By DENNIS OVERBYE, JUNE 17, 2015
Astronomers said Wednesday that they had discovered a lost generation of monster stars that ushered light into the universe after the Big Bang and jump-started the creation of the elements needed for planets and life before disappearing forever.
Modern-day stars like our sun have a healthy mix of heavy elements, known as metals, but in the aftermath of the Big Bang only hydrogen, helium and small traces of lithium were available to make the first stars.
Such stars could have been hundreds or thousands of times as massive as the sun, according to calculations, burning brightly and dying quickly, only 200 million years after the universe began. Their explosions would have spewed into space the elements that started the chain of thermonuclear reactions by which subsequent generations of stars have gradually enriched the cosmos with elements like oxygen, carbon and iron.
Spotting the older stars in action is one of the prime missions of the James Webb Space Telescope, to be launched by NASA in 2018. The discovery of such stars “would be wonderful,” James Peebles, a Princeton professor and one of the fathers of modern cosmology, said recently.
Now, in a paper to be published in The Astrophysical Journal, an international crew of astronomers led by David Sobral of the University of Lisbon, in Portugal, and the Leiden Observatory, in the Netherlands, said they had spotted the signature of these first-generation stars in a recently discovered galaxy that existed when the universe was only about 800 million years old. Its light has been traveling to us for 12.9 billion years, while succeeding generations of stars have worked their magic to make the universe interesting.
The galaxy, known as CR7, is three times as luminous as any previously found from that time, the authors said. Within it is a bright blue cloud that seems to contain only hydrogen and helium.
In an email, Dr. Sobral called this the first direct evidence of the stars “that ultimately allowed us all to be here by fabricating heavy elements and changing the composition of the universe.”
In a statement from the European Southern Observatory, he said, “It doesn’t really get any more exciting than this.”
Garth Illingworth, an astronomer at the University of California, Santa Cruz, and a veteran of the search for early galaxies, pointed out, however, that these stars were appearing far later in cosmic history than theory had predicted.
Dr. Sobral and his colleagues were using the Very Large Telescope of the Southern Observatory in Chile and the W. M. Keck Observatory in Hawaii, among other big telescopes, to build on an earlier search for glowing clouds of hydrogen that might represent very early galaxies. Galaxy CR7 — short for Cosmos Redshift 7, after the method by which distant objects in the universe are dated — stood out.
In an expanding universe, the farther away or back in time an object is, the faster it is receding, which causes the wavelength of light from it to lengthen, the way the pitch of a siren sounds lower after it passes. In astronomy, this lengthening is known as redshifting.
The galaxy’s name, Dr. Sobral said, was also inspired by the great Portuguese soccer player Cristiano Ronaldo, a.k.a. CR7.
As in much of astronomy, the nomenclature of these star generations is awkwardly rooted in history and Earth-centered. Modern stars like the sun, with healthy abundances of so-called metals (anything heavier than helium), are now called Population I, mainly because they were the first known. They mostly inhabit the spiral arms and younger parts of galaxies like the Milky Way. In the middle of the 20th century, however, the astronomer Walter Baade noticed that the stars in older parts of the galaxy, like its core or globular clusters, are older and have fewer metals. He called them Population II.
The advent of the Big Bang theory of the origin of the universe forced astronomers to realize that the first stars must have had no metals at all; those are known as Population III.
Stars of both Population II and Population III are probably present in CR7, Dr. Sobral and his team report.
While the blue cloud is metal-free, according to spectral measurements, the color of the rest of the galaxy is consistent with more evolved stars making up most of its mass. This suggests, they write, that the Population III stars there are late bloomers of a sort, forming from leftover clouds of pristine material as the galaxy was sending out its light 12.9 billion years ago.
The only alternative explanation, Dr. Sobral said, is something so spectacular and unlikely that astronomers do not know if it has ever happened, namely a primordial cloud bypassing the star stage and collapsing directly into a black hole. That, he noted, is impossible to rule out because nobody really knows what it should look like.
Further observations with the Hubble Space Telescope and the James Webb could help resolve the issue.