For decades, scientists have moved slowly toward an explanation of how the heaviest elements in nature are made. Now, by observing the starlight from a dwarf galaxy 98,000 light-years away, researchers have found the first direct evidence for their source.
The lighter elements, from hydrogen to iron, are forged in the hot, high-pressure interiors of stars, but the heavier an element is, the more difficult it is to create. The creation of these heavy elements, called “r-process” elements, requires extreme circumstances; understanding where they come from is “one of the hardest problems in nuclear physics,” according to MIT physicist Anna Frebel. “The production of these really heavy elements takes so much energy that it’s nearly impossible to make them experimentally. The process for making them just doesn’t work on Earth. So we have had to use the stars and the objects in the cosmos as our lab.”
Since the 1950s, physicists have been searching the cosmos for the forge of r-process elements like lead, uranium and gold. “It is very difficult to see these elements shine when they’re created in the universe because they are very rare,” explained Enrico Ramirez-Ruiz, a professor of astronomy and astrophysics at the University of California at Santa Cruz. “For example, gold is only one part in a billion in the Sun. So even though the necessary physical conditions needed to make these elements were clear to physicists more than 50 years ago, it was a mystery as to what sort of objects and astrophysics would provide these conditions.”
The physical conditions required to make r-process elements are freely-moving neutrons, to build up the atoms, and extreme heat and pressure, to force the neutrons to fuse to the atoms’ nuclei. The scientists’ search has focused on the most explosive, energetic astronomical events: supernovae, which occur when a giant star explodes, and collisions between extremely dense neutron stars. The higher the energy of an explosion or
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