Milky Way a Cannibal, Ancient Star Confirms
Artist's conceptions of the extremely metal-poor red giant star S1020549 surrounded by other stars the Sculptor dwarf galaxy.
Credit: David Aguilar, CfA.

An ancient star discovered in a nearby dwarf galaxy looks to be long-sought key evidence confirming a leading theory that our Milky Way galaxy grew by "cannibalizing" other, smaller galaxies orbiting around it.

The star has a low metal composition, a signal that it formed among the second generation of stars born after the formation of the universe, which occurred 13.7 billion years ago. The start was discovered in a dwarf galaxy, whereas such stars had only been known in center of the Milky Way, back up the idea that the old stars present in the Milky Way were gobbled up from smaller dwarf galaxies.

"The original idea that the halo of the Milky Way was formed by destroying a lot of dwarf galaxies does indeed appear to be correct," said study team member Josh Simon, an astronomer at the Observatories of the Carnegie Institution. Dwarf galaxies are small galaxies that contain just a few billion stars, as compared to the hundreds of billions found in the Milky Way.

In 1978, astronomers Leonard Searle and Robert Zinn proposed a theory of galaxy formation ? called the "bottom-up model" ? in which large galaxies like the Milky Way attained their great size by swallowing up neighboring dwarf galaxies over billions of years.

"If you watched a time-lapse movie of our galaxy, you would see the swarm of dwarf galaxies buzzing around it like bees around a beehive," said astronomer Anna Frebel of the Harvard-Smithsonian Center for Astrophysics, who led the study that found the old star. "Over time, those galaxies smashed together and mingled their stars to make one large galaxy ? the Milky Way."

If this was indeed the case, then the same kinds of stars should be found in both large galaxies such as the Milky Way and dwarf galaxies, especially older, "metal-poor" stars. Old stars have fewer metals (in astronomy, these are any elements heavier than hydrogen and helium) because it took generations of stars to forge the first heavy elements in their nuclear cores.

Stars in the Milky Way's halo are very metal-poor, with metal abundances as much as 100,000 times less than those of the sun, an average star. But surveys over the past decade have failed to turn up any such metal-poor stars in dwarf galaxies.

"The Milky Way seemed to have stars that were much more primitive than any of the stars in any of the dwarf galaxies," Simon said. "If dwarf galaxies were the original components of the Milky Way, then it's hard to understand why they wouldn't have similar stars."

Simon and his colleagues suspected that the methods used to find metal-poor stars in dwarf galaxies were biased in a way that caused them to miss most of these stars. To find these elusive stars, team member Evan Kirby, of Caltech, developed a method to estimate the metal abundances of large numbers of stars at a time, making the search for metal-poor stars more efficient.

When the team conducted the search on the dwarf galaxy Sculptor (about 290,000 light-years away), lo and behold, they found a star that seemed to fit the bill.

Spectroscopic measurements of the star's light, taken with Carnegie's Magellan-Clay telescope in Chile, showed that the star had a metal abundance more than 4,000 times lower than that of the sun ? five times lower than that of any other star found so far in a dwarf galaxy. The low metal count means that the star may be from the second generation of stars to form after the theoretical Big Bang.

"This star is likely almost as old as the universe itself," Frebel said.

The researchers expect that other searches will discover even more metal-poor stars in dwarf galaxies, though one challenge to locating them is the distance and faintness of such stars. Larger optical telescopes could help remove that obstacle.

The research is detailed in the March 4 issue of the journal Nature.