When neutron stars collide
Physicists have listened to and watched one of the most powerful and violent events in the universe. It unfolded exactly as their theories had predicted.
IN A GALAXY 130 million light-years away, two neutron stars were caught in a fatal embrace. They were remnants of exploded stars, only about 20 km wide and so dense that a teaspoon of their stuff weighed the same as Mt Everest. Each a little heavier than the Sun and spinning 1,000 times a second like a giant pair of kitchen blenders, they churned up spacetime as they drew closer together, sending gravitational waves rippling through space.
Their situation was not uncommon. The universe is studded with pairs of dangerously attracted neutron stars. Eventually, whirling almost as fast as light itself, our pair surrendered to gravity and collided.
An immense shock wave radiated out into the universe. One hundred and thirty million years later, it reached Earth. Astronomers had long predicted that such collisions would be commonplace, occurring somewhere about once every second. But this one was the first they were able to catch in the act.
At 12:41 universal time on 17 August 2017, three gravitational wave detectors on planet Earth picked up the shock wave. It sounded like a chirp. Over the course of 100 seconds its frequency climbed to a thousand cycles per second, a soprano’s high C – the predicted crescendo of a neutron star collision.
Two of the detectors, located 3000 km apart in opposite corners of the US, belonged to the Laser Interferometer Gravitational Wave Observatory (LIGO). The third was the Virgo detector in Italy.
About 1.7 seconds after alerts went off at LIGO and Virgo, the Fermi space telescope detected a short burst of intense gamma rays coming from the same part of the sky. Researchers believed that gamma ray bursts like this were also caused by neutron star collisions.
Using all three gravitational wave detectors and the gamma ray flash, astronomers were able to triangulate the source of the shockwave to a 30-degree arc of southern sky in the vicinity of the Hydra constellation. As dusk fell in Chile, a team from the University of California, Santa Cruz, started scanning the sky with the one-metre Swope telescope. About 10 hours later they found something conspicuous in a galaxy called NGC 4993: a new dot of bluish light that faded, turned red and disappeared over the course of several days.
What the astronomers saw was called a kilonova: intense light radiating from the halo of material that exploded out from the neutron star collision.
Modelling by Brian Metzger at Columbia University suggests the emissions of different colours of light are the signatures of heavy atoms being forged in this cosmic furnace. In the first few days the glow was bluish, corresponding to the formation of ‘lighter’ heavy metals. In the following days the glow becomes reddish, the signature of the heaviest elements like gold and platinum.
Physicists are ecstatic about their observations. They confirmed long-standing theories about neutron star collisions and the source of most of the bling in the universe. Metzger estimates the amount of gold created in this particular collision was 40 to 100 times the mass of the Earth.
Gold and platinum – symbols of undying passion – turn out to have been forged in the consuming passion of neutron stars.