Astronomy is sometimes similar to zoology. It keeps an eye on new stars, while we discover new species. On January 5 and 15, 2020, astronomers discovered an unprecedented object, as they announced on June 29 Astrophysical Journal Letters. For the first time, a pair consisting of a black hole and a neutron star, spiraling around each other, was observed.
Separately, representatives of these two families have already been seen. A black hole, a star so dense that it prevents all matter and even all light from escaping its gravity, was first imaged in 2019. Likewise, neutron stars, which concentrate the mass of one or two of our Sun’s equivalent into a sphere with a radius of only about ten kilometers , are regularly observed in our galaxy, as “beacons” pulsing with their radiation like metronomes. Hence their name pulsar. These two objects are actually the remnants of very massive stars, which are running out of fuel, and are collapsing on themselves.
But these two works, the behavior of which is still mysterious to physicists, were never considered a pair. In 2015, two black holes surprised each other to form one. Then in 2017, the ballet spotted two neutron stars. “But the neutron star and the black hole binary were missing from our hunting chart. We’re glad we found them! We even saw two,” Astrid Lamberts, an astrophysicist at the National Center for Scientific Research at the Côte d’Azur Observatory, is excited.
Explosion of energy
On January 5, 2020, a black hole the size of nine suns engulfed a neutron star four times lighter. Ten days later, another event collected two champions weighing 5.7 and 1.5 suns, respectively. Either way, the ballet didn’t last long. In a few tens of seconds, the largest of them, the black hole, devoured the weakest neutron star. In the end, not only a black hole remains, but also a blast of energy, powerful enough to shake spacetime (the kind of jelly that makes up the universe), to generate waves, in the same way as a pebble in water, a signal that reached Earth a billion years later.
However, these tremors, as light as caresses, were enough to move the detectors, by adjusting the lengths of two laser arms 3 to 4 km in length. For the first event, one detector, LIGO in Livingston, Louisiana, saw so-called gravitational wave passing. For the second, the wave, a relic of the catastrophe, shook three detectors, the second is located in Hanford (Washington state) and the Virgin in Italy.
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