At least a few stars that formed not long after the universe began are swirling around the center of the Milky Way, astronomers have found. Such very old stars are known to reside around the Milky Way’s edges but haven’t previously been detected in the galaxy’s central bulge.
Observations of the galactic core using the telescopes in Australia and Chile revealed 23 stars with especially low amounts of metals — a signature of early stars — an international team of astronomers reports online November 11 in Nature. Those stars probably formed from the remnants of a supersized supernova, or hypernova, the researchers say.
“We didn’t think there would be so many,” says coauthor Andrew Casey, an astrophysicist at the University of Cambridge. Because there are a lot of stars and gases in the galaxy’s jam-packed center, finding these stars is like “finding a needle in a haystack.”
First, Casey and his team used the SkyMapper Telescope at Australian National University to identify 14,000 candidate stars for potential low metal abundance. Then, the team used the Anglo-Australian Telescope to discover that around 500 of those stars had very little iron. Finally, using the high-resolution scrutiny of the Magellan Clay Telescope in Chile, the team analyzed 23 stars with an especially low abundance of iron.
The star with the least amount of iron had one ten-thousandth as much iron as the sun. That’s like if you took all the iron in the sun and compressed it, you’d get a fist-sized rock. In comparison, if you did the same thing to the low-iron star, you’d get a tiny pebble, Casey says.
Stars that have lots of metals, like iron, were born after the universe had been around awhile; their metals originally formed in primordial stars that eventually exploded. So stars with little iron could be among the earliest stellar spawn in the universe. It’s likely that the observed low-iron star formed in the aftermath of an early hypernova, an explosion at least 10 times as powerful as a typical supernova, the researchers say.
Another indication of the stars’ early birth is their tight orbits around the center of the Milky Way. At least 14 of the low-metal stars identified orbit within the Milky Way’s central bulge. Early stars had been expected to reside there because the bulge is the oldest part of the galaxy.
It’s not surprising that these stars exist there, says Anna Frebel, an astronomer at MIT. What’s more surprising, she says, “is that it has taken this long to find them.”
The researchers couldn’t tag an exact age to the stars. “It’s really difficult to obtain numbers for old stars,” says Frebel, who has dated one star to 13.2 billion years old. Radioactive elements such as uranium that decay over time are usually needed to estimate stellar age. A true, first star that broke the cosmic dawn still awaits discovery. Casey says the stars in this study were the progeny of the first stars.
The new findings help confirm the accuracy of researchers’ ideas about how the Milky Way might have formed, Frebel says. “Plugging in holes here and there with actual data … shows that we’re on the right track to modeling the Milky Way and all its components in great detail.”