Gold may be a glittery leftover from a newborn black hole’s messy first meal.
Gold is a heavy element. So is platinum. And uranium. These and many other heavy elements might form when rapidly spinning, massive stars collapse into newly formed black holes. Known as collapsars, these stars get their name from that collapse. And as this last dying stage of their lives take place, layers of gas around them explode. That collapse and explosion leave a disk of material swirling around each new black hole. When that black hole devours the surrounding material, the conditions become just right for gold, platinum and other heavy elements to form, scientists now report.
“Black holes in these extreme environments are fussy eaters,” says Brian Metzger. He is an astrophysicist at Columbia University in New York City. These black holes can gulp down only so much matter at a time. What they don’t swallow blows off in a wind. This wind has lots of neutrons — subatomic particles having no electric charge. With a lot of them in the wind, it makes just the right conditions for the creation of heavy elements, Metzger says. At least that’s what simulations from his team’s new computer model suggest.
Metzger and his colleagues described those simulations and their results online May 8 in Nature.
Cooking up the elements
Their team has been trying to answer an age-old question: Where do the heaviest elements in the universe come from?
Astronomers know certain elements form inside stars and then spew into space when dying stars explode. These are elements such as carbon, oxygen and iron. Scientists call these light elements as they have less mass than those such as gold and platinum.
Stars can’t make elements that are heavier than iron (like gold and platinum). To get such heavies there’s got to be a lot of neutrons. And they’ve got to be packed together tightly, creating an extreme environment. And in it, the centers of atoms — nuclei — absorb neutrons. After absorbing a lot of them, an atom’s nucleus will become unstable. To stabilize itself once more, it undergoes radioactive decay. In that decay, a neutron changes into a proton. And that makes a new element. Astrophysicists refer to this chain of reactions as the r-process.
Scientists had suspected that elements made this way could emerge when two stars collide. Specifically, it would happen when the smashup involves two dead stars known as neutron stars.
Good evidence for the idea came out almost two years ago. That’s when astronomers spotted a collision between two neutron stars. It made waves that stretched and squeezed spacetime — the fabric of space. Astronomers call the ripples gravitational waves. Studying the smashup showed the neutron stars did spew out heavy elements, including gold, silver and platinum.
But the neutron-star idea explanation has shortcomings. Dense, dead stars can take a long time to collide. Heavy elements, however, have been found in ancient stars, ones that formed in the early universe. It’s not clear whether a neutron-star merger could happen that early in the history of the universe. But it would have to in order to explain the elements’ presence in those early stars.
Very old black holes
So if there weren’t neutron-star smashups back then, what made the heavy elements? Scientists think spinning stars that collapse into black holes — collapsars — could have occurred in the early days of the universe. And that process could be a prolific maker of heavy elements.
A single collapsar might generate 30 times as much r-process material as a neutron-star merger. A collapsar could generate a few hundred times the Earth’s mass in gold, Metzger now says. Collapsars, therefore, might be responsible for 80 percent of elements made by the r-process. Neutron-star mergers would make up the rest, Metzger and his colleagues suggest.
Their results shed new light on a 2016 discovery about a dwarf galaxy called Reticulum II. Its stars are rich in heavy elements. That means some type of catastrophe happened in the galaxy billions of years ago to make all those heavy elements. Scientists had thought an ancient neutron-star merger seeded this galaxy with those elements. A collapsar now becomes another candidate.
“It’s very exciting,” says Anna Frebel. She is an astrophysicist at the Massachusetts Institute of Technology in Cambridge and a coauthor of the 2016 study on Reticulum II. Neutron star mergers are rare. So “it felt a little bit like we were proposing to win the lottery,” she says. But collapsars are even more rare. For every 10 neutron-star mergers, there’s one collapsar. So, she notes, if collapsars are the explanation, “it feels like we’ve won the lottery twice.”
It’s still not clear if collapsars happen frequently enough to explain the abundances of heavy elements seen throughout the universe. It’s also not clear if they produce the right amount of material. “I think the jury’s still out,” says Alexander Ji. He is an astrophysicist at Carnegie Observatories in Pasadena, Calif. He coauthored the 2016 paper on Reticulum II.
“Now we’re really excitedly thinking about how you might be able to tell the difference,” Ji says — whether collapsars or neutron stars better explain galaxies such as Reticulum II. Future observations of the aftermath of collapsar explosions could be helpful. They could help nail down collapsars’ role in shaping such galaxies. The observations also might reveal whether collapsars really do litter the universe with heavy elements.
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.
atom The basic unit of a chemical element. Atoms are made up of a dense nucleus that contains positively charged protons and uncharged neutrons. The nucleus is orbited by a cloud of negatively charged electrons.
black hole A region of space having a gravitational field so intense that no matter or radiation (including light) can escape.
carbon The chemical element having the atomic number 6. It is the physical basis of all life on Earth. Carbon exists freely as graphite and diamond. It is an important part of coal, limestone and petroleum, and is capable of self-bonding, chemically, to form an enormous number of chemically, biologically and commercially important molecules.
coauthor One of a group (two or more people) who together had prepared a written work, such as a book, report or research paper. Not all coauthors may have contributed equally.
collapsar A rapidly spinning, massive star that in the final stage of its life collapses into a newly formed black hole. In the process, layers of gas surrounding the star will explode. As a black hole now devours all of this material, conditions can develop that spawn heavy elements, such as gold and platinum. Those new elements can live on throughout the life of the universe.
colleague Someone who works with another; a co-worker or team member.
computer model A program that runs on a computer that creates a model, or simulation, of a real-world feature, phenomenon or event.
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.
electric charge The physical property responsible for electric force; it can be negative or positive.
element A building block of some larger structure. (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.
environment The sum of all of the things that exist around some organism or the process and the condition those things create.
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.
gravitational waves A ripple in spacetime that can travel throughout the universe. It takes a very strong collision between celestial bodies to create gravitational waves that can be felt across long distances and time.
heavy element (to astronomers) Any element other than hydrogen (or possibly helium).
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.
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.
matter Something that occupies space and has mass. Anything on Earth with matter will have a property described as "weight."
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.
nucleus (in astronomy) The rocky body of a comet, sometimes carrying a jacket of ice or frozen gases. (in physics) The central core of an atom, containing most of its mass.
online (n.) On the internet. (adj.) A term for what can be found or accessed on the internet.
oxygen A gas that makes up about 21 percent of Earth's atmosphere. All animals and many microorganisms need oxygen to fuel their growth (and metabolism).
particle A minute amount of something.
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.
proton A subatomic particle that is one of the basic building blocks of the atoms that make up matter. Protons belong to the family of particles known as hadrons.
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.
simulation (v. simulate) An analysis, often made using a computer, of some conditions, functions or appearance of a physical system. A computer program would do this by using mathematical operations that can describe the system and how it might change over time or in response to different anticipated situations.
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.
subatomic Anything smaller than an atom, which is the smallest bit of matter that has all the properties of whatever chemical element it is (like hydrogen, iron or calcium).
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.
Journal: D.M. Siegel, J. Barnes and B.D. Metzger. Collapsars as a major source of r-process elements. Nature. Published online May 8, 2019. doi: 10.1038/s41586-019-1136-0.
Journal: A.P. Ji et al. R-process enrichment from a single event in an ancient dwarf galaxy. Nature. Vol. 531, March 31, 2016, p. 610. doi: 10.1038/nature17425.