There were giants in the very early Universe, or so it has been predicted. Stars so massive they not only put the Sun to shame, but potentially also every other star that’s been observed to currently exist anywhere in the Universe.

These first stars, predicted by models of a Universe with little beyond hydrogen, could have been greater than 300 times the mass of the Sun. Compare that to Eta Carinae. It's one of the largest stars ever observed, but Eta Carinae is only ~100 to 150 times the mass of the Sun. There’s evidence that stars don’t get much bigger than this, at least not in the present Universe (the exceptions being where multiple stars have merged).

The existence of these primordial behemoths, however, is not yet certain. While we’ve lacked even indirect evidence of these stars, a new study seems to have uncovered evidence that those supergiants did exist after all. Using new high-definition spectrographic data they obtained with the Subaru Telescope High Dispersion Spectrograph, researchers studied the star SDSS J001820.5-093939.2, which has a chemical composition significantly different than any previously discovered.

Explosions!

When massive stars run out of fuel, they explode in a supernova. Supernovae are so energetic that they can create elements that are heavier than those produced in stellar cores. And supernovae generated by the early, supergiant stars are even more energetic than anything we’re familiar with, producing heavier elements still.

So if supergiant stars really did pervade the early Universe, scientists would expect to find those elements throughout the present-day Universe in the chemical compositions of existing stars. But in previous studies of stars in the Milky Way, no clear indication of these heavy elements has been found. More recent models predicted that supergiants were rare, if they existed at all, in the early Universe.

But this new analysis of star SDSS J001820.5-093939.2 revealed low abundances of calcium, magnesium, strontium, and barium compared with similar stars. Since these elements come from the explosion of earlier stars, this suggests that J001820 is a very ancient object. If so, it likely contains evidence of nucleogenesis from the death of a single, even older star—a first-generation star. Younger stars, such as the Sun, are the result of multiple cycles of stellar death and rebirth, but SDSS J001820.5–093939.2 seems to have only one parent, according to the study.

While it is possible that the unusual composition of the star arises from a complicated series of conventional supernovae, the scientists determined that this is unlikely. For one thing, such supernovae do not produce the exact abundances of elements observed in SDSS J001820.5–093939.2. In addition, type IA supernovae, which would be required for its creation, are currently understood to only occur later in the Universe’s history.

A single progenitor star, the paper’s authors conclude, is a simpler and more likely explanation.

Lone giant?

According to current models, there is a mechanism that prevents most stars in the early Universe from gaining too much mass, keeping them in the area of tens of solar masses. But those same models show a small fraction of those early stars, several percent, might have grown larger, even exceeding 300 solar masses in some cases.

Such a massive star might explode long before it naturally runs out of fuel in what’s called a pair-instability supernova. Pair-instability supernovae arise because so many energetic photons are produced in a star’s core that some of them are converted into mass—electrons and positrons. This process consumes energy that otherwise would go to keeping the star’s core from collapsing, triggering the supernova.

It’s not clear whether such a massive star would actually produce a supernova, but a pair-instability supernova would neatly explain the elemental abundances seen in SDSS J001820.5–093939.2.

Significance

Although this explains SDSS J001820.5–093939.2‘s characteristics, it raises the question: why does this one star bear evidence of having had a supergiant progenitor, whereas no evidence has been found elsewhere?

If these massive giants existed, they were probably very rare, but nonetheless their existence provides an important clue as to the dynamics of the earliest materials in the Universe. This can be used in future simulations and theoretical studies. And understanding the early mass distribution of stars is key to the larger goal of understanding how the large-scale structure of the Universe was formed and how chemicals were distributed throughout it.

Science, 2014. DOI: 10.1126/science.1252633 (About DOIs).