Three cool, important discoveries from merging neutron stars
Meteorologist/Science Writer
Tuesday, October 17, 2017, 3:24 PM - Monday's announcement of the detection of merging neutron stars, and the very first sighting of a kilonova explosion from that merger, was groundbreaking for the science community. Here's the three coolest parts of this discovery.
On August 17, 2017, telescopes and observatories around the world, and even in space, took in every aspect of neutron star merger, GW170817, that was possible. From the swath of data, covering nearly the entire electromagnetic spectrum, the scientists involved unveiled a string of discoveries - the first gravitational wave detection of a neutron star merger ever detected, the first 'kilonova' explosion spotted, the confirmation that merging neutron stars cause gamma ray bursts and that they may be the primary source of so called 'short gamma ray bursts'.
There were a few very important parts of this discovery that slipped through with less fanfare, however.
1. This neutron star merger created a new black hole
Neutron stars and stellar mass black holes share a common origin. Both are known to be the end result of a massive star running out of fuel, collapsing in on itself and causing a 'supernova' explosion.
If the leftover stellar core is massive enough (between 3 and 4 times the mass of the Sun), it will collapse so fast that the particles will blow past any obstacles, and the entire core will crush down to a singularity. The result is a black hole.
If the core is only up to 3 times the mass of the Sun, the collapse is halted by what's known as "neutron degeneracy pressure". The gravitational pull on the matter towards the centre of the core isn't strong enough to force the neutrons together, so the collapse stops. The result is a neutron star, roughly 10 km wide.
In the case of binary neutron stars, though, as they spiral in towards one another and merge, the gravitational waves carry away energy, and the explosion spews out neutrons and heavy elements (see below), but only a few per cent of the total mass of both neutron stars is actually explodes into space. The leftover combined mass of both neutron stars exceeds that 3 solar mass limit, so it overcomes the degeneracy pressure and the mass crushes down into a black hole.
2. This discovery proves a fundamental truth about our universe
When LIGO and VIRGO picked up the gravitational waves emitted by this binary neutron star pair, just as they began to merge, and then the Fermi and INTEGRAL telescopes picked up the pulse of gamma rays emitted during the merger a couple of seconds later, it provided a fundamental confirmation for one of the basics of our physical understanding of the universe.
Gravitational waves travel at the speed of light.
An artist's conception animation of the gravitational waves emitted by binary neutron stars. Credit: Robert Hurt/Caltech/JPL
This may not seem like an overwhelming revelation, but before this, with the other four confirmed gravitational wave detections, the scientists involved only knew that they had seen gravitational waves passing by, and that they came from the merger of black holes. They didn't know how far away they were generated (there was no pulse of light accompanying them), nor did they know how long it took them to travel to us. So, there was no way to tell exactly what speed the waves were travelling through space, on their way to get here.
It was generally thought that gravitational waves would travel at the speed of light. Einstein predicted this roughly 100 years ago, and his physics has held up to pretty much every test since then. Still, science wants evidence. Simply thinking that something is right isn't enough.
So, when the gravitational waves were detected, and the gamma ray burst showed up seconds later, right on schedule, based on the timing of when the two would have been generated during the neutron star merger, it confirmed that both the waves and the gamma rays were travelling at the same speed. And since gamma rays travel at the speed of light, it followed that the gravitational waves were also travelling at the speed of light!
Another point for Einstein!
3. We now know the origin of most of the universe's gold, silver, platinum, uranium, etc
If you are wearing anything gold, silver or platinum right now, chances are that the atoms in those objects were produced by the merger of two neutron stars - exactly like what happened with the pair that generated the GW170817 event reported on Monday.
"The origin of the really heaviest chemical elements in the universe has baffled the scientific community for quite a long time," Hans-Thomas Janka, senior scientist at the Max Planck Institute for Astrophysics (MPA), said in a MPA press release. "Now we have the first observational proof for neutron star mergers as sources; in fact, they could well be the main source of the r-process elements."
This was another idea presented some time ago - 28 years ago, to be more exact. In the wake of the first discovery of a binary pair of neutron stars, Professor Tsvi Piran, at the Hebrew University of Jerusalem, along with his colleagues, investigated the idea of what would happen to such a pair as they orbited one another.
Their work resulted in the prediction that the pair's mutual gravitational pull would cause them to spiral inward towards each other, setting off gravitational waves in the process, and eventually a gamma ray burst and kilonova as the pair merged to form a black hole. Additionally, their research indicated that the explosion that accompanied the merger would also produce a variety of heavy, so called 'rapid neutron capture' or 'r-process', elements, which would be blasted out into space.
These r-process elements include precious metals like gold, silver and platinum (which are also produced in smaller quantities by supernovae), and these mergers are solely responsible for all the bismuth, radium, thorium and uranium (among other elements) that we find here on Earth.
The periodic table of the first 92 elements, revealing their origins. Bright yellow are produced from merging neutron stars. Elements with more than one colour are formed by different events, with the relative amount of the element's square covered by the colour denoting the relative amount of the element created by the process. Credit: Robert Hurt/IPAC/Caltech
This idea was resisted by many in the scientific community at the time, and although some further research in the years since has supported those conclusions, it wasn't until this event was detected that there was solid evidence.
The broad spectrum of light that was gathered from the kilonova, by the numerous telescopes and observatories around the world, pointed to the presence of an abundance of these r-process elements, showing that the neutron star merger was, indeed, the source of these elements.
"I am exhilarated by this confirmation of a prediction we made nearly thirty years ago," Prof. Tsvi Piran said in a Hebrew University of Jerusalem press release. "I also remember how difficult it was to convince the scientific community of our idea: at the time it was against the standard model that was published even in freshman textbooks on astronomy. When we made this prediction in 1989, we did not expect it to be confirmed within our lifetimes. But with continued curiosity and the development of new technologies, we are able learn ever deeper truths about the nature of our Universe."
Sources: Science News | NASA GSFC/CI Lab | MPA | HUJI
