Gravity and Light: When Neutron Stars Collide

Posted on Mon, October 16, 2017

The discovery announced today was a twofer.

For the first time ever, on August 17, 2017, astronomers detected the collision of two neutron stars. Not satisfied with that, they caught the cosmic smashup using both gravitational waves and light – another breakthrough.

An artist’s illustration of two merging neutron stars.

Artist’s illustration of two merging neutron stars.

What’s the big deal? To borrow an analogy attributed to France Córdova, director of the National Science Foundation: Regular astronomy is like watching a silent film. This is what humans have been doing since we have first gazed up at the sky. In recent centuries, we have innovated better ways to collect light, using telescopes, sophisticated detectors, and other tools like those visitors can see in our Explore the Universe gallery. We can now even study the universe using light invisible to our eyes, like ultraviolet rays and radio waves.

Less than two years ago, astronomers used a new instrument called LIGO to detect gravitational waves: ripples in the universe predicted by Albert Einstein. These ripples were created from moving masses, like two black holes spiraling toward each other. As these ripples pass over us, they warp distances.

A mathematical animation showing the gravitational waves produced as two black holes spiral toward each other and then collide.

This mathematical animation shows the gravitational waves produced as two black holes spiral toward each other and then collide.

The catch is that the ripples are tiny: Even a violent collision of two black holes generates ripples a thousandth of the diameter of a proton. After Einstein’s prediction, it took 99 years for scientists and engineers to develop an instrument sensitive enough to discover gravitational waves. And they did, in September 2015. Returning to Dr. Córdova’s analogy, detecting gravitational waves is kind of like hearing sound for the very first time. It is an entirely new kind of astronomy. But gravitational waves are hard to localize, so it’s like hearing sounds in a dark room.

Today’s announcement put those pieces together. For the first time, astronomers made a detection with gravitational waves, then immediately worked together to point many telescopes to that part of the sky and succeeded in making the same detection using light. Many kinds of light, actually – visible light, ultraviolet, radio, and more. The flash of light from this collision has already faded, but now we have the film and the soundtrack, too.

Comparing images from the Hubble Telescope and the Swope Telescope.

Left: a Hubble Space Telescope image of the area near the galaxy NGC 4993 in April 2017.  Right: an image of the same area from the Swope Telescope in Chile, in August 2017.  The new dot is light from the colliding neutron stars. 

And what is this first-ever complete picture showing us? The “chirp” of gravitational waves detected by LIGO on August 17 lasted about 100 seconds, much longer than chirps from detections of the collisions of black holes. This was an immediate tell-tale to the LIGO scientists that the waves could be from a kilonova: the collision of two neutron stars.

Neutron stars are the corpses of mid-sized stars, larger than the Sun. Formed almost entirely of neutrons, they are among the densest objects in the universe. If you could scoop out a teaspoonful of neutron star material, it would weigh as much as Mount Everest. Good luck getting it out, though, because the surface of a neutron star is 10 billion times stronger than steel, and with such a powerful gravitational pull that the teaspoon – and you – would be smashed onto its surface as a fine coating of atomic particles.

Simulation of the merger of two neutron stars. 

As you may be able to imagine, two neutron stars smashing together is quite a spectacle. Astronomers always thought it would be, and now they have the proof. The observations of the August kilonova, taken with a wide variety of telescopes, are being used by astronomers to show that most of the heavy elements in the universe – the gold in a ring, the uranium in a nuclear bomb – were created during neutron star collisions.

This is the beginning of a new era, one that weds gravity and light.