When a massive star runs out of fuel and dies, it may go out in a blaze of glory, exploding as a supernova.
But supernovas aren't the only big booms out there. Enter the "kilonova." It's 1,000 times brighter than a nova (when a white dwarf erupts) but not as bright as a supernova. A kilonova is triggered by the collision of two stellar corpses. These events produce the most powerful electromagnetic explosions in the universe and are responsible for showering the universe in gold.
Neutron stars are the stellar corpses in question. Produced by supernovas, these extremely dense husks are left behind after massive stars end their lives. They are composed mainly of neutrons and measure about a dozen miles wide. But don't let their relatively diminutive size fool you. They pack the mass of an entire star (more massive than our sun) into their tiny volumes and possess intense magnetic fields. This means neutron stars are among the most extreme objects in the known universe. A teaspoonful of neutron star material weighs a cool 1 billion tons (907 million metric tons).
Neutron star matter doesn't act like normal matter. These gravitationally dominated objects crush all that they are made of into a "degenerate" state. That is, the pressures are so extreme that quantum mechanics is the only thing preventing their mass from collapsing in on itself and creating a black hole.
So, if two neutron stars collided, it would obviously be an incredibly violent and destructive event. On Aug. 17, scientists saw the aftermath of such a collision courtesy of the Laser Interferometer Gravitational-wave Observatory (Advanced LIGO) in the U.S. and the Virgo gravitational wave observatory in Italy. These advanced gravitational wave observatories detected a very strange, weak signal emanating from a galaxy called NGC 4993, 130 million light-years away.
Until that moment, gravitational wave detectors had only discerned the merger of black holes billions of light-years away, so to measure a weak signal at a comparatively close distance came as a surprise. After analysis of the telltale gravitational wave "chirp" (a rapid increase in frequency as two massive objects spin around each other, eventually colliding and merging), scientists realized that the signal, called GW170817, wasn't a black hole merger, it was in fact the merger of two neutron stars. The stars, with masses of only 1.1 and 1.6 suns, had become trapped in a gravitational dance, spiraling in on one another and colliding.
When the detection was made, NASA's Fermi gamma-ray observatory and Europe's INTEGRAL space telescope also recorded a powerful flash of gamma-ray radiation blasting from NGC 4993, known as a short gamma-ray burst (GRB).
Although scientists have theorized that short GRBs are generated by colliding neutron stars, only with the help of gravitational wave detectors could this be confirmed. This is the first time that scientists have measured both the gravitational waves and electromagnetic waves from a single cosmic event, connecting a GRB with a neutron star merger and opening a brand-new way to study the universe – known as "multi-messenger astronomy."
The gravitational waves helped us connect the GRB with the collision of neutron stars, but what created the GRB?
The neutron star merger that generated GW170817 was undoubtedly a violent one. As the two masses rapidly spun around each other and made contact, huge quantities of super-hot neutron star material were blasted into space. When this happened, it set the stage for some kilonova fireworks.
As neutron stars are composed mainly of neutrons, and neutrons are a key component (along with protons) of atomic nuclei, there were suddenly a LOT of subatomic building blocks flying around immediately after the neutron star smashup. The conditions were so extreme that this environment was ripe for chunks of radioactive neutron star material to stick together, creating new elements. Through a process called rapid neutron capture ("r-process"), neutrons attached themselves to the newly minted elements before they could radioactively decay. The creation of new elements generated an astounding amount of energy, erupting with powerful gamma-ray radiation, generating the GRB astronomers saw from 130 million light-years away.
Follow-up studies of the turbulent blast site by the Hubble Space Telescope, Gemini Observatory and ESO Very Large Telescope revealed spectroscopic evidence for the r-process having taken place. And this is special: In the remnants of the kilonova blast, vast quantities of heavy elements, like gold, platinum, lead, uranium and silver had been synthesized.
Scientists have long wondered how elements heavier than iron are created in our universe (elements lighter than iron are created via stellar nucleosynthesis in the cores of stars), but now we have observational evidence that these cataclysmic kilonovas are also cosmic foundries where the heaviest — and most precious — elements are seeded.
Editorial note: This article was corrected on Oct. 20, to rectify an inaccuracy introduced by the editor, misstating the brightness of kilonovas. Supernovas are, in fact, the brightest, followed by kilonovas and novas, respectively.