To probe the dawn of time, astronomers usually peer far away; but now they've made a notable discovery close to home. An ancient star a mere thousand light-years from Earth bears chemical elements that may have been forged by the death of a star that was both extremely massive and one of the first to arise after the big bang. If confirmed, the finding means that some of the universe’s first stars were so massive they died in exceptionally violent explosions that altered the growth of early galaxies.
Theorists have long suspected that the universe's very first stars were massive, because early gas clouds favored the formation of heavy stars. The big bang produced only hydrogen, helium, and a little lithium, and gas clouds containing only these elements can't cool. Cooling requires heavier elements that didn't exist then. When clouds cool, they split into smaller parts and collapse, forming smaller stars. Because the primordial gas clouds stayed warm, more gravity was needed to overwhelm the gas pressure and make them collapse, so they spawned massive stars. And some of these brilliant stars, those born 140 to 300 times as massive as the sun, exploded in a way unseen in the Milky Way today.
Wako Aoki, an astronomer at the National Astronomical Observatory of Japan in Tokyo, and his colleagues have discovered a star bearing signs of just such an explosion, they report online today in Science. "This is a unique example," Aoki says. They saw that the star has a high abundance of elements with even atomic numbers compared with odd ones. "We were very surprised by the chemical composition," he says.
His team searched for 18 chemical elements in SDSS J0018-0939, a dim orange star in the constellation Cetus that emits less light than the sun. The star belongs to the Milky Way's stellar halo, the ancient population that surrounds the galaxy's bright disk. Like other halo stars, it has little iron, because it arose before most of the stellar explosions that spewed the element into space.
Aoki says the star likely formed from a gas cloud enriched by a pair-instability explosion, which is 10 to 100 times more violent than an ordinary supernova. Such a blast marks the death of a star so luminous that photons hold up its weight; but the star is so hot that the photons can convert themselves into pairs of electrons and antielectrons, which exert little outward pressure. So the star starts to collapse, which heats the gas, promoting more nuclear reactions, which heat the gas further, inducing yet more nuclear reactions, until the whole thing blows up. During the explosion, helium nuclei bombard one another, creating elements as heavy as nickel. Because helium is atomic number 2, elements with even atomic numbers vastly outnumber odd-numbered ones, which is exactly the pattern that Aoki's team finds in the star in Cetus.
Volker Bromm, an astronomer at the University of Texas, Austin, calls the discovery very important. "It really is a new window into star and element formation in the early universe," he says. He adds that pair-instability explosions were so powerful they altered the growth of galaxies shortly after the big bang.
"It's always interesting to see a star with abundances like no other," says Stan Woosley, an astronomer at the University of California, Santa Cruz. But he's not fully convinced those abundances signify a pair-instability supernova rather than an ordinary one. To distinguish between the two, he'd like observations of additional elements.