Dwarf galaxy spawned heavy elements

SALT LAKE CITY, Utah — In the earliest universe, a violent event churned up a dwarf galaxy. This upheaval left a lasting mark on the stars that formed there. That’s what scientists concluded after finding traces of elements heavier than iron that had been left behind by that event.

The dwarf galaxy where this occurred is called Reticulum II. “It might be the first time where we can distinctly point to something and say, ‘One thing happened in this galaxy 13 billion years ago,’” says Alexander Ji. He works at the Massachusetts Institute of Technology in Cambridge. He presented the research April 19 at a meeting, here, of the American Physical Society. 

Many of the heaviest elements in our universe formed through a chain of chemical reactions. It’s known as the r-process. (The “r” stands for “rapid.”) Atoms’ nuclei— their central cores — swallow up particles called neutrons. The more neutrons they swallow, the more mass they gain. Radioactive decay transforms some neutrons into charged particles called protons. Since an atom’s properties depend on the number of protons it contains, this process forms new elements.

But not all scientists agree on where in the universe that r-process takes place. To find out, Ji and his coworkers dug into the past of nine stars in the Reticulum II dwarf galaxy. The team used the Magellan telescopes in Chile to pick apart the stars’ chemical signatures. Seven of the stars contained heavy elements. Among these were elements 60, 63 and 64 — or neodymium, europium and gadolinium. These elements in the stars were present in the same proportions as would be made by the r-process.                                                       

Most similar dwarf galaxies show no clear evidence of r-process elements. So the scientists deduced that the event that spawned those heavy elements must be rare. That idea helped them pin down what produced the elements. They ruled out relatively common occurrences such as a type of exploding star known as a core-collapse supernova. Instead, the key event in the Reticulum II galaxy was probably a collision of two neutron stars, the researchers say. Another possibility is a rare type of exploding star that spews jets of material.

Power Words

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atom    The basic unit of a chemical element. Atoms are made up of a dense nucleus that contains positively charged protons and neutrally charged neutrons. The nucleus is orbited by a cloud of negatively charged electrons.

dwarf galaxy    A small galaxy, containing several billion stars. By comparison, our own Milky Way galaxy has hundreds of billions of stars.

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.

europium   A rare chemical element that appears as a silver metal when it is pure. It is found in some minerals, and can be used to trace the source of mineral grains carried long distances by water or wind.

gadolinium    An element with the atomic number 64. The silvery-white metal is soft and malleable, easily reshaped. In medicine, this element is used as a contrast agent — something that identifies structures in the body, such as neural tissues, that accumulate this element.

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.

Milky Way   The galaxy in which Earth’s solar system resides.

neodymium   A chemical element which appears as a soft, silvery metal when it is pure. It is found in some minerals, and can be used to trace the source of mineral grains carried long distances by water or wind. Its scientific symbol is Nd.

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    Plural is nuclei. (in physics) The central core of an atom, containing most of its mass.

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

r process     A rapid chain of events creating elements heavier than iron. This process typically would occur during the explosion of an aging star (supernova) or collision of two heavy stars.

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).

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.

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).