Ushering in the beginning of a new era in astronomy and physics, scientists on Monday announced they have for the first time detected the spacetime ripples known as gravitational waves from the collision of two neutron stars. Streaming in from the sky over the Indian Ocean on August 17, the waves registered at the twin detecting stations of the U.S.-based Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO), as well as a European detector called Virgo located in Italy. This is the fifth time in the last two years that scientists have confirmed spotting such waves, a phenomenon that Einstein first predicted more than a century ago—and that led to this year’s Nobel Prize in Physics for three of LIGO’s leaders.
All of the previously detected gravitational waves, however, came from merging pairs of black holes. These objects are so dense that light cannot escape their grasp, making such mergers essentially invisible to normal telescopes despite the prodigious gravitational waves they generate in the final moments of their incredibly violent death spirals. Without a much-larger network of gravitational wave observatories, astronomers cannot pin down the precise locations of merging black holes, let alone deeply investigate them.
But neutron-star mergers begin with objects that in comparison to black holes can be featherweights. A neutron star is the highly compressed core of an expired massive star, and is formed in the aftermath of a supernova explosion. Its gravitational field is strong enough to squeeze and break down an entire sun’s worth of matter into a city-sized orb of neutrons, making it less a true “star” and more an atomic nucleus as big as Manhattan. But a neutron star’s gravity is still too weak to trap light. So the flash from two of them slamming together can escape into the cosmos, producing not just gravitational waves but also one of the universe’s most brilliant fireworks displays for anyone who cares to look.
In this case, after the initial chirp of gravitational waves signaling the onset of the merger, the “fireworks” consisted of a two-second-long gamma ray burst (GRB) followed by a weeks-long, multi-wavelength afterglow—and “anyone” proved to be nearly every astronomer and physicist on Earth who had found out about the event. Julie McEnery, project scientist for the Fermi Gamma-ray Space Telescope, which spotted the GRB, called August 17 “the most exciting morning of the nine-year Fermi mission.”
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