By engaging with all the videos within this series, you will effectively complete a full undergraduate course in astronomy, equipping yourself with the knowledge and skills necessary to navigate the night sky with confidence, learning all the basics and many advanced topics! Now we delve into the captivating realm of gravitational waves, a groundbreaking discovery that has revolutionized our understanding of the cosmos. This exploration commenced with the inaugural detection of gravitational waves in 2016, a pivotal moment that has since refined our comprehension of the universe. In a previous lecture, we explored how the discovery of gravitational waves was the culmination of approximately four to five decades of meticulous research, with the Laser Interferometer Gravitational-Wave Observatory (LIGO) playing a pivotal role. The initial detection earned the Nobel Prize for the scientists involved in this groundbreaking achievement. Since that historic moment, six additional confirmed observations of gravitational waves have been made, each providing novel insights into cosmic phenomena. Gravitational waves, as predicted by Einstein’s general theory of relativity, can be detected using terrestrial interferometers such as LIGO. These waves are generated by exceptionally energetic events, including rotating neutron stars experiencing ‘starquakes,’ supernovae, and notably, the collisions of compact objects such as black holes and neutron stars. Neutron stars represent incredibly dense remnants of supernova explosions, typically possessing between one and three solar masses, yet compressed into a sphere with a diameter of only a few kilometers. Due to their immense density and powerful magnetic fields, neutron stars can emit beams of electromagnetic radiation, manifesting as pulsars. The narrative of gravitational wave detection traces back to 1974, when Russell Hulse and Joseph Taylor identified the first binary pulsar. Their observations of variations in the pulsar’s emissions provided evidence that the system was losing energy through gravitational waves, as Einstein had predicted. This inquiry leads us to consider the consequences of neutron star collisions, which will be the subject of our next exploration. I will elucidate how such collisions not only generate gravitational waves but also produce gamma-ray bursts, which are brief and intense emissions of gamma radiation. In August 2017, a significant discovery occurred when the LIGO and Virgo observatories detected gravitational waves resulting from a neutron star collision. This event, designated as GW170817, initiated a global effort to locate and investigate the source. Within hours, astronomers identified the host galaxy, NGC 4993, situated approximately 130 million light-years away. This marked the first instance of observing light from a gravitational wave source, providing valuable data. Upon the merger of neutron stars, a phenomenon known as a kilonova is produced, which is approximately a thousand times more luminous than an average supernova. Overall, the segment emphasizes clear definitions, underlying geometry, and practical observing guidance so viewers can connect the concept to the real sky.