Population III stars are largely theoretical and have never been directly observed. Population I stars are the fairly young stars that are abundant in our cosmic neighborhood, and they're full of the heavy elements -- oxygen, nitrogen, carbon and iron, for example -- that make life possible. Pop II stars, which also carry heavy elements but in much lower abundance, have also been observed. However, we know that these elements weren't present at the dawn of the universe. In fact, they're created by the life cycle of stars.
Scientists have long theorized that there must have been a third type of star -- Population III -- that didn't have any of these heavy, (relatively) new elements. There must have been a first generation of star that kicked off the process of creating these elements for future galaxies. They'd be huge -- maybe even a thousand times bigger than the sun -- and hot, bursting into supernovae within just 2 million years or so. That, many astronomers believe, is why we have such a hard time finding any. Maybe Pop II stars were massive by default, leading them to early and explosive deaths, and none of them had masses low enough to stick around for us to see them.
But in observing a galaxy so distant that we're seeing it as it was just 800 million years after the Big Bang, scientists believe they've found signs of these original stars.
In an exceptionally bright old galaxy called CR7, the researchers report, there are signs of strong ionized helium emission, but no signs of the heavier elements. That could mean there are stars with no heavy elements in there, churning away and emitting helium.
"The discovery challenged our expectations from the start," study author David Sobral of the Institute of Astrophysics and Space Sciences said in a statement, "as we didn't expect to find such a bright galaxy. Then, by unveiling the nature of CR7 piece by piece, we understood that not only had we found by far the most luminous distant galaxy, but also started to realize that it had every single characteristic expected of Population III stars. Those stars were the ones that formed the first heavy atoms that ultimately allowed us to be here. It doesn't really get any more exciting than this."
If they're right about the presence of Pop III stars in CR7, the authors report, it could mean that the rare stars are actually easier to spot than we thought. They'll be following up with data from the Very Large Telescope and other space monitoring equipment, such as ALMA and the Hubble, to confirm their suspicions.