Neutron Star

A neutron star weighs slightly more than our Sun, but crams that mass into an object fewer than 12 miles (less than the length of Manhattan) across. Because they’re so incredibly dense—one teaspoon of neutron star would weigh about six billion tons—the matter in these stars doesn’t look like the familiar atoms that make up all the stuff on Earth. Instead, neutron stars contain the particles you’d normally find in an atom’s nucleus, a densely-packed crowd of neutrons (hence the name) with a smattering of protons mixed in.

Unlike black holes, neutron stars create a visible spectacle when they collide: Theorists have predicted that they would shoot out jets of gamma rays and hurl glowing matter into space. So when LIGO heard GW170817, its researchers alerted astronomers all over the globe to start scanning the skies. Thanks to Virgo, they could also tell observatories roughly where to look. The signal that reached each detector had a slightly different strength and arrived at a slightly different time, and researchers could analyze these disparities to narrow down the area of space where the waves must have originated: a 30-square-degree region visible from Earth’s southern hemisphere.



That wide variety of eyes on the sky fulfills one of the most valuable promises of gravitational wave astronomy: multi-messenger events. By watching the neutron-star merger through gravitational waves, gamma rays, and the full electromagnetic spectrum, researchers are already making breakthroughs about how neutron stars, as well as many of the elements in the universe, formed. As they continue to analyze their data, more papers are sure to follow.

“We’re just beginning to explore the gravitational universe,” David Reitze, executive director of the LIGO Laboratory at Caltech, told Popular Science. “It’s very likely that we’re going to see something that no one anticipated, that really shakes the paradigms of established scientific theory.” (Reitze’s number-one goal: “Proving Einstein wrong, with all due respect, would be a big deal. We might be able to see cracks in the edifice of the theory of general relativity. That may give us a clue for how to bring general relativity and quantum theory together in an uber-theory, a theory of everything.”)

In the meantime, here are a few of the discoveries that scientists are already celebrating.



First non-black-hole gravitational waves

Scientists are buzzing about the fact that this marks the first-ever multi-messenger event—an astronomical event observed using multiple kinds of signals. That hype is well-deserved; after all, this represents the beginning of a new type of astronomy. But in the excitement, don’t forget that this also marks the first-ever detection of gravitational waves from a neutron star merger.

That means this collision was the first event for which multi-messenger observations were even possible (black holes don't produce light). Beyond that, it gives researchers a better sense of how often neutron-star mergers occur—relatively speaking. Based on this finding, LIGO could expect to see anywhere from one to two neutron star mergers per year to one per week.


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