The origin of the universe's heaviest elements has mystified scientists, but after Monday's (Oct. 16) historic announcement of the detection of gravitational waves produced by two colliding neutron stars, astronomers have struck gold — literally.
Researchers know that stars fuse light atomic nuclei to create heavier nuclei. Elements in the universe heavier than hydrogen (but lighter than iron) are created by a process known as stellar nucleosynthesis: nuclear reactions that occur deep inside stars' cores. But it has been a long-standing mystery as to where in the universe elements heavier than iron are synthesized, researchers said in a statement from the Max Planck Institute for Astrophysics (MPA) in Germany.
Though astrophysicists have theorized processes for how heavy elements like gold, platinum and lead are created in the cosmos, observational evidence has been scarce — until now. [Gravitational Waves from Neutron Star Crashes: The Discovery Explained]
"The origin of the really heaviest chemical elements in the universe has baffled the scientific community for quite a long time," Hans-Thomas Janka, a senior scientist at MPA, said in the statement. "Now, we have the first observational proof for neutron star mergers as sources; in fact, they could well be the main source of the r-process elements," which are elements heavier than iron, like gold and platinum.
After black holes, neutron stars are the densest known objects in the universe. Each is the size of a city, with a mass greater than that of Earth's sun; a teaspoon of this dense material would therefore weigh a billion tons. Neutron stars are created after stars more massive than Earth's sun explode as supernovas, leaving behind superdense magnetized balls of spinning matter composed mainly of neutrons, neutral particles that, along with protons, are found inside atomic nuclei.
Neutron stars therefore contain some of the building blocks of atomic nuclei. If these neutrons are somehow released from a neutron star, they might undergo reactions that allow them to stick together, creating elements heavier than iron.
For this process to work, however, it must be rapid, the researchers said.
Newly formed particles will be highly unstable and will lose neutrons, radioactively decaying into lighter particles. But if the surrounding environment is dense in free neutrons, more neutrons can be captured before the nuclei will decay, so heavier and heavier elements can be formed. And if a neutron star smashes into another neutron star, clumps of neutrons are blasted into space and can rapidly synthesize heavy elements like gold via a mechanism called rapid neutron capture process, or "r-process," according to an article released Oct. 16 in the journal Nature.
So, when astronomers confirmed the detection of the gravitational wave signal GW170817 that emanated from the site of a gamma-ray burst in a galaxy 130 million light-years away, they realized they were looking at an intense cosmic collision called a "kilonova." This was a ripe environment for the r-process to take place, the researchers said. Kilonovas are powerful explosions that unleash gamma-rays and have been long theorized to occur when neutron stars collide.
By comparing observations made using the Hubble Space Telescope and Gemini Observatory with theoretical models, astronomers have now confirmed that the r-process occurs in kilonovas, observing the spectroscopic fingerprint of heavy elements being created in the explosion's afterglow.
Researchers are witnessing a distant heavy-element factory synthesizing "maybe hundreds of Earth masses' [worth] of gold and … maybe 500 Earth masses' worth of platinum," theoretical astrophysicist Daniel Kasen, of the University of California, Berkeley, said in a new video.
With the help of the new gravitational wave signal, researchers now estimate that neutron star collisions may be responsible for the creation of most of the r-process heavy elements, like gold, found in galaxies, the Nature article said.
So, to paraphrase famed astronomer Carl Sagan, while we may be made of "star stuff," the ring on your finger is made of "neutron star stuff."
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Ian O'Neill is a media relations specialist at NASA's Jet Propulsion Laboratory (JPL) in Southern California. Prior to joining JPL, he served as editor for the Astronomical Society of the Pacific‘s Mercury magazine and Mercury Online and contributed articles to a number of other publications, including Space.com, Space.com, Live Science, HISTORY.com, Scientific American. Ian holds a Ph.D in solar physics and a master's degree in planetary and space physics.