A new study reveals that some ancient stars in the outer reaches of the Milky Way have unusually large amounts of heavy metals like gold, platinum and uranium.

This discovery has puzzled astronomers as it is the younger generations of stars that typically have an abundance of heavy metals, scientists said.

Researchers tried to unravel the mystery by observing these ancient stars for several years using the European Southern Observatory's array of telescopes in Chile, training them on 17 "abnormal" stars in the Milky Way that were found to be rich in the heaviest chemical elements.

"In the outer parts of the Milky Way there are old 'stellar fossils' from our own galaxy's childhood," said the study's lead author Terese Hansen, an astrophysicist at the Niels Bohr Institute at the University of Copenhagen. "These old stars lie in a halo above and below the galaxy's flat disc. In a small percentage -- approximately 1 to 2 percent of these primitive stars -- you find abnormal quantities of the heaviest elements relative to iron and other 'normal' heavy elements."

According to the researchers, there are two possible theories to explain these discoveries about ancient stars and both are about supernova explosions.

One theory focuses on the formation of the first stars shortly after the universe was created. The universe then was dominated by light elements like hydrogen and helium and the first stars were formed as clouds of these gasses gathered and collapsed under their own gravity.

The hydrogen and helium in the stars then merged and formed the first heavy elements like carbon, nitrogen and oxygen.

Supernova explosions cause these stars to die, and the newly formed elements are spread as gas clouds into space, which eventually formed into new stars containing heavier elements. This process have made the newer generations of stars more abundant with heavy elements.

A second theory is that early supernovas disperse these elements in different directions, which eventually formed some of the stars in the Milky Way. This explains how many of the old stars became abnormally rich in heavy elements, the researchers said.

"In the supernova explosion the heavy elements like gold, platinum and uranium are formed and when the jets hit the surrounding gas clouds, they will be enriched with the elements and form stars that are incredibly rich in heavy elements," Hansen said.

The study was published in the Astrophysical Journal Letters.

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