stars
© NASA
In the beginning, there was just hydrogen and helium. Things were light. Gold and lead and other heavy elements were not created when the universe sprang into existence with a Big Bang.

Since then, a lot has happened inside stars to create the other ingredients needed to make planets, people, wedding rings and fishing weights.
Astronomers know this, and they have a long-standing theory about how the giant element factories - stars - operate. But within this theory is a subtheory describing how the heaviest elements originate, those weighing as much as iron or more. The subtheory is on less solid ground, and scientists continue to search for evidence to support it.

Research published in the journal Nature supports the idea that a lot of the heavy metal in the world today is the product of a slow burn, deep inside stars of a certain type.

Bombs and Stars

The raw materials for the production of heavy metals are lighter elements, and the process involves simply packing things together more tightly. It all happens at the atomic level.

In each atom of hydrogen, the lightest element, one electron orbits around a nucleus that contains just one proton. In heavier elements, protons in the nucleus share space with neutrons, and more electrons are in orbit.

On a periodic table, an element is given an atomic number based on the number of protons in its nucleus. Hydrogen is 1 and lead is 82. The heaviest elements have more than 100 protons packed in their nuclei.

In laboratories more than five decades ago, researchers learned that if they smashed two atoms of a light element together violently, nuclear fusion could occur, resulting in the creation of heavier elements. This was the basis for the hydrogen bomb.

But some 13 billion years before modern science tackled fusion, the universe itself was struggling with the same process. After hydrogen and helium were created, there wasn't much chance for heavier elements to form. Because the universe was expanding and cooling, things didn't run into each other all that often.

Lucky for us, gravity eventually brought material together. Knots of it collapsed under their own weight, and stars were born.

Hot and Dense

Inside a star, where things are hot and dense, the nuclei of light atoms are smashed together more often. This process is an efficient way to convert hydrogen into helium - it's something our Sun does every day. It's also a nifty way to create heavier elements, up to and including iron.

But heavier elements - things like zirconium, tungsten and lead - don't form this way, said Sophie Van Eck, a researcher at the Université Libre de Bruxelles, Belgium, who led the new study. Instead, they are thought to develop in gentler, slower process that scientists call it the s-process.

The s-process occurs deep in the core of stars roughly the mass of our Sun, where neutrons are gathered together to seed nuclei of iron. Another more rapid method, called the r-process, also creates heavy metals in violent explosions called supernovae, which end the lives of the most massive stars.

This s-process is thought to occur in a specific phase late in the life cycle of stars, called the AGB phase (Asymptotic Giant Branch), Van Eck explains. This AGB phase occurs just before a star similar in size to our Sun expels its gaseous envelope into space and dies, becoming a dim white dwarf often surrounded by rings and bubbles of gas that astronomers call planetary nebulae - the subject of many Hubble Space Telescope images.

Heavy metals churn to the surface over time and are expelled by stellar winds during the AGB phase, and also during a deadly final "superwind" phase.

Telltale Signs

Exactly how all this works is not entirely known. But computer models predict that AGB stars that formed early in the life of our Milky Way Galaxy should build up an abundance of the heaviest elements like lead. It should therefore be possible to spot such stars by detecting the pile-up of lead contrasting with low quantities of the lighter metals like iron and barium.

But stars like this are rare in our galaxy.

So Van Eck and her colleagues studied instead a handful metal-poor stars about 1,600 light-years away.

These metal-poor stars have not evolved enough to become AGB stars, and so they cannot produce heavy elements of their own. But past studies have found their atmospheres to be fairly rich in certain heavy metals. For more than 15 years, scientists have known that the metal in these stars was siphoned off an AGB star long ago.

Imagine two stars, orbiting each other in what astronomers call a binary system. One is an AGB star, and when it dies, it expels some of its heavy metals into the companion metal-poor star. The AGB star is nowadays a white dwarf.

With several such binary systems available to study, the history of AGB stars is, in a way, written in the stars.

"The composition that we can observe today on the surface of a [metal-poor star] actually reflects the nucleosynthesis taking place deep inside its companion star in the past," Van Eck said.

In three metal-poor stars observed from the European Southern Observatory in Chile, Van Eck and her colleagues found the overabundance of lead they were looking for. Each had more lead than any other chemical element heavier than iron.

Van Eck said the result agrees with predictions about how lead would build up inside metal-poor AGB stars, thus adding credence to the s-process idea.

The stars studied are named HD 187861, HD 196944 and HD 224959.

The results do not rule out the other method of heavy metal production. Some stars die a little more fantastically than AGB stars, ending their lives in a furious explosion called a supernova. Researchers believe that some heavy metals, such as silver, gold and platinum, form during this violent explosion in a rapid fashion (called the r-process).

Van Eck said the two processes are complementary. While some elements can only be built by the s-process or the r-process, other heavy metals are the result of both.