By Ron Cowen
Astronomers have found new evidence that the Milky Way is a cannibal, devouring streams of stars from its nearest galactic neighbors. The finding lends further support to the idea that instead of forming wholesale, our galaxy grew bit by bit and is even today snaring matter from other galaxies (SN: 4/22/00, p. 261).
The study also raises questions about how and when globular clusters, dense groupings of the oldest stars in our galaxy, were formed. According to a widely accepted view, those clusters that have the lowest abundance of what astronomers refer to as metals–any element heavier than helium–are slightly older residents of the Milky Way than the others. That’s because they were presumably born at the very start of the galaxy, when interstellar space wasn’t yet polluted with heavy elements.
However, some metal-poor globular clusters might in fact be younger refugees from smaller galaxies in which star formation got a later start, Suk-Jin Yoon and Young-Wook Lee of Yonsei University in Seoul, South Korea, report in the July 26 Science.
Yoon and Lee base their findings on a reexamination of a 60-year-old puzzle about globular clusters. The puzzle has to do with RR Lyrae stars, bright stars of the Milky Way that pulsate every few hours. In 1939, Dutch astronomer P. Oosterhoff found that in some globular clusters, the RR Lyrae stars take longer to pulsate, on average, than in other clusters. If the clusters all formed at about the same time, the RR Lyrae stars shouldn’t blink at such different rates, he noted.
Using the colors of the unusually bright stars called HB stars, Yoon and Lee computed the clusters’ relative ages. They found that several of the clusters with the most metal-poor stars are about a billion years younger than the clusters richer in metals. Moreover, those younger clusters contain the longer-period RR Lyrae stars.
The researchers propose that the clusters formed in another galaxy and were later captured by the Milky Way. Supporting that assertion, they note that several of the young clusters appear to move along with the Large Magellanic Cloud galaxy, a close neighbor of the Milky Way that orbits it.
Gerry Gilmore of the University of Cambridge in England says that if Yoon and Lee are correct, some of the oldest groupings of stars in the universe might have been formed with little fanfare in small galaxies–rather than in a brilliant fireball of star birth sparked by violent galactic collisions common in the young, dense cosmos. If the very first stars had a quiet birth, then the proposed successor to Hubble, the Next Generation Space Telescope, might have a hard time observing them, Gilmore says.
But he notes that other teams, using different methods, don’t agree with the globular-cluster ages reported by Yoon and Lee. Moreover, because of the limits of existing telescopes, any similarity of travel path between clusters and nearby galaxies remains unproven, he says. Measuring the full three-dimensional motion of globular clusters awaits the launch around 2010 of two proposed observatories–the European Space Agency’s GAIA and NASA’s Space Interferometry Mission.
