Astronomers have discovered a star that may have been among the second generation of stars to form after the Big Bang.
Located in the dwarf galaxy Sculptor some 290,000 light-years away, S1020549 has a remarkably similar chemical make-up to the Milky Way's oldest stars. Its presence supports the theory that our galaxy underwent a "cannibal" phase, growing to its current size by swallowing dwarf galaxies and other galactic building blocks. The discovery of the new star is detailed in Nature.
Dwarf galaxies are small galaxies with just a few billion stars, compared to hundreds of billions in the Milky Way. In the "bottom-up model" of galaxy formation, large galaxies attained their size over billions of years by absorbing their smaller neighbors.
"If you watched a time-lapse movie of our galaxy, you would see a swarm of dwarf galaxies buzzing around it like bees around a beehive," explained Anna Frebel, lead author of the paper. "Over time, those galaxies smashed together and mingled their stars to make one large galaxy - the Milky Way."
The newly discovered red giant star S1020549 dominates this artist's conception. The primitive star contains 6,000 times less heavy elements than our Sun, indicating that it formed very early in the Universe's history. Located in the dwarf galaxy Sculptor some 290,000 light-years away, the star's presence supports the theory that our galaxy underwent a "cannibal" phase, growing to its current size by swallowing dwarf galaxies and other galactic building blocks.
(Photo Credit: David A. Aguilar / CfA)
If dwarf galaxies are indeed the building blocks of larger galaxies, then the same kinds of stars should be found in both kinds of galaxies, especially in the case of old, "metal-poor" stars. To astronomers, "metals" are chemical elements heavier than hydrogen or helium. Because they are products of stellar evolution, metals were rare in the early Universe, and so old stars tend to be metal-poor.
Old stars in the Milky Way's halo can be extremely metal-poor, with metal abundances 100,000 times poorer than in the Sun, which is a typical younger, metal-rich star. Surveys over the past decade have failed to turn up any such extremely metal-poor stars in dwarf galaxies, however.
"The Milky Way seemed to have stars that were much more primitive than any of the stars in any of the dwarf galaxies," says co-author Josh Simon of the Observatories of the Carnegie Institution. "If dwarf galaxies were the original components of the Milky Way, then it's hard to understand why they wouldn't have similar stars."
The team suspected that the methods used to find metal-poor stars in dwarf galaxies were biased in a way that caused the surveys to miss the most metal-poor stars. Team member Evan Kirby, a Caltech astronomer, developed a method to estimate the metal abundances of large numbers of stars at a time, making it possible to efficiently search for the most metal-poor stars in dwarf galaxies.
Among stars found in the Sculptor dwarf galaxy was one faint, 18th-magnitude speck designated S1020549. Spectroscopic measurements of the star's light with Carnegie's Magellan-Clay telescope in Las Campanas, Chile, determined it to have a metal abundance 6,000 times lower than that of the Sun; this is five times lower than any other star found so far in a dwarf galaxy.
The researchers measured S1020549's total metal abundance from elements such as magnesium, calcium, titanium, and iron. The overall abundance pattern resembles those of old Milky Way stars, lending the first observational support to the idea that these galactic stars originally formed in dwarf galaxies.
The researchers expect that further searches will discover additional metal-poor stars in dwarf galaxies, although the distance and faintness of the stars pose a challenge for current optical telescopes. The next generation of extremely large optical telescopes, such as the proposed 24.5-meter Giant Magellan Telescope, equipped with high-resolution spectrographs, will open up a new window for studying the growth of galaxies through the chemistries of their stars.
Citation: Frebel et al., 'Linking dwarf galaxies to halo building blocks with the most metal-poor star in Sculptor', March 2010, Nature, 464, 72-75; doi:10.1038/nature08772
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