The first stars may have appeared when the Universe was only 100 million years old, or less than 1 percent of its current age. Since then, the rapid expansion of space has stretched their light into oblivion, leaving us to look for clues to their existence in cosmic sources closer to home.
By analyzing the light emerging from the clouds around a distant quasar, researchers from Japan, Australia and the United States have discovered that a “characteristic mixture of heavy elements” could come from a single source: the colossal supernova of a first-generation star .
All stars that we can observe are classified as either Population I or Population II, depending on their age. Population I stars are younger and contain more heavy elements, while Population II stars are older with fewer heavy elements.
The first stars – described as Population III – are even older, their existence coinciding with cosmic distances that put them beyond the reach of even our best technologies. For now, we can only theorize what they might look like.
Scientists believe that these first stars were extremely hot, bright and massive, perhaps hundreds of times the mass of our Sun.
Without a history of powerful cosmic events to create elements heavier than lithium, Population III stars would be composed entirely of the simplest gases. Back then, the only available materials in the Universe were hydrogen, helium and some lithium, found in the primordial gas left over from the Big Bang. Only once the first stars themselves collapsed with intense violence could heavier elements appear.
These first stars likely ended their lives in pair instability supernovae, a theoretical type of super-supernova only possible in such massive stars. Unlike other supernovae, this one won’t leave behind stellar debris like a neutron star or black hole, instead blasting everything outward in an ever-expanding cloud.
This explosion could have seeded ancient interstellar space with the heavy elements needed to form rocky worlds like ours—thus enabling life as we know it—so the net effect is positive.
However, for astronomers on Earth now hoping to learn about Population III stars, the light from these ancient mega-explosions has faded into the distance, leaving little more than a diffuse cloud containing a complex mix of elements.
Given time, this mix of material could break down into something new. To find signs of such a concentration of stardust, the authors of the new study used near-infrared spectrograph data from one of the most distant known quasars—a type of active galactic nucleus, or the extremely bright center of a young galaxy.
The light from this quasar sped through space for 13.1 billion years before reaching Earth, the researchers note, meaning we are seeing the quasar as it looked when the Universe was only 700 million years old.
A spectrograph is an instrument that captures and divides incoming light, in this case from a celestial object, into its constituent wavelengths. This can reveal what elements are present in a distant object, although gathering this information is not always easy.
The brightness of lines in astronomical spectra can depend on factors other than the abundance of an element, the authors point out, which can complicate efforts to identify specific elements.
However, two of the study’s authors – astronomers Yuzuru Yoshii and Hiroaki Sameshima, both from the University of Tokyo – had already developed a trick to overcome this problem.
Their method, which involves using wavelength intensity to estimate the abundance of elements, allowed the research team to analyze the composition of the clouds around this quasar.
The analysis revealed a strangely low ratio of magnesium to iron in the clouds, which had 10 times more iron than magnesium compared to our Sun. That was a clue, the researchers say, suggesting it was material from the cataclysmic explosion of a first-generation star.
“It was obvious to me that the candidate supernova for this would be a pair instability supernova of a Population III star, in which the entire star explodes without leaving any remnant behind,” says co-author Yuzuru Yoshii, an astronomer at the University. of Tokyo.
“I was excited and somewhat surprised to find that a single-star pair instability supernova with a mass about 300 times that of the Sun provides a magnesium-to-iron ratio that agrees with the low value we derived for the quasar.”
At least one other possible trace of a population III star was reported in 2014, Yoshi and his colleagues note, but they argue that this new finding is the first to provide such strong evidence.
If they are right about what they found, this research could reveal a great deal about how matter has evolved throughout the history of the Universe. But to be sure, they add, more observations will be needed to check for similar features in other celestial objects.
These observations may not all have to come from such distant quasars. Even if there are no more Population III stars left in the Universe, the longevity of supernova remnants means that evidence could be lurking almost anywhere – including the local Universe around us.
“We now know what to look for; we have a pathway,” says co-author Timothy Beers, an astronomer at the University of Notre Dame.
“If this was happening locally in the very early Universe, which it should have, then we would expect to find evidence of it.”
The findings were published in The Astrophysical Journal.