Astronomers finally find the cosmic source of gold and silver | Science News for Students

Astronomers finally find the cosmic source of gold and silver

Gravitational waves point to answers for some longstanding mysteries of the universe
Oct 16, 2017 — 4:21 pm EST

A smashup of two neutron stars is shown in this artist’s rendering. Such a collision sent out gravitational waves and a burst of gamma rays. The ejected debris also left a glow.

NSF, LIGO, A. Simonnet/Sonoma State Univ.

WASHINGTON, D.C. — Some 130 million years ago, the ultradense cores of two dead stars collided. The first evidence of the cataclysmic smashup were gravitational waves. They reached Earth on August 17. As astronomers rushed to home in on their source, they turned up a trove of riches. It is helping explain, among other things, the source of such precious metals as silver, gold and platinum.

This smashup marked the first direct sighting of a collision between two neutron stars. Researchers announced the event today at a news conference in Washington, D.C. Neutron stars are the remnants of aged stars that died in an explosion. The corpses of these stars are spectacularly dense. A single teaspoon of such neutron-rich material would carry a mass that on Earth would weigh roughly one billion tons.

Churning debris produced in the afterglow of the collision included newly created gold, silver and platinum. There was also a smattering of other heavy elements. Among those: uranium. Until now, the birthplace of such elements had been unknown. 

“It really is the last missing piece” of the periodic table, says Anna Frebel. An astronomer, she works at the Massachusetts Institute of Technology, in Cambridge. She was not involved in the new research.

Story continues below image.

elemental origins periodic table
The light emitted by a neutron star collision showed signs this event was a factory for new-elements (yellow). Some elements are produced in different ways, including through the explosion of huge stars and dying low-mass stars.
Robert Hurt/IPAC/Caltech

The extreme conditions produced in the newfound collision forged heavier elements than the parent stars had hosted. “This is heaven for anyone working in the field,” Frebel says. And this new-element factory was quite effective. For instance, scientists have just calculated that the smashup produced about 10 to 100 times Earth’s mass in gold!

It took a scientific village

To probe the event, scientists used data gathered from about 70 different observatories. What they learned from poring over those data resulted in at least 20 new research papers. All were released today. Included among their data had been a tremor of gravitational waves. The Advanced Laser Interferometer Gravitational-Wave Observatory, or LIGO, spotted it two months ago. This was the first sign of the smashup.

light wavelengths emitted by neutron stars merging
The afterglow of two merging neutron stars emitted a variety of wavelengths of light that were captured by telescope. Those emissions ranged from the ultraviolet (left, image from NASA’s Swift satellite) and infrared (middle, image from the Gemini South telescope) to radio waves (right, image from the Very Large Array).
Robert Hurt/IPAC/Caltech, Mansi Kasliwal and Gregg Hallinan/Caltech, Phil Evans/NASA and the GROWTH collaboration

The National Science Foundation funds LIGO. Observes France Córdova, NSF’s director, “Already [LIGO] is transforming our understanding of the universe.” She says it's offering “a fresh narrative of the physics of stars in their death throes.”

Like musical instruments taking turns in a symphony, a sequence of various types of electromagnetic radiation (light) followed that first gravitational trill. It started with a burst of gamma rays. Then came a glow of visible and infrared light. These were first spotted about 12 hours after the gravity waves. More than a week later, as those wavelengths faded away, X-rays crescendoed. Radio waves then followed.

Astrophysicists have long dreamed of being able to combine gravitational waves with light to study a neutron-star merger. “The picture that you can put together by having all of those sources is synergistic,” says David Shoemaker. By that he means it’s greater than the sum of the parts. Shoemaker, at MIT, is a spokesperson for LIGO.

It started with gravity waves

LIGO consists of two detectors. One sits in Washington State, another in Louisiana. Both registered an unmistakable sign of the upheaval. It was a shimmying of space itself. It lasted for about 100 seconds before cutting off. It was the strongest and longest series of spacetime ripples LIGO had ever seen.

At once, scientists knew they had something big, says Vicky Kalogera. She’s a LIGO member at Northwestern University in Evanston, Ill. Emails quickly circulated, she said, saying “Oh my God, this is it.”

That ripple in spacetime was an indication of a cosmic crash. As if on an ill-fated merry-go-round, two orbiting neutron stars whirled around each other. They spiraled closer and closer. Eventually, they merged. Each neutron star had a mass a bit greater than that of the sun. The pairing probably collapsed into a black hole (although LIGO scientists can’t be certain). LIGO has previously spotted mergers of swirling black holes having masses tens of times that of the sun. The smaller masses of the orbiting duo suggested that this time the smashup involved neutron stars. And because black holes aren’t expected to emit light, the fireworks that followed confirmed neutron stars as the source.

A sister gravity wave detector, Virgo, exists in Italy. It saw only a faint signal. That helped narrow down where to look for signs of that smashup. It was “a part of the sky that was a blind spot of Virgo,” Kalogera says. Knowing that helped astronomers find the event’s general location in the southern sky.

Just two seconds after the gravity-wave signal, NASA’s Fermi space telescope spotted a glimmer of gamma rays in the same part of the sky. Quickly, other telescopes swung into action. One by one, they began picking up a glow where none had been.

neutron star collision pinpoint light
Telescopes used the gravitational waves from the collision of two neutron stars to find the pinpoint of light (right, indicated with red lines) that marks the event. It was light where none had been before (left).
P.K. Blanchard, E. Berger, Pan-STARRS, DECam

“We saw what looked like a new star,” says Edo Berger. He’s an astronomer at Harvard University in Cambridge, Mass. He led a team in South America that spotted the light using a camera on the Blanco telescope in Chile. His was one of several teams that observed the blast’s light. That detection pinpointed in which galaxy the collision had occurred. It was NGC 4993. The actual smashup now appears to have taken place 130 million light-years from Earth in the constellation Hydra.

“There was this moment of disbelief” after putting all of that data together. Then, Berger recalls thinking, “Wow, we actually did it. We found it!”

Precious element factory

That afterglow also revealed the birth of elements. As the collision spurted neutron-rich material into space, a variety of heavy elements formed through a chain of nuclear reactions known as the “r-process.” It requires an environment crammed with neutrons. When the conditions are right, atomic nuclei rapidly gobble up neutrons and then undergo radioactive decay. This transforms old elements into newer, bigger ones, before they resume their neutron gorge-fest. The r-process is thought to produce about half of all elements heavier than iron.

In follow-up observations, astronomers picked up the characteristic glow of this process. It’s known as a kilonova (KIL-uh-NO-vuh). “Until this event, we had never directly seen anywhere in nature these heavy elements being forged. Now we have,” says Brian Metzger. He’s a theoretical astrophysicist at Columbia University in New York City. Suddenly, he says, it seems “like you’ve discovered some kind of secret of nature.”

Astrophysicists had disagreed about where the r-process takes place. Two top candidates were exploding stars, called supernovas, and neutron-star mergers. Although scientists can’t yet say whether all r-process elements come from colliding neutron stars, the amount of heavy elements that such smashups should produce appears large enough to explain the amounts of these heavy elements that exist in our universe.

gamma rays
NASA’s Fermi space telescope detected gamma rays from within the region (outlined in yellow) that gravity waves suggested the neutron-star collision occurred. Visible light from the crash allowed scientists to pinpoint the galaxy where this occurred (red dot).
Stefano Valenti/UC Davis, DLT40 survey

The collision also yielded something known as a short gamma-ray burst. This brief spurt of high-energy light lasted less than two seconds. Such bursts appear in the sky about 50 times a year. Understanding where they come from has been “a long-standing problem in astrophysics,” says Rosalba Perna. She’s a theoretical astrophysicist at Stony Brook University in New York. The new data have now clinched it: Short gamma-ray bursts come from neutron star mergers.

By studying how the neutron stars spiraled inward, astrophysicists also got their first chance to test the “squishiness” of neutron-star material. Scientists don’t fully understand how that material responds when squeezed. Although the results couldn’t pin down whether the neutron stars were squishy, some theories had predicted they could be ultrasquishy. The new data rule that out.

Finally, the new union of neutron stars allowed researchers to gauge the rate at which the universe is expanding. They did it by measuring the distance of the collision using gravity waves. Then they compared this to how much the wavelength of light from the galaxy was stretched by that expansion.

Scientists had measured this property — known as the Hubble constant — before. But they used other means. And those measurements disagreed with each other. That puzzled scientists. Now, scientists have “a totally different, independent measurement,” says Daniel Holz. He’s a LIGO-collaboration member at the University of Chicago in Illinois. The new measurement indicates that distantly separated galaxies are spreading apart at about 70 kilometers (43.5 miles) per second for each megaparsec between them. (A megaparsec is a distance in space equal to 3.3 million light-years.) The new estimate falls squarely between the two previous estimates. And future neutron-star mergers could help improve the measurement.

“These are all just unbelievable, major advances,” Holz says. “It’s really been this insane thrill!”

The excitement has yet to die down. Take it from astronomer Ryan Foley of the University of California, Santa Cruz. His team was the first to spot visible light from the merger. And he says: “This is certainly the biggest discovery of my career and probably will be the biggest discovery of my entire life.”

Caption: Researchers announced October 16 that Advanced LIGO (the Laser Interferometer Gravitational-Wave Observatory) and its sister experiment, Advanced Virgo, have detected gravitational waves from colliding neutron stars. It was a cosmic crash also observed by more than 70 observatories around the world.
Helen Thompson/Science News/YouTube

Power Words

(more about Power Words)

astronomy     The area of science that deals with celestial objects, space and the physical universe. People who work in this field are called astronomers.

astrophysics     An area of astronomy that deals with understanding the physical nature of stars and other objects in space. People who work in this field are known as astrophysicists.

atomic     Having to do with atoms, the smallest possible unit that makes up a chemical element.

black hole     A region of space having a gravitational field so intense that no matter or radiation (including light) can escape.

cataclysm     An enormous, violent, natural event. A meteor hitting Earth and wiping out most living species would qualify as a cataclysmic event.

constant     Continuous or uninterrupted.

constellation     Patterns formed by prominent stars that lie close to each other in the night sky. Modern astronomers divide the sky into 88 constellations, 12 of which (known as the zodiac) lie along the sun’s path through the sky over the course of a year. Cancri, the original Greek name for the constellation Cancer, is one of those 12 zodiac constellations.

core     Something — usually round-shaped — in the center of an object.

cosmic     An adjective that refers to the cosmos — the universe and everything within it.

debris     Scattered fragments, typically of trash or of something that has been destroyed. Space debris, for instance, includes the wreckage of defunct satellites and spacecraft.

decay      (for radioactive materials) The process whereby a radioactive isotope — which means a physically unstable form of some element — sheds energy and subatomic particles. In time, this shedding will transform the unstable element into a slightly different but stable element. For instance, uranium-238 (which is a radioactive, or unstable, isotope) decays to radium-222 (also a radioactive isotope), which decays to radon-222 (also radioactive), which decays to polonium-210 (also radioactive), which decays to lead-206 — which is stable. No further decay occurs. The rates of decay from one isotope to another can range from timeframes of less than a second to billions of years.

electromagnetic     An adjective referring to light radiation, to magnetism or to both.

electromagnetic radiation     Energy that travels as a wave, including forms of light. Electromagnetic radiation is typically classified by its wavelength. The spectrum of electromagnetic radiation ranges from radio waves to gamma rays. It also includes microwaves and visible light.

element     (in chemistry) Each of more than one hundred substances for which the smallest unit of each is a single atom. Examples include hydrogen, oxygen, carbon, lithium and uranium.

field     An area of study, as in: Her field of research was biology. Also a term to describe a real-world environment in which some research is conducted, such as at sea, in a forest, on a mountaintop or on a city street. It is the opposite of an artificial setting, such as a research laboratory.

galaxy     A massive group of stars bound together by gravity. Galaxies, which each typically include between 10 million and 100 trillion stars, also include clouds of gas, dust and the remnants of exploded stars.

gamma rays     High-energy radiation often generated by processes in and around exploding stars. Gamma rays are the most energetic form of light.

gauge     A device to measure the size or volume of something. For instance, tide gauges track the ever-changing height of coastal water levels throughout the day. Or any system or event that can be used to estimate the size or magnitude of something else. (v. to gauge) The act of measuring or estimating the size of something.

heavy element     (to astronomers) Any element other than hydrogen (or possibly helium).

Hubble Constant      A measure of the rate at which our universe appears to be expanding.we

infer     (n. inference) To conclude or make some deduction based on evidence, data, observations or similar situations.

infrared light     A type of electromagnetic radiation invisible to the human eye. The name incorporates a Latin term and means “below red.” Infrared light has wavelengths longer than those visible to humans. Other invisible wavelengths include X-rays, radio waves and microwaves. Infrared light tends to record the heat signature of an object or environment.

iron     A metallic element that is common within minerals in Earth’s crust and in its hot core. This metal also is found in cosmic dust and in many meteorites.

laser     A device that generates an intense beam of coherent light of a single color. Lasers are used in drilling and cutting, alignment and guidance, in data storage and in surgery.

light-year     The distance light travels in one year, about 9.46 trillion kilometers (almost 6 trillion miles). To get some idea of this length, imagine a rope long enough to wrap around the Earth. It would be a little over 40,000 kilometers (24,900 miles) long. Lay it out straight. Now lay another 236 million more that are the same length, end-to-end, right after the first. The total distance they now span would equal one light-year.

LIGO     (short for Laser Interferometer Gravitational wave Observatory) A system of two detectors, separated at a great geographical distance, that are used to register the presence of passing gravitational waves.

mass     A number that shows how much an object resists speeding up and slowing down — basically a measure of how much matter that object is made from.

metal     Something that conducts electricity well, tends to be shiny (reflective) and malleable (meaning it can be reshaped with heat and not too much force or pressure). 

NASA     Short for the National Aeronautics and Space Administration. Created in 1958, this U.S. agency has become a leader in space research and in stimulating public interest in space exploration. It was through NASA that the United States sent people into orbit and ultimately to the moon. It also has sent research craft to study planets and other celestial objects in our solar system.

neutron     A subatomic particle carrying no electric charge that is one of the basic pieces of matter. Neutrons belong to the family of particles known as hadrons.

neutron star     The very dense corpse of what had once been a star with a mass four to eight times that of our sun. As the star died in a supernova explosion, its outer layers shot out into space. Its core then collapsed under its intense gravity, causing protons and electrons in its atoms to fuse into neutrons (hence the star’s name). Astronomers believe neutron stars form when large stars undergo a supernova but aren’t massive enough to form a black hole. A single teaspoonful of a neutron star, on Earth, would weigh a billion tons.

nuclear reaction     Events that physically alter the nucleus of an atom. (This is in contrast to chemical reactions that affect the electrons orbiting an atom.) Some nuclear reactions will transmute an atom, change it into a different chemical element, such as through fission (also known as atom splitting). Others may involve the capture of energy by bombardment with electromagnetic radiation or subatomic particles. Nuclear reactions are not affected by temperature and pressure (as chemical reactions may be). Instead, they are driven primarily by the energy of the particle that hits them or by the intensity of the radiation prompting the reaction.

observatory     (in astronomy) The building or structure (such as a satellite) that houses one or more telescopes.

platinum     A naturally occurring silver-white metallic element that remains stable (does not corrode) in air. It is used in jewelry, electronics, chemical processing and some dental crowns.

pressure     Force applied uniformly over a surface, measured as force per unit of area.

radiation     (in physics) One of the three major ways that energy is transferred. (The other two are conduction and convection.) In radiation, electromagnetic waves carry energy from one place to another. Unlike conduction and convection, which need material to help transfer the energy, radiation can transfer energy across empty space.

radio     To send and receive radio waves, or the device that receives these transmissions.

radioactive     An adjective that describes unstable elements, such as certain forms (isotopes) of uranium and plutonium. Such elements are said to be unstable because their nucleus sheds energy that is carried away by photons and/or and often one or more subatomic particles. This emission of energy is by a process known as radioactive decay.

radioactive decay     A process by which an element is converted into a lighter element through the shedding of subatomic particles (and energy).

radio waves     Waves in a part of the electromagnetic spectrum. They are a type that people now use for long-distance communication. Longer than the waves of visible light, radio waves are used to transmit radio and television signals. They also are used in radar.

remnant     Something that is leftover — from another piece of something, from another time or even some features from an earlier species.

spacetime     A term made essential by Einstein’s theory of relativity, it describes a designation for some spot given in terms of its three-dimensional coordinates in space, along with a fourth coordinate corresponding to time.

star     The basic building block from which galaxies are made. Stars develop when gravity compacts clouds of gas. When they become dense enough to sustain nuclear-fusion reactions, stars will emit light and sometimes other forms of electromagnetic radiation. The sun is our closest star.

sun     The star at the center of Earth’s solar system. It’s an average size star about 26,000 light-years from the center of the Milky Way galaxy. Also a term for any sunlike star.

supernova     (plural: supernovae or supernovas) A massive star that suddenly increases greatly in brightness because of a catastrophic explosion that ejects most of its mass.

telescope     Usually a light-collecting instrument that makes distant objects appear nearer through the use of lenses or a combination of curved mirrors and lenses. Some, however, collect radio emissions (energy from a different portion of the electromagnetic spectrum) through a network of antennas.

theoretical     An adjective for an analysis or assessment of something that based on pre-existing knowledge of how things behave. It is not based on experimental trials. Theoretical research tends to use math — usually performed by computers — to predict how or what will occur for some specified series of conditions. Experimental testing or observations of natural systems will then be needed to confirm what had been predicted.

trill     A fluttering but fast song, usually oscillating between two or more notes.

universe     The entire cosmos: All things that exist throughout space and time. It has been expanding since its formation during an event known as the Big Bang, some 13.8 billion years ago (give or take a few hundred million years).

uranium     The heaviest naturally occurring element known. It’s called element 92, which refers to the number of protons in its nucleus. Uranium atoms are radioactive, which means they decay into different atomic nuclei.

wave     A disturbance or variation that travels through space and matter in a regular, oscillating fashion.

wavelength     The distance between one peak and the next in a series of waves, or the distance between one trough and the next. Visible light — which, like all electromagnetic radiation, travels in waves — includes wavelengths between about 380 nanometers (violet) and about 740 nanometers (red). Radiation with wavelengths shorter than visible light includes gamma rays, X-rays and ultraviolet light. Longer-wavelength radiation includes infrared light, microwaves and radio waves.

X-ray     A type of radiation analogous to gamma rays, but having somewhat lower energy.


Journal: B. P. Abbott et al. GW170817: Observation of gravitational waves from a binary neutron star inspiral. Physical Review Letters. Published online October 16, 2017. doi: 10.1103/PhysRevLett.119.161101.

Journal: B. P. Abbott et al. Multi-messenger observations of a binary neutron star merger. The Astrophysical Journal Letters. Published online October 16, 2017. doi: 10.3847/2041-8213/aa91c9.

Journal: B. P. Abbott et al. Gravitational waves and gamma-rays from a binary neutron star merger: GW170817 and GRB 170817A. The Astrophysical Journal Letters. Published online October 16, 2017. doi: 10.3847/2041-8213/aa920c.

Journal: I. Arcavi et al. Optical follow-up of gravitational-wave events with Las Cumbres Observatory. The Astrophysical Journal Letters. Published online October 16, 2017. doi:10.3847/2041-8213/aa910f.

Journal: I. Arcavi et al. Optical emission from a kilonova following a gravitational-wave-detected neutron-star merger. Nature. Published online October 16, 2017. doi:10.1038/nature24291.

Journal: D.A. Coulter et al. Swope Supernova Survey 2017a (SSS17a), the optical counterpart to a gravitational wave sourceScience. Published online October 16, 2017. doi: 10.1126/science.aap9811.

Journal: S. Covino et al. The unpolarized macronova associated with the gravitational wave event GW 170817. Nature Astronomy. Published online October 16, 2017. doi: 10.1038/s41550-017-0285-z.

Journal: M.C. Diaz et al. Observations of the first electromagnetic counterpart to a gravitational-wave source by the TOROS collaboration. The Astrophysical Journal Letters. Published online October 16, 2017. doi: 10.3847/2041-8213/aa9060.

Journal: M.R. Drout et al. Light curves of the neutron star merger GW170817/SSS17a: Implications for r-process nucleosynthesisScience Published online October 16, 2017. doi: 10.1126/science.aaq0049.

Journal: P.A. Evans et al. Swift and NuSTAR observations of GW170817: Detection of a blue kilonovaScience. Published online October 16. doi: 10.1126/science.aap9580.

Journal: G. Hallinan et al. A radio counterpart to a neutron star mergerScience. Published online October 16, 2017. doi: 10.1126/science.aap9855.

Journal: J. Hjorth et al. The distance to NGC 4993: The host galaxy of the gravitational-wave event GW170817. The Astrophysical Journal Letters. Published online October 16, 2017. doi: 10.3847/2041-8213/aa9110.

Journal: D. Kasen et al. Origin of the heavy elements in binary neutron star mergers from a gravitational wave event. Nature. Published online October 16, 2017. doi:1038/nature24453.

Journal: M.M. Kasliwal et al. Illuminating gravitational waves: A concordant picture of photons from a neutron star mergerScience. Published online October 16, 2017. doi: 10.1126/science.aap9455.

Journal: C.D. Kilpatrick et al. Electromagnetic evidence that SSS17a is the result of a binary neutron star mergerScience. Published online October 16, 2017. doi: 10.1126/science.aaq0073.

Journal: The LIGO Scientific Collaboration and The Virgo Collaboration, The 1M2H Collaboration, The Dark Energy Camera GW-EM Collaboration and the DES Collaboration, The DLT40 Collaboration, The Las Cumbres Observatory Collaboration, The VINROUGE Collaboration & The MASTER Collaboration. A gravitational-wave standard siren measurement of the Hubble constant. Nature. Published online October 16, 2017. doi:10.1038/nature24471.

Journal: E. Pian et al. Spectroscopic identification of r-process nucleosynthesis in a double neutron-star merger. Nature. Published online October 16, 2017. doi: 10.1038/nature24298.

Journal: B.J. Shappee et al. Early spectra of the gravitational wave source GW170817: Evolution of a neutron star mergerScience. Published online October 16, 2017. doi: 10.1126/science.aaq0186.

Journal: S.J. Smartt et al. A kilonova as the electromagnetic counterpart to a gravitational wave source. Nature. Published online October 16, 2017. doi: 10.1038/nature24303.

Journal: E. Troja et al. The X-ray counterpart to the gravitational-wave event GW170817. Nature. Published online October 16, 2017. doi: 10.1038/nature24290.