Neutron stars are the remnants of explosions of supernovae. Neutron stars are most dense in the universe so these are quite small in size. For example the size of London. They are so dense that a spoonful of neutrons weighs as much as 100 000 Eiffel towers.
It so happens that when two neutron stars approach each other, they turn faster and faster generating the gravitational waves by modifying the space-time around them. Ultimately they collide and fuse. After this the eject lots of matter.
How such events in occuring in the universe are detected? Electromagnetic rays emitted during such an event in the form of visible and invisible energy waves can be tracked by the modern telescopes and spectrometry.
Another technique has been added and it measures the gravitational waves which are generated in such violent events involving massive bodies. Such instruments has been installed in US. Called LIGO (Laser interferometer Gravitational-Wave Observatory) can record the gravitational waves.
One such event of fusion of two neutron stars happened 130 million years ago. The gravitational ripples were detected by LIGO on August 17 at 1241 GMT for 100 seconds. Then after 2 seconds a Gamma rays burst was observed. After hours and days following the event, X-rays, UV rays and infrared rays were detected.
This is the first successful observation of fusion of two neutron stars through Gravitational Waves and as well as through different kind of rays that are emitted.
It also confirmed the premise that gravitational waves travel with speed equal to that of light. This event was made possible with the collaboration of 3674 scientists from 953 institutions all around the world.