A Giant Space Collision Let Scientists Measure Size of Superdense Neutron Star for the First Time

An artist's rendering of the collision of two neutron stars. Carnegie Institution for Science

In August, scientists caught their first-ever glimpse of two superdense neutron stars colliding, setting off ripples of gravitational waves that reached Earth. Now, a team of researchers has taken that data and calculated that one of the stars must have been at least 6.6 miles across before its fiery demise.

Their estimate, published in The Astrophysical Journal Letters, is unprecedented in its precision for neutron star measurements and is based on the giant burst of light that accompanied the collision, which scientists call a kilonova. That kilonova means the collision resulted in a neutron star, rather than a black hole, which would have meant much less bright images from the collision.

Scientists had already had an approximate sense of how large the neutron stars in question were. The initial characteristics they measured during the collision indicated that put together, the two stars consisted of about 2.74 times the amount of stuff as our sun. But neutron stars are much, much denser than our sun: The sun is more than 850,000 miles wide, wide enough for 100 Earths to stretch across, whereas a neutron star is about the size of a large terrestrial city.

Neutron stars are zombies of a sort, formed after stars about 10 times the mass of our sun exploded. During that explosion, the star flung most of its contents deep across the universe and crammed what it kept down into the superdense neutrons that give the star's second form its name.

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Having a better sense of neutron star sizes is important because it can help scientists evaluate different theories about how the stars are structured. Even just knocking out sizes under 6.6 miles across eliminated a few models, although there are still a few to choose from.

And of course, although August's merger was the first such event scientists have spotted, they hope to catch plenty more similar incidents. The same gravitational wave detectors used to identify the two neutron stars colliding has already spotted five mergers of black holes since the first detection in February 2016, the most recent of which was announced last month.

More collisions, and more collisions with slight differences among them, will help astrophysicists pin down precisely what's happening inside of these superdense neutron stars.