On October 16, the physicists and astronomers who comprise the LIGO and Virgo research teams announced the detection of a fifth gravity wave event. Radio astronomer Alessandra Corsi at Texas Tech University called this detection “a big gift that nature has given us” and “a life-changing event.”1
The “big gift” was that this detection was the first-ever observation of gravity waves from the merger of two neutron stars. The previous four gravity wave detections were all the result of the merging of two black holes. What made this gift especially big was that the gravity waves from the merging of two neutron stars was observed by both the LIGO and Virgo gravity wave telescopes. The three locations of the gravity wave detectors (Hanford, Livingston, and Pisa) allowed the researchers to roughly pinpoint the location of the gravity wave event and promptly alert astronomers around the world to perform follow-up observations at multiple wavelengths.
The neutron star merging event occurred on August 17 at 12:41 universal time 6,200 light-years from the nucleus of the galaxy NGC 4993.2 NGC 4993 is 130 million light-years from Earth in the Hydra constellation.3 It is a little closer to us than the center of the Virgo cluster of galaxies.
Unlike neutron stars, black holes contain no matter that might radiate light. Thus, since the previous four gravity wave detections were of the merging of two black holes, the events produced no emissions of light. In contrast, astronomers calculated that the merging of two neutron stars should spew debris that emits light at all wavelengths.
Two seconds after the detection of the gravity wave signal, the Fermi Gamma-Ray Space Telescope detected a brief gamma-ray burst from the event.4 A few hours later, astronomers at five observatories identified the light source of the event. Over the course of several days, they observed the neutron star merging event fade from a bright blue color to a dim red color. About two weeks later, the merging event began to emit both x-ray and radio waves.
About a third of all the world’s astronomers—4,600+, from 952 different research institutions and at 70 different observatories—studied the merging event.5 The wealth of accumulated data led to several outstanding discoveries.
Origin of Short Gamma-Ray Bursts
First, the wealth of data on the neutron star merging event solved the mystery of the origin of short gamma-ray burst events. Gamma-ray bursts range from 10 milliseconds to a few hours in duration. Several hundred have been observed. Orbiting satellites currently detect an average of one gamma-ray burst per day. None of have been detected any closer than 130 million light-years away. This lack of nearby gamma-ray bursts is good since they are deadly for life. In fact, the frequency of gamma-ray bursts, by itself, rules out 90 percent of all galaxies as candidates to harbor life.
Theoretical physicists had determined that the only conceivable source of gamma-ray bursts shorter than two seconds in duration was the merging of two neutron stars to form a black hole. The multi-wavelength observations of the August 17 gravity wave detection clinched this conclusion. From now on, astronomers can be confident that whenever a short gamma-ray burst accompanies a gravity wave detection, they are looking at the merger of two neutron stars.
Reality of Kilonovae
For decades, astronomers have hypothesized the existence of kilonovae or macronovae. These events are thought to briefly outshine ordinary novae by thousands of times. Theoreticians had calculated that the merging of two neutron stars should result in such furious radioactive decay of heavy radioisotopes as to produce an emission of light at least a thousand times brighter than a nova.
Furthermore, theoreticians had determined that such radioactive decay should produce the emission of bright blue light that, over a few days, transitions to dimmer red light as the daughter products of the radioactive decay absorb the blue wavelengths of the initial light emission. Multi-wavelength observations of the August 17 gravity wave event confirmed all of the theoreticians’ conclusions.6
Source of r-Process Elements
The periodic table contains 94 naturally occurring elements. (Plutonium and neptunium, though present on the early earth, have long since decayed away.) Several decades ago, astronomers and physicists established with high accuracy how stars and supernovae manufacture the elements lighter than iron and half the elements that are heavier than iron—the s-process elements. They also successfully determined the means by which the other half of the elements heavier than iron—the r-process elements—are formed. However, they were stymied in their attempts to explain the source of the r-process elements.
The r-process is shorthand for rapid neutron capture process. The process entails a succession of rapid neutron captures that results in lighter elements being transformed into heavier elements. The process is viable only where there is a very high flux of free neutrons. The neutron-rich debris thrown off from the merging of two neutron stars provides such a high flux of free neutrons.
The multi-wavelength observations of the August 17 gravity wave event affirmed that the merger of the two neutron stars indeed efficiently produced r-process elements.7 Moreover, the observations established that most, if not nearly all, the r-process elements that exist on Earth and elsewhere in the universe came from neutron star merging events.
The r-process elements include such valuable metals as silver, gold, platinum, palladium, osmium, thorium, and uranium. Earth’s superabundance of thorium and uranium explains to a large degree its enduring strong magnetic field, crucial for preventing the erosion of its atmosphere and the protection of life from deadly solar and cosmic radiation. Earth’s superabundance of thorium and uranium also explains its enduring plate tectonics, which transformed it from a water world into a planet with both surface oceans and surface continents.
If it were not for primordial Earth being strongly salted by r-process elements produced by neutron star merger events, any kind of advanced life would be impossible. If it were not for silver, gold, platinum, palladium, and osmium produced by neutron star merger events, our advanced civilization and high standard of health could not be sustained.
Thanksgiving Day is just a few weeks away. On that day I encourage you to thank God for giving astronomers the capability of learning so much about neutron star merging events and therein to uncover more of the glory of God in creation. I encourage you, too, to thank God for exposing primordial Earth to such a rich source of r-process elements from neutron star mergers and to presently protecting us from any nearby neutron star merging events. The heavens indeed proclaim the glory and righteousness of God.
Featured image: Artist’s Rendition of a Binary Neutron Star Merging Event. Image credit: NASA/Goddard Space Flight Center; for an animation of a binary neutron star merging event see below:
Resources
All 23 of the peer-reviewed research papers on the observations of the August 17 gravity wave event are available in their entirety for free in the October 20, 2017 edition of the Astrophysical Journal Letters.
Endnotes
Adrian Cho, “Merging Neutron Stars Generate Gravitational Waves and a Celestial Light Show,” Science (October 16, 2017): doi:10.1126/science.aar2149.
Y.-C. Pan et al., “The Old Host-Galaxy Environment of SSS17a, the First Electromagnetic Counterpart to a Gravitational-Wave Source,” Astrophysical Journal Letters 848 (October 16, 2017): id. L30, doi:10.3847/2041-8213/aa9116.
Jens Hjorth et al., “The Distance to NGC 4993: The Host Galaxy of the Gravitational-Wave Event GW170817,” Astrophysical Journal Letters 848 (October 16, 2017): id. L31, doi:10.3847/2041-8213/aa9110.
B. P. Abbott et al., “Gravitational Waves and Gamma-Rays from a Binary Neutron Star Merger: GW170817 and GRB170817a,” Astrophysical Journal Letters 848 (October 16, 2017): id. L13, doi:10.3847/2041-8213/aa920c.
B. P. Abbott et al., “Multi-Messenger Observations of a Binary Neutron Star Merger,” Astrophysical Journal Letters 848 (October 16, 2017): id. L12, doi:10.3847/2041-8213/aa91c9.
P. S. Cowperthwaite et al., “The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/Virgo GW170817. II. UV, Optical, and Near-Infrared Light Curves and Comparison to Kilonova Models,” Astrophysical Journal Letters 848 (October 16, 2017): id. L17, doi:10.3847/2041-8213/aa8fc7.
R. Chornock et al., “The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/Virgo GW170817. IV. Detection of Near-Infrared Signatures of r-Process Nucleosynthesis with Gemini-South,” Astrophysical Journal Letters 848 (October 16, 2017): id. L19, doi:10.3847/2041-8213/aa905c.
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