Scientists examined long-ago collision of two neutron stars. Measurements of the light and other energy emanating from the crash have helped scientists explain how planet-killing gamma ray bursts are born, how fast the universe is expanding, and where heavy elements like platinum and gold come from. It started in a galaxy called NGC 4993, seen from Earth in the Hydra constellation. Two neutron stars, collapsed cores of stars so dense that a teaspoon of their matter would weigh 1 billion tons, danced ever faster and closer together until they collided. The crash, called a kilonova, generated a fierce burst of gamma rays and a gravitational wave, a faint ripple in the fabric of space and time, first theorized by Albert Einstein. Scientists finally now know what happens when an unstoppable force meets an immovable object and it’s a kilonova. The crash happened 130 million years ago, while dinosaurs still roamed on Earth, but the signal didn’t arrive on Earth until Aug. 17 after traveling 130 million light-years. A light-year is 5.88 trillion miles. Signals were picked up within 1.7 seconds of each other, by Nasa’s Fermi telescope, which detects gamma rays, and gravity wave detectors in Louisiana and Washington state that are a part of the LIGO Laboratory, whose founders won a Nobel Prize earlier this month. A worldwide alert went out to focus telescopes on what became the most well-observed astronomical event in history. Before August, the only other gravity waves detected by LIGO were generated by colliding black holes. But black holes let no light escape, so astronomers could see nothing. This time there was plenty to see, measure and analyse: matter, light, and other radiation. The Hubble Space Telescope even got a snapshot of the afterglow. Finding where the crash happened wasn’t easy. Eventually scientists narrowed the location down to 100 galaxies, began a closer search of those, and found it in the ninth galaxy they looked at. The colliding stars spewed bright blue, super-hot debris that was dense and unstable. Some of it coalesced into heavy elements, like gold, platinum and uranium. Scientists had suspected neutron star collisions had enough power to create heavier elements, but weren’t certain until they witnessed it. Calculations from a telescope measuring ultraviolet light showed that the combined mass of the heavy elements from this explosion is 1,300 times the mass of Earth. And all that stuff - including lighter elements - was thrown out in all different directions and is now speeding across the universe. Perhaps one day the material will clump together into planets the way ours was formed, maybe ones with rich veins of precious metals. The crash also helped explain the origins of one of the most dangerous forces of the cosmos - short gamma ray bursts, focused beams of radiation that could erase life on any planet that happened to get in the way. These bursts shoot out in two different directions perpendicular to where the two neutron stars first crash. Scientists knew that the universe has been expanding since the Big Bang. By using LIGO to measure gravitational waves while watching this event unfold, researchers came up with a new estimate for how fast that is happening, the so-called Hubble Constant. Before this, scientists came up with two slightly different answers using different techniques. The rough figure that came out of this event is between the original two. The first optical images showed a bright blue dot that was very hot, which was likely the start of the heavy element creation process amid the neutron star debris. After a day or two that blue faded, becoming much fainter and redder. And after three weeks it was completely gone. Scientists involved with the search for gravitational waves said this was the event they had prepared for over more than 20 years.