A Cosmic Detective Story
Hiding in the dense center of our galaxy are stars that formed just after the Big Bang. Here's how astronomers found them.
Some of the oldest stars in the universe are still around. And not in that cosmic way, where light from some distant object remains, even though its source winked out long ago.
These ancient stars are still alive, still burning, still pouring fresh light into the universe. Some of the oldest live locally, in the dense star fields in our galaxy’s center. But it’s hard to spot them. Images of the galactic core look like they were painted by pointillists. Gazing at so many billions of brilliant stars, it’s hard to know which formed at the beginning of time, and which burst into existence last week.
But astronomers are a crafty lot. If they spot a suspect-looking star, they can run its light through a special prism, and look for fine lines in the resulting rainbow. These fine lines reveal a star’s metallicity—and generally, the more metallic the star, the newer it is.
Let me explain: Shortly after the Big Bang, the universe was a sea of hydrogen gas, cold clouds of which condensed to form the first stars. In the cores of these stars, atoms fused together, making metals that exploded out into the universe when they died. The metallic shrapnel from these explosions seeded the hydrogen clouds that spawned succeeding generation of stars, which made still more metals in their cores.
The process repeated, over and over. The elements that make up the universe became ever heavier. The gas clouds that birthed young stars like our sun were dusty and metal-rich enough to form rocky planets. When astronomers run sunlight through their special prisms, they find all kinds of fine lines.
Older generations of stars have less metal. The oldest are composed almost entirely of hydrogen. Astronomers call them “pristine.” The stars that made up the ancient, proto-galaxy that became the Milky Way were mostly pristine—but during the last 13 billion years, the Milky Way has merged with many of its neighbors, and it has eaten scores of smaller satellite galaxies. Time and gravity have intermingled our galaxy’s many stellar generations, scattering its oldest stars across the galactic disc.
There are several teams of astronomers looking for these stars. You can’t talk to them at any length without hearing the phrase, “needle in a haystack.” This phrase is often relayed in an Australian accent, because ancient stars are best glimpsed from the Southern Hemisphere.
The Northern Hemisphere’s window on the universe looks out to the Milky Way’s outer edge, and out to the wider universe beyond, where billions of other galaxies can be seen floating in the void. There are ancient stars in the outer galactic reaches, but they are mixed in with impostors, which are poor in metals for a different reason: because they formed way out in the galactic exurbs, where there is very little exploded star shrapnel.
In the Southern Hemisphere, you can peer down into the Milky Way’s core, its central city of lights. And if you find pristine stars there, you can bank on their antiquity—because the galactic core is rich in metals, and has been for some time. If you find a pristine star in the galactic core, you can be fairly certain that it’s a relic, from the hydrogen-fog-filled epoch that followed the Big Bang.
In 2014, an international team of astronomers searched a series of dense star surveys, captured by Australian National University’s SkyMapper telescope, which sits on a rock formation, 300 miles northwest of Sydney. The astronomers were looking for stars whose light looked pristine. They identified 14,000 candidates.
Earlier this year, the team used a larger telescope, on a high plain in Chile’s Atacama desert, to zoom into the 23 most promising candidates. They had chosen well. Of the 23 suspect stars, nine were 0.1 percent as metallic as our sun—and one was .01 percent as metallic.
Astronomers think this star may have formed more than 13 billion years ago, only 300 million years after the Big Bang. If they’re right, this star has had a front-row seat for the grand pageant of cosmic evolution. It has watched, epoch after epoch, as the universe slowly organized itself into a web of galaxies, which is expanding faster and faster, every day.
This star may live for another 10 billion years. In that time, many astronomers may lay eyes on it. It’s possible that not all of them will be human. The cosmos is just entering its fertile period. Billions of temperate, rocky planets will form in the future. On the surfaces of some, processes approximating Earth’s natural history may play out. A few might even mimic our planet’s cultural history, and spawn a new species of astronomers. Maybe one of these astronomers will spot this most ancient of stars as it circles the galactic core, its pristine light a long-burning reminder of time’s unfathomable depths.