Early Universe's First Stars Spied in Distant Galaxies

Kate Ravilious
for National Geographic News
July 10, 2007
More than 13 billion years ago, some of the first stars in our universe flickered into life.

Now, for the first time, astronomers think they may have spotted light from these early stars in the most distant known galaxies, which lie more than 13 billion light-years away.

(Related: "Earliest Galaxies in the Universe Spied by Astronomers" [September 15, 2006].)

The finding will help scientists understand what the universe was like in its infancy and what made the first stars "switch on."

"It's really exciting to think that these very early stars might finally have been detected for the first time," said Marek Kukula, an astronomer at The University of Edinburgh in Scotland who wasn't involved in the new discovery.

Tomorrow Richard Ellis of the California Institute of Technology (Caltech) will present images of the distant galaxies at a cosmology conference at the Geological Society in London.

The conference, titled "From IRAS to Herschel/Planck: Cosmology with Infrared and Submillimetre Surveys," is being held from July 9 to 11.

Trick of the Light

Ellis and colleagues discovered the far-off galaxies using one of the most powerful telescopes in the world: the W. M. Keck Observatory on Mauna Kea in Hawaii.

The team spent three years searching the sky using a technique called gravitational lensing.

This method involves looking for light from distant bodies that has been bent as it passes through the "lens" created by the gravitational fields of nearby massive objects.

"The amount of bending depends on the distance of the source and the power of the lens," Ellis said.

"Light from far away tends to be deflected more and so lies further from the center of the lens."

Using a massive cluster of nearby galaxies as their lens, the scientists were able to spot six star-forming galaxies that appear to be 13 billion light-years away.

That means light from stars in those galaxies was first emitted about 13 billion years ago.

"This corresponds to a time when the universe was only 500 million years old, or less than 4 percent of its present age," Ellis said.

Cosmic Dawn

Proving that these galaxies really are as far away—and thus as old—as they appear is difficult.

But in addition to thorough checks of their work, Ellis and colleagues say they have supporting evidence.

One clue is that slightly closer and younger galaxies detected by NASA's Spitzer Space Telescope contain surprisingly old stars.

(See morphing images of stellar nurseries taken by Spitzer.)

"To produce these old stars requires significant earlier activity, most likely in the fainter star-forming galaxies we have now seen," said Dan Stark, a graduate student at Caltech who helped Ellis gather and analyze the latest observations.

The new discovery may help astronomers understand how the universe pulled itself out of its initial period of darkness, which started about 300,000 years after its birth and lasted for about 500 million years.

During this time—often called the dark ages—there were no stars, only hydrogen clouds that didn't shine.

"At some point those hydrogen clouds collapsed, and in their cores nuclear reactions would ignite to form stars," research presenter Ellis said.

One of the biggest challenges now is to pinpoint this "cosmic dawn," when the first stars began to shine. But Ellis and his team believe they are close.

"We think we may be detecting traces of a 'magic moment,'" Ellis said, "where the sky was ablaze with these young systems."

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