Four years ago, Anna Frebel, a young astronomer at the Massachusetts Institute of Technology, found an ancient star in a neighboring galaxy whose chemical composition proved nearly identical to some unusual stars on the outskirts of our own galaxy, which are older than the Milky Way itself.
It was a striking discovery, suggesting that the relatively young Milky Way is growing by conquest – “cannibalizing” nearby older dwarf galaxies. And it underscored the importance of a new way of learning how the universe evolved from the Big Bang to the modern cosmos.
Traditionally, astronomers study the early universe by looking back in time – peering deeper and deeper into space for vestiges of light from billions of years ago. But in the last decade, Frebel and others have used powerful telescopes and high-resolution spectroscopes to study the chemical composition of very old stars closer to home, in the Milky Way’s halo, producing a wealth of information about the creation of elements and the formation of the first stars and galaxies.
These astronomers are like Egyptologists combing the desert for relics of bygone civilizations, and call themselves stellar archaeologists. Their work relies on the fact that the rare, primordial stars they are looking for have very few atoms heavier than hydrogen and helium, the gases from which they came together. By contrast, our sun and other relatively young stars are rich in other elements, which astronomers collectively refer to as metals.
Astronomers believe that some of the old stars formed from the chemically enriched dust left over from the explosive deaths of the very first generation of stars. And their atmospheres contain important information about their forebears, like DNA passed from parent to offspring.
The hunt for these scarce antiquities goes back to the early 1950s, when scientists recognized that not all stars have the same metal-rich chemical composition as the sun.
“At the time, they didn’t know what to do with the metal-poor stars,” Frebel, 33, said. But astronomers have since established what she called “a framework for the chemical evolution of the universe.”
The first stars were made up entirely of hydrogen, helium and negligible traces of lithium. With no heavy elements to cool the gas clouds, they grew massive, rapidly burned through their fuel and exploded in supernovas.
During various burning stages of those first stars’ evolution, before and after they exploded, their intense heat fused the hydrogen and helium atoms into heavier elements – the first metals – which in turn enabled the formation of long-lived, low-mass stars.
Some of those early second- and third-generation stars eventually made their way to our corner of the universe, where they long remained unnoticed by astronomers among a sea of even younger stars. Most of the stars we see in the sky are relatively rich in metals like iron and are known as Population I stars because they were once thought to be the only type existing.
That changed in 1951, when the astronomers Joseph W. Chamberlain and Lawrence H. Aller found the first two Population II stars, with about one three-hundredth the amount of iron as our sun. Three decades later, in 1984, two astronomers at Mount Stromlo Observatory in Australia, John Norris and Mike Bessell, chanced upon a star with just one ten-thousandth the iron abundance of the sun.
Iron content closely corresponds to what astronomers call metallicity, which is expressed on a logarithmic scale: Minus 2.0 is one-hundredth the iron abundance of the sun, -3.0 is one-thousandth, and so on. The newly discovered star was -4.0.
It was a serendipitous find, according to Norris, who along with Bessell would become a Ph.D. adviser to Frebel.
In a paper in Nature, Frebel and a group of colleagues, including Stefan Keller of Australian National University, the lead author, described a star in the Milky Way constellation Hydrus with a metallicity of less than -7.1. The star, SMSS 0313-6708, is presumably very old, perhaps the oldest yet identified. The astronomers who found it estimate that it formed more than 13 billion years ago.
Astronomers, Frebel said, “are finding stars that are over 13 billion years old – what we think are plausible second-generation stars. So now we’re trying to decipher their chemical composition in order to answer questions like: How massive were the first stars? How many were there? How and where were the elements produced? How did they explode? And how did the first low-mass stars form?”