One of the most significant astronomical events in recent history occurred on August 17, 2017, when astronomers detected the collision of two neutron stars some 130 million light-years away.
This event, known as a kilonova or a neutron star merger, provided groundbreaking insights into several areas of astrophysics, including the origin of heavy elements, the nature of gamma-ray bursts, and the expansion of the universe. A kilonova is an astronomical event that occurs when two neutron stars merge and release an enormous amount of energy in the form of light and other electromagnetic radiation. It is a powerful explosion that occurs following the collision of these dense stellar remnants. The term “kilonova” reflects the fact that the event is about a thousand times brighter than a classical nova. Kilonovae play a crucial role in the production of heavy elements in the universe, such as gold and platinum.
The observation of a kilonova associated with the neutron star merger detected in 2017 provided valuable insights into the origin of these elements and deepened our understanding of the processes involved in stellar evolution and cosmic nucleosynthesis. The detection of the neutron star collision was made possible by the collaborative efforts of several observatories and gravitational wave detectors around the world.
The Laser Interferometer Gravitational-Wave Observatory (LIGO) in the United States and the Virgo Interferometer in Italy first detected the gravitational waves produced by the merging neutron stars. Gravitational waves are ripples in the fabric of spacetime that propagate outward from accelerating massive objects, such as colliding neutron stars. The detection of gravitational waves confirmed a key prediction of Albert Einstein’s general theory of relativity.
The Laser Interferometer Gravitational-Wave Observatory (LIGO) is a groundbreaking scientific instrument designed to detect gravitational waves. It operates based on the principle of interferometry[1], where two perpendicular arms, each several kilometers long, contain mirrors at their ends. A laser beam is split and sent down both arms, bouncing off the mirrors and returning to a central point. In the absence of gravitational waves, the returning laser beams perfectly cancel each other out.
However, when a gravitational wave passes through, it causes tiny changes in the lengths of the arms, altering the time it takes for the laser beams to return. This results in an interference pattern, allowing the detection of the gravitational wave. LIGO’s precise measurements are possible due to the use of advanced technologies, including high-power lasers, extremely stable mirrors, and sophisticated detectors.
Following the detection of the gravitational waves, a worldwide alert was issued to observatories to search for electromagnetic signals associated with the event. The detection of gamma-ray bursts by NASA’s Fermi Gamma-ray Space Telescope just 1.7 seconds after the gravitational wave signal provided further confirmation of the neutron star collision. Subsequently, a flurry of observations was made using a wide range of telescopes across the electromagnetic spectrum, including X-ray, optical, and radio telescopes.
The observations revealed a wealth of information about the aftermath of the neutron star collision. The intense gravitational forces generated by the merger caused the emission of a vast amount of energy across different wavelengths. The resulting explosion, known as a kilonova, produced a bright burst of light that gradually faded over time. This event was crucial in understanding the origin of heavy elements like gold, platinum, and uranium, as the collision of neutron stars is believed to be one of the primary sources of these elements in the universe.
Such objects are the skeletons of massive stars that have cataclysmically exploded, leaving behind a core so dense that just a teaspoon’s worth can weigh millions of tons. Observations revealed the event forged roughly 50 Earth masses’ worth of silver, 100 Earth masses of gold, and 500 Earth masses of platinum.
The gold forged alone is worth about 100 octillion dollars at today’s market price, according to Brian Metzger, an astrophysicist at Columbia University, or $100,000,000,000,000,000,000,000,000,000 written out (1 followed by 29 zeroes). The detection of the neutron star collision also provided a unique opportunity to measure the rate of the expansion of the universe. By comparing the distance to the host galaxy of the merger with the observed redshift of its light, astronomers were able to refine their understanding of the Hubble constant, a fundamental parameter that describes the rate at which the universe is expanding.
The discovery of the neutron star collision marked a significant milestone in multi-messenger astronomy, where information from both gravitational waves and electromagnetic waves is used to study cosmic events. It demonstrated the power of combining different observational techniques to gain a more comprehensive understanding of the universe. The remnants of the collision may also include a newly formed black hole, or if the combined mass is below a certain threshold, a more massive neutron star.
Additionally, the merger can have profound effects on the surrounding space, such as generating shockwaves, accelerating cosmic rays, and impacting the dynamics of the host galaxy.
Footnotes
- Interferometry is a measurement technique that utilizes the interference of waves to extract valuable information about the waves themselves or the objects that produced them. By splitting a wave into two or more separate paths and allowing them to recombine, an interference pattern is formed. This pattern contains details about the phase and amplitude of the waves, enabling precise measurements and analysis. Interferometry finds applications in various fields, including physics, optics, and astronomy, where it plays a crucial role in instruments like the Laser Interferometer Gravitational-Wave Observatory (LIGO) and the Virgo Interferometer for detecting gravitational waves. The technique provides a powerful means of studying wave phenomena and acquiring detailed insights into the nature of waves and their sources. [Back]
Further Reading
Sources
- “How a watershed space explosion created trillions of tons of gold” (OCTOBER 18, 2017) https://www.salon.com/2017/10/18/how-a-watershed-space-explosion-created-trillions-of-tons-of-gold/
- “Astronomers detected 100 Earths’ worth of gold being forged in space — here’s how much it’s worth” (Oct 16, 2017) https://www.businessinsider.com/how-much-gold-created-in-neutron-star-collision-2017-10
- Tanvir, N. R., et al. (2017). The Emergence of a Lanthanide-rich Kilonova Following the Merger of Two Neutron Stars. The Astrophysical Journal Letters, 848(2), L27. doi: 10.3847/2041-8213/aa90b6
- Abbott, B. P., et al. (LIGO Scientific Collaboration and Virgo Collaboration). (2017). Gravitational Waves and Gamma-Rays from a Binary Neutron Star Merger: GW170817 and GRB 170817A. The Astrophysical Journal Letters, 848(2), L13. doi: 10.3847/2041-8213/aa920c
- Goldstein, A., et al. (Fermi GBM and LAT Collaborations). (2017). An Ordinary Short Gamma-Ray Burst with Extraordinary Implications: Fermi-GBM Detection of GRB 170817A. The Astrophysical Journal Letters, 848(2), L14. doi: 10.3847/2041-8213/aa8f41
- Chornock, R., et al. (2017). Electromagnetic Evidence that SSS17a is the Result of a Binary Neutron Star Merger. The Astrophysical Journal Letters, 848(2), L19. doi: 10.3847/2041-8213/aa8f41
- Aasi, J., et al. (LIGO Scientific Collaboration). (2015). Advanced LIGO. Classical and Quantum Gravity, 32(7), 074001. doi: 10.1088/0264-9381/32/7/074001
- Grier, C. J. (2003). Interferometry. In Encyclopedia of Optics (2nd ed., Vol. 2, pp. 137-146). Academic Press. doi: 10.1016/b978-012369395-2/50059-7
- “Hunting Gravitational Waves: The LIGO Laser Interferometer Project in Photos” (April 11, 2014) https://www.space.com/25423-ligo-gravitational-waves-observatory-photos.html
- “Collision of Two Neutron Stars Spews Impossible Jets at 99.97% the Speed of Light!” (October 14, 2022) https://weather.com/en-IN/india/space/news/2022-10-14-neutron-stars-collision-spews-jets-at-9997-percent-light-speed
