“Galactic archaeology” provides clues to star formation, and origin of gold
Old stars in our own galaxy are yielding information that illuminates conditions in the early universe. Richard A Lovett reports.
Cosmologists looking for fingerprints of the early universe need look no further than old stars in our own galaxy and its neighbours, astronomers say.
Not that these stars date back to the dawn of time, but a few were formed when the universe was only a fraction of its current age, and the composition of their atmospheres reveals much about how conditions have changed between then and the time, much later, when our own sun was formed. They can even reveal the origin of important elements such as silver and gold.
It’s a type of study that Gina Duggan, a graduate student in astrophysics at California Institute of Technology, Pasadena, in the US, calls galactic archaeology. “[It] uses elements in stars alive today to probe the galaxy’s history,” she says.
In fact, adds Timothy Beers, an astrophysicist at The University of Notre Dame, in Indiana, US, it’s not just our own galaxy’s history that can be probed in this manner. Such stars provide clues to conditions throughout the early universe.
Both researchers recently presented their ideas to the annual meeting of the American Astronomical Society in Denver, US.
The first stars, cosmologists believe, were composed entirely of hydrogen and helium — the only elements formed directly in the Big Bang. These elements still compose the bulk of today’s stars; the sun, for instance, is 98% hydrogen and helium.
But there’s a big difference between 98% and 100%. Pure hydrogen and helium stars tend to be hot and big, burning bright and dying young in giant explosions. In the process, they spray other elements into the cosmos – elements that enrich the next generation of stars, building toward the 2% of them found in the sun.
Such chemically enriched stars, Beers says, don’t necessarily burn as brightly or die as young. Some can be smaller, with lifetimes of 10 billion or more years. “These low-mass stars we can still see today,” he says.
Spectroscopic analysis can determine how much “pollution” these stars picked up from materials ejected by their predecessors. This allows astronomers to pick out early second-generation stars from other stars populating the Milky Way galaxy and its neighbours, allowing them to be used as cosmological time capsules.
“We can learn about the chemistry of the very early universe right in our own backyard, not just from studying faint sources more than 10 billion light years away,” Beers says.
In fact, one of these stars, known as BD+44:493, is only 600 light years away.
“It’s visible with binoculars,” Beers exclaims. “But it’s preserving stuff from the early universe!”
Kris Youakim of the Leibniz Institute for Astrophysics in Potsdam, Germany, adds that such stars can also be used to study the way large galaxies like our own were formed by mergers of numerous smaller ones. Such mergers, he says, tore the smaller galaxies apart, producing long “spaghettified” streamers.
But by using old stars similar to those studied by Beers as markers, he adds, it’s possible find these streamers and trace the history of how our galaxy came together.
Other researchers believe that nearby dwarf galaxies that have not yet merged into larger galaxies are good laboratories for understanding processes in the early universe, where dwarf galaxies dominated.
“This is an under-utilised but important way to get at where and how the first stars might have formed and the kind of galaxies that helped,” says Aparna Venkatesan of the University of San Francisco, California, US.
But the most exciting find involves the origin of the Earth’s gold.
Geologically, of course, we know it comes from gold mines. But before the Earth was formed there had to have been gold in the dust cloud that created the solar system, and there are two theories for how that gold could have been made.
One, says Duggan, is that it was formed in the heart of giant stellar explosions called magnetorotational supernovae. Another is that it was made in an equally titanic process: the collision of the remnants of dead stars known as neutron stars.
The former tended to occur early in the universe’s history, when giant stars met their catastrophic ends. The latter mostly came later, following the deaths of later-generation stars.
To figure out which it was, Duggan’s team looked at the concentration of a related element, barium, in stars of a variety of ages. By comparing the amount of barium to that of iron, which is known to build up steadily with each new generation of stars, she was able to determine if it, acting as a proxy for gold, appeared on the scene early – a sign that they were produced by magnetorotational supernovae – or more recently, a sign that they came from neutron star collisions.
Evan Kirby, a researcher on the project, calls it another example of galactic archaeology in operation.
“This study … used elements present in stars today to ‘dig up’ evidence of the history of element production in galaxies,” he says.
“By measuring the ratio of elements in stars with different ages, we are able to say when these elements were created.”
The conclusion: gold and related elements were largely formed later on, in neutron star collisions.
Without such impacts, perhaps everything from gold rushes to the history of precious coins might have been entirely different.