Short answer: On August 17, 2017, the LIGO and Virgo detectors picked up gravitational waves from two neutron stars colliding about 130 million light-years away, and telescopes saw the same event in light less than two seconds later. It was the first cosmic event observed in both gravitational waves and light, opening the era of multimessenger astronomy.
Here is what happened, why it mattered, and what it means for students who love physics.
The LIGO and Virgo press conference announcing the neutron star merger discovery in October 2017.
Key points
- Two neutron stars: Ultradense remnants of collapsed stars spiraled together and merged.
- Seen two ways: Gravitational wave detectors and telescopes caught the same event.
- A kilonova: The collision produced a bright, fast-fading explosion never seen before.
- Gold and platinum: Mergers like this forge many of the universe’s heavy elements.
What did scientists detect?
At 8:41 a.m. Eastern time on August 17, 2017, a gravitational wave signal reached the Virgo detector in Italy and the two LIGO detectors in Louisiana and Washington state. Unlike the split-second “chirps” from black hole mergers, this signal lasted about 100 seconds, a sign of smaller, lighter objects.
Less than two seconds later, NASA’s Fermi Space Telescope recorded a short burst of gamma rays from the same patch of sky. Within hours, observatories around the world were searching that region.
Astronomers found a new point of light next to the galaxy NGC 4993. According to the LIGO press release, about 70 ground- and space-based observatories went on to study it.
What is a kilonova?
A kilonova is the explosion that happens when two neutron stars merge. Neutron stars are the collapsed cores of massive stars, made almost entirely of neutrons and incredibly dense.
Until 2017, kilonovas were only predicted by theory. This one glowed blue at first, then turned red and faded within days, just as models predicted.
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Why was this a new era for astronomy?
- Multimessenger astronomy: For the first time, scientists observed one event with both gravity and light, so each could fill in what the other missed.
- Where gold comes from: The light from the kilonova showed signatures of heavy elements, supporting the idea that neutron star mergers make much of the universe’s gold and platinum.
- Einstein passes another test: Because the gamma rays arrived so soon after the gravitational waves, the event confirmed that gravitational waves travel at the speed of light.
- A new way to measure the universe: Events like this give an independent way to measure how fast the universe is expanding.
The discovery came just weeks after three LIGO founders won the 2017 Nobel Prize in Physics for the first detection of gravitational waves.
Tutor tip: Gravitational waves come straight out of Einstein’s general relativity. If this story sparks your curiosity, start with our theory of relativity series, and for stargazing at home see astronomy for beginners.
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Frequently asked questions
What are gravitational waves?
Gravitational waves are ripples in space-time produced by massive objects accelerating, such as black holes or neutron stars orbiting and merging. Einstein predicted them about a century ago, and LIGO first detected them in 2015.
When were gravitational waves from a neutron star merger first detected?
On August 17, 2017, when the LIGO and Virgo detectors recorded a signal from two neutron stars merging about 130 million light-years away.
What is multimessenger astronomy?
Multimessenger astronomy studies the same cosmic event through different signals, such as gravitational waves and light. The 2017 neutron star merger was the first event observed both ways.
Do neutron star mergers create gold?
Observations of the 2017 kilonova showed signatures of heavy elements, supporting the idea that neutron star mergers produce much of the universe’s gold and platinum.
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