Galactic archaeologists have uncovered new insights into the formation of the earliest galaxies by observing the chemical signatures of ancient ‘fossil’ stars.

The finding has shed some light on the early history of our stellar neighbourhood - the Milky Way.

Older stars are more present in the halos that encircle a galaxy, such as those around the Milky Way.

The older stars are made of more substances present immediately after the Big Bang, whereas later generations of stars tend to contain more heavy metals.

Both the outer and inner halos of the Milky Way contain carbon-enhanced metal-poor (CEMP) stars, but new research shows a bit more complexity.

The team of Australian and US researchers further observed that the outer halo contains a greater number of CEMP stars with low levels of elements heavier than iron (CEMP-no).

In contrast, the inner halo contain high quantities of CEMP stars that are also enhanced in elements heavier than iron (CEMP-s).

CEMP-no and CEMP-s stars had different ancestors.

In the case of CEMP-no stars, it was the explosion of massive first stars. The stellar progenitors of CEMP-s were intermediate-mass stars with a low mass companion, with which it underwent mass transfer.

“Understanding the formation of the CEMP-no and CEMP-s and their chemical structure in combination tells us how massive the initial gas clouds were, and how many stars were present in these clouds,” says Macquarie University researcher Daniela Carollo.

“It is this difference between the inner and outer halo stars’ chemical signatures that reveals much about the formation of galaxies and the first stars,” she said.

“Around 400 million years after the Big Bang, the first stars appeared, composed of the elements the Big Bang produced – hydrogen, helium and lithium. This pristine population has never been observed, but they left their chemical signature in the next generation of stars.

“We believe that this signature is carried by the CEMP-no class of stars. Indeed almost all the most metal deficient stars in the Milky Way belong to this category.

“The most metal poor CEMP-no stars we observe were born before the Milky Way formed, in gas clouds polluted by the explosions of the first stars, which had produced and left behind heavier elements.

“Essentially, the ‘purer’ the star’s chemical signature, or the lower the presence of heavy metals, the older the star, and the closer we come to the start of the universe.

“We now see that the outer halo stars contain the chemical signature of the first stars.”

The discovery suggests the gas clouds that formed the inner halo of the Galaxy were massive and crowded with stars two to three times the mass of the Earth’s sun, while the gas clouds that formed the outer halo were smaller and containing one or few stars that lived a short period (a few million years) before exploding and producing the distinctive pattern of heavier metals.

The article “Carbon-Enhanced Metal-Poor Stars: CEMP-S And CEMP-No Sub-Classes in the halo system of the Milky Way” (PDF) has been published in The Astrophysical Journal.