4 min read

When Dead Stars Collide!

Gravity has been making waves — literally. In October 2017, the Nobel Prize in Physics was awarded for the first direct detection of gravitational waves two years earlier. Also in that month, astronomers announced a huge advance in the field of gravitational waves: For the first time, they had observed light and gravitational waves from the same source. Let’s look at what happened.

Two glowing stars with red and black surfaces sit in the middle of a starfield.
Two neutron stars are on the verge of colliding in this illustration.
NASA's Goddard Space Flight Center

There was a pair of orbiting neutron stars in a galaxy (called NGC 4993). Neutron stars are the crushed leftover cores of massive stars (stars more than 8 times the mass of our sun) that long ago exploded as supernovae. There are many such pairs of binaries in this galaxy, and in all the galaxies we can see, but something special was about to happen to this particular pair.

Two blue spheres circle each other on a grid representing space-time. As the spheres orbit, ripples propagate outward along the grid, representing gravitational waves.
An animation of gravitational wave propagation.
R. Hurt/Caltech/JPL

Each time these neutron stars orbited, they would lose a teeny bit of gravitational energy to gravitational waves. Gravitational waves are disturbances in space-time — the very fabric of the universe — that travel at the speed of light. The waves are emitted by any mass that is changing speed or direction, like this pair of orbiting neutron stars. However, the gravitational waves are very faint unless the neutron stars are very close and orbiting around each other very fast.

Two bright spheres orbit each other, and pale arcs of blue, representing gravitational waves, ripple away from the spheres. The spheres get closer with each orbit, and as they do they distort, turning into teardrop shapes, with the points pointing toward the center. Then they touch and finally merge in a bright, white explosion.
Doomed neutron stars whirl toward their demise in this illustration. Gravitational waves (pale arcs) bleed away orbital energy, causing the stars to move closer together and merge.
NASA's Goddard Space Flight Center/Conceptual Image Lab

The teeny energy loss caused the two neutron stars to get a teeny bit closer to each other and orbit a teeny bit faster. After hundreds of millions of years, all those teeny bits added up, and the neutron stars were very close. So close that … BOOM! … they collided. And we witnessed it on Earth on August 17, 2017.

At the center of this illustration is a bright region of light that looks like two balls that haven’t quite merged into one. Two rays of white and orange light emanate from that central collision, one up and to the right, the other down and to the left, though you can’t see the one to the left quite as well because there is also a disk of swirling material blocking the view. There is also a faint grid across the entire image, representing space-time. Ripples in the grid can be seen at the edges of the image, showing gravitational waves that had been emitted by the merger.
Illustration of two merging neutron stars. The rippling space-time grid represents gravitational waves that travel out from the collision. Narrow beams show the burst of gamma rays that are shot out just seconds after the gravitational waves. The swirling clouds of material are ejected from the merging stars.
National Science Foundation/LIGO/A. Simonnet (Sonoma State Univ.)

A couple of very cool things happened in that collision, and we expect they happen in all such neutron-star collisions. Just before the neutron stars collided, the gravitational waves were strong enough and at just the right frequency that the National Science Foundation’s Laser Interferometer Gravitational-Wave Observatory (LIGO) and European Gravitational Observatory’s Virgo could detect them. Just after the collision, those waves quickly faded out because there are no longer two things orbiting around each other!

LIGO and Virgo are ground-based detectors waiting for gravitational waves to pass through their facilities on Earth. When it is active, it can detect them from almost anywhere in space.

This animation of a gamma-ray burst shows two jets of material that look like two orange cones connected by their points and facing in opposite directions, one opening up and to the right of the center, the other opening down and to the left. The ends of the cones have bright magenta light, which represent an expanding shock wave. At the center is a wrapped-candy-shaped blue structure lined up with the jets which represents the kilonova, the neutron-rich debris of the explosion.
This illustration shows a snapshot of a gamma-ray burst caused by the merger of two neutron stars. Powerful jets (orange) emerge and plow into their surroundings, causing shock waves (pink). Just emerging at the center is the kilonova, the neutron-rich debris of the explosion (blue) powered by the decay of newly forged radioactive elements.
NASA's Goddard Space Flight Center/Conceptual Image Lab

The other thing that happened was what we call a gamma-ray burst. When they get very close, the neutron stars break apart and create a spectacular, but short, explosion. For a couple of seconds, our Fermi satellite saw gamma rays from that explosion. Fermi’s Gamma-ray Burst Monitor is one of our eyes on the sky, looking out for such bursts of gamma rays that scientists want to catch as soon as they’re happening.

And those gamma rays came just 1.7 seconds after the gravitational wave signal. The galaxy this occurred in is 130 million light-years away, so the light and gravitational waves were traveling for 130 million years before we detected them.

This animation GIF has text “Swift Ultraviolet light,” and shows the UV light Swift detected on August 18 and 29, fading between the two. The image shows the sky, black with several small circles of light. On August 18, the central source appears as a small yellow blob with a second white ball just to the side of it. On August 29, the white ball has disappeared, leaving just the larger yellow source.
NASA's Neil Gehrels Swift Observatory imaged the kilonova produced by merging neutron stars in the galaxy NGC 4993 (box) on Aug. 18, 2017, about 15 hours after gravitational waves and the gamma-ray burst were detected. Inset: Magnified views of the galaxy.

After that initial burst of gamma rays, the debris from the explosion continued to glow, fading as it expanded outward. Our Swift, Hubble, Chandra, and Spitzer telescopes, along with a number of ground-based observatories, were poised to look at this afterglow from the explosion in ultraviolet, optical, X-ray, and infrared light. Such coordination between satellites is something that we’ve been doing with our international partners for decades, so we catch events like this one as quickly as possible and in as many wavelengths as possible.

This animated GIF shows the region of the sky where the gravitational waves and gamma-ray burst were detected as seen by Hubble in visible light and Chandra in X-ray light, fading between the two. In visible light, there is a bright oval-shaped galaxy that takes up most of the image with a bright, white center region that fades into gray clouds around it. The site of the gamma-ray burst is outlined in a box, and shows a dim source in visible light about half way between the center of the galaxy and its edge. In X-ray light, the galaxy’s center and a couple of other sources appear as dots encircled in blue. The galaxy itself does not show up in X-rays. The site of the gamma-ray burst is a bright blue source.
The kilonova associated with GW170817 (box) was observed by NASA's Hubble Space Telescope and Chandra X-ray Observatory. Hubble detected optical and infrared light from the hot expanding debris. Nine days later, Chandra detected the X-ray afterglow emitted by the jet directed toward Earth after it had spread into our line of sight.

Astronomers have thought that neutron star mergers were the cause of one type of gamma-ray burst — a short gamma-ray burst, like the one they observed on August 17. It wasn’t until we could combine the data from our satellites with the information from LIGO/Virgo that we could confirm this directly.

When this animation opens, there are concentric rings of pale blue the expand away and off the screen. At the center is a bright ball of light with two narrow cones of orange, fiery-looking material extend in opposing directions, tilted just to the right. During the first few seconds, there are magenta flashes of light that seem to be pushed along with the ends of the orange cones. The central ball expands into a puffy, electric blue cloud. The sequence represents the events that happened after two neutron stars merged, exploding in a gamma-ray burst.
This animation captures phenomena observed over the course of nine days following the neutron star merger known as GW170817, detected on Aug. 17, 2017. They include gravitational waves (pale arcs), a near-light-speed jet that produced gamma rays (magenta), expanding debris from a kilonova that produced ultraviolet (violet), optical and infrared (blue-white to red) emission, and, once the jet directed toward us expanded into our view from Earth, X-rays (blue).
NASA's Goddard Space Flight Center/Conceptual Image Lab

That event began a new chapter in astronomy. For centuries, light was the only way we could learn about our universe. Now, we’ve opened up a whole new window into the study of neutron stars and black holes. This means we can see things we could not detect before.

This animated GIF shows a fun animation of what happened on Aug. 17, 2017. The scene shows Earth on the left side with a cartoon depiction of the Fermi satellite near the center. The image appears to ripple, starting from a dot on the upper right of the image. A speech bubble raises from two site on Earth that says, “Did you hear that?” Then, 1.7 seconds after the ripple, a magenta “blast” of light appears where the ripple originated, and the Fermi telescope has a speech bubble that says, “I sure saw it!”
On Aug. 17, gravitational waves from merging neutron stars reached Earth. Just 1.7 seconds after that, NASA's Fermi saw a gamma-ray burst from the same event. Now that astronomers can combined what we can “see” (light) and what we can “hear” (gravitational waves) from the same event, our ability to understand these extreme cosmic phenomena is greatly enhanced.
NASA's Goddard Space Flight Center

The first LIGO detection was of a pair of merging black holes. Mergers like that may be happening as often as once a month across the universe, but they do not produce much light because there’s little to nothing left around the black hole to emit light. In that case, gravitational waves were the only way to detect the merger.

The neutron star merger, though, has plenty of material to emit light. By combining different kinds of light with gravitational waves, we are learning how matter behaves in the most extreme environments. We are learning more about how the gravitational wave information fits with what we already know from light — and in the process we’re solving some long-standing mysteries!