Thursday, March 29, 2007

Starchart - Quasars


In the begining, hydrogen, helium and a few other light elements pervaded space. There were no heavier atoms, such as carbon, oxygen and iron. Heavy elements, which astronomers collectively refer to as metals, were forged later in the interior of stars, where nuclear fusion melded together hydrogen and helium atoms to make larger and more complex ones.

Current telescopes are not powerful enough to reach back to a time when hydrogen and helium were the only elements in the universe. No matter where we look, we never see pristine matter. Everything we look at is always contaminated with every element.

To determine when the first stars formed, the Keck telescope examines the light from quasars as it passed through gas clouds on its way toward Earth. These clouds are believed to be the progenitors of some of the first stars and galaxies. Certain wave lengths of quasar light are absorbed by the ancient gas clouds. We can determine the elements inside clouds by studying the wavelengths they absorb.

By measuring the shift of the quasar's light waves toward the red end of the light spectrum, astronomers can determine how far away the quasar and gas clouds are. In an expanding universe, quasars are moving away from Earth. By the time their light waves reach us, they have stretched out. The higher the red shift, the farther away the object is from Earth and the further back in time it lies.

To determine how fast stars formed in these primordial gas clouds, the quasar spectra can be used to calculate the rate at which the gas clouds cooled. Because these progenitor galaxies existed in a steady state – the energy flowing into them equaled the energy flowing out. The heating rate of the clouds is proportional to the amount of ultraviolet radiation the stars in the clouds emitted. And that amount of radiation is in proportion to the rate at which these stars formed.

Studies suggest that stars formed much more slowly than previously believed. The universe, it appears, was seeded with heavy elements not all at once, but steadily over time.

Although they can't look back to the emergence of the first stars, astronomers have speculated what may have happened. Starting with neutral hydrogen, atoms clouded the expanding and cooling universe, absorbing all light.

This hydrogen "fog" was not distributed uniformly. As it piled up in distinct regions, the gas collapsed under its growing gravity to form the first stars. Over time, the intense ultraviolet radiation streaming from these first stars gave the neutral hydrogen an electrical charge and made it transparent to light.

This has been considered a rapid change from darkness to light, as if a bank of fog suddenly succumbed to bright sunshine. But more recent measurements suggest it took longer to lift the shroud sufficiently to allow the light from rapidly forming stars to illuminate the universe.

Keck Observatory on the 13,800-foot summit of Mauna Kea in Hawaii
Scientists look back in time for clues to how the first stars formed
Arthur Wolfe, chancellor's associates professor of physics at UCSD

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