SEOUL, July 25 (UPI) -- Some of the oldest stars in the Milky Way are younger than they should be and may have originated in other galaxies, South Korean astronomers report.
The new findings might help resolve "one of the long-standing problems in modern astronomy" -- the age of the galaxy -- astronomer Young-Wook Lee, with Yonsei University, told United Press International.
According to current theory, the Milky Way condensed from a whirling cloud of gas that collapsed into a disk some 11 to 13 billion years ago -- some 2-4 billion years after the so-called Big Bang that gave birth to the universe.
Among the first objects to emerge in our newborn galaxy were 150 to 200 globular clusters, each composed of 10,000 to one million stars. The clusters reached sizes of up to 200 light-years in diameter, a distance that would take a supersonic fighter jet about 90 billion years to cross.
The oldest stars in our galaxy lurk inside the clusters. By analyzing the spectrum of light emitted from super-hot matter inside those stars, astronomers have determined they contain much less metal than our sun, explained Christine Clement of the University of Toronto.
The heavier elements comprising our sun -- as well as the other planets, Earth, and even our bodies -- are inheritances forged by nuclear reactions deep in the hearts of the galaxy's first supermassive stars, which exploded eons ago in catastrophic events called supernovae. Based on this knowledge, astronomers assume the metal-poor stars in the clusters were among the first to emerge in the Milky Way, forming "at an earlier epoch when the interstellar medium was less enriched in the heavy elements," Clement told UPI.
Lee and colleague Suk-Jin Yoon's new evidence suggests, however, some of the stars in these metal-poor globular clusters might be substantially younger than previous data have predicted. If so, they might have been captured "from a satellite galaxy about one billion years younger than the Milky Way," Lee said, one of the hundreds of galaxies thought to orbit the Milky Way much as the moon orbits Earth.
Lee and Yoon reached this conclusion by looking at unique interstellar pulses of light. Globular clusters contain so-called variable stars, which can flare in brightness for hours. Astronomers have found two distinct kinds of globular clusters -- those whose stars flare for about 13 hours, and others that flare for 15 hours.
Young stars tend to flare longer and older stars less. But for decades astronomers have been confounded because some metal-poor stars -- ostensibly the oldest ones -- flared as if they were young.
Lee and Yoon discovered most of these anomalous clusters line up with two of the Milky Way's satellite galaxies, the Draco dwarf galaxy and the Large Magellanic Cloud. They also found evidence these globular clusters follow an orbit similar to the Cloud. This suggests these clusters were captured from other galaxies that once passed near or through the Milky Way.
Clement said astronomers might be able to use these clusters to help determine the galaxy's age more accurately. This "addresses an important problem in stellar evolution," Clement said.
The scientists describe their findings in the July 26 issue of the journal Science.
(Reported by Charles Choi, UPI Science News, in New York.)
