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A dozen and a half dwarf galaxies orbit the mighty Milky Way, including tiny Segue 1 and Segue 2, with about a thousand stars apiece.


Fossil Galaxy Provides a Window Into the Early Universe

The dwarf galaxy Segue 1 has remained largely unchanged for billions of years.

A tiny galaxy lying just beyond the edge of the Milky Way has failed to evolve normally over the last several billion years, say astronomers, and it represents a sample of the universe as it was in its infancy, around 13 billion years ago.

The small, faint galaxy known as Segue 1 may thus be a sort of fossil, which scientists can study to understand the nature of the earliest structures in the universe, researchers reported in the May 1 Astrophysical Journal.

"People have speculated about what those first galaxies might have looked like," said the paper's lead author, MIT astronomer Anna Frebel. Segue 1, she explained, is "like a cosmic Rosetta Stone" that will allow astronomers to trace the evolution of galaxies from the very beginning to the present day.

Astronomers already had good theoretical reasons to believe that the earliest galaxies in the universe were puny little things, far smaller and less massive than the Milky Way and its modern kin. Over billions of years, say the theorists, gravity forced these small clumps of stars and gas to merge together. Eventually they assembled themselves into the majestic spirals and bulbous elliptical galaxies we see today. (See "Photo Gallery: Galaxies.")

Thanks to all of that merging, nearly all of these primordial mini-galaxies have long since been dismembered. But a few remain, including Segue 1.

How Segue 1 Was Unmasked

Segue 1, discovered by the Sloan Digital Sky Survey in 2006, is one of the smallest galaxies ever found. The galaxy has only about 1,000 stars, compared with the Milky Way's 100 billion or more. But it's not the size that convinced Frebel and her colleagues that Segue 1 was so ancient—it's the composition of its stars.

Unlike the sun, the stars in Segue 1 are notably deficient in elements heavier than hydrogen and helium. That means that the gas clouds out of which the stars were born were also deficient in these elements.

That's an important clue. The very first stars in the cosmos were made almost entirely of hydrogen and helium, the only elements to emerge in any quantity from the big bang. Thermonuclear reactions in the cores of these stars forged the hydrogen and helium into heavier atoms, such as as carbon, nitrogen, and oxygen. When those stars exploded as supernovae, they enriched the surrounding gas with the heavy atoms, which were then incorporated into a second generation of stars.

The new stars forged even heavier elements, such as aluminum and iron, and exploded in turn, creating a new supply of element-rich gas for making stars like the sun—a third-generation star that was born about 4.6 billion years ago.

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Star-poor Segue 1, orbiting the Milky Way on its fringe, has been called "a cosmic Rosetta Stone" to understanding the evolution of galaxies.

But observations with the powerful Keck I telescope, in Hawaii, and the Magellan and Very Large telescopes, in Chile, found no evidence that Segue 1 is home to any third-generation stars. "It may have tried to make them," said Frebel, "but for some reason it failed."

The best guess, according to Volker Bromm, a theorist at the University of Texas, Austin, who wasn't involved in the new study, is that at least some of Segue 1's second-generation stars did explode, but the element-rich gas clouds they created escaped the tiny galaxy's weak gravitational pull and puffed off into deep space. "There was no chance for another round of star formation," he said. "It was a kind of suicide."

That makes Segue 1 unique. It's not the only dwarf galaxy that orbits the Milky Way—there are more than a dozen of them. But of the others studied so far, all are massive enough to have held onto their gas and formed third-generation stars. When that happens, said Bromm, the story of chemical evolution has too many layers. "It's very difficult," he said, "to disentangle the message" and understand which stars belong to which generation—or even whether a particular star might have formed from a mix of first- and second-generation gas clouds.

Frebel is already on the search for more galaxies like Segue 1, which may lead to insights into the early universe. But for now, Segue 1 appears to be an outlier. "For some reason, this little guy has managed to survive for 13 billion years," said Frebel. "It's truly amazing."