/a010543/Merge_Horizontal.00038_print.jpg


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