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Accompanying those minute fluctuations in radiation, scientists believe, were tiny fluctuations of matter, or, more precisely, baryonic matter, mainly hydrogen and helium gas. Gravitational attraction between the atoms concentrated them into faint clouds of gas. As the universe expanded, the surrounding matter gradually thinned out, with the result that the internal gravity of the gas clouds grew relatively stronger.
Slowly, then faster and faster, the clouds pulled in more and more material from the surrounding medium. Eventually, the clouds began to collapse under their own gravity, evolving into galaxies. About one billion years after the Big Bang, the first galaxi es and the stars they contain were born.
Galaxies: Snapshots in Time
Peering back12 billion years in time and space, scientists using the Hubble Space Telescope recently observed what appear to be "baby
galaxies." Despite their "age," these 12 billion year-old galaxies have
patterns of light emission remarkably similar to today's fully formed
elliptical galaxies -- round collections of stars which formed fairly rapidly.
JPEG Image (43.9 KB); Caption, Credits and Copyrights
Distant Galaxy Clusters
Spiral galaxies, such as our own Milky Way, contain a combination of young
and old stars, and appear to have formed later in cosmic history.
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Galaxies in the Young Universe
The Hubble telescope has also detected nascent spiral galaxies 9 billion light years away.
Far from being neat spirals, these early galaxies resemble a zoo of tadpole-like objects, disturbed systems called
"train-wrecks" and a multitude of other strange formations. The bluish color of these early galaxies,
corresponding to hot gas, indicates active starbirth.
JPEG Image (57.7 KB); Caption, Credits and Copyrights
Taken together, these findings give scientists important clues about the age of the universe.
The billion years it took for galaxies to form, added to the initial one billion years after the Big Bang and the 12 billion years it took for the galaxy's light to reach the Hubble Telescope, implies that the universe is about 14 billion years old. This estimate is in approximate agreement with other estimates based on the age of the oldest known stars. But it stands in stark contrast with other observations, also based on Hubble telescope observations, suggesting that th e universe is only 8 billion years old.
Quasars--quasi-stellar radio sources--may be another kind of infant galaxy. They look like bright stars, and yet they are billions of light years from earth. They are relatively small--about the size of a solar system--and yet they emit prodigious amounts of energy, particularly in the radio region of the electromagnetic spectrum. One quasar may emit as much energy as hundreds of galaxies put together!
Quasar PKS 2349
What could power the enormous amounts of radio energy pouring out of these distant objects? One distinct possibility is that
gigantic black holes weighing up to a hundred million suns
lurk at the centers of rapidly rotating disks of gas from which galaxies later evolve.
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Some scientists theorize that such supermassive black holes may form when a young galaxy is only a mass of gas and contains but a few stars. A star in the densely packed core explodes, its core collapsing into a black hole which then grows rapidly, devou ring neighboring stars and gas. Some of the gas sucked in by the black hole is transformed into vast amounts of energy, particularly radiation at radio wavelengths. It's this radiation we can detect with the aid of radio telescopes.
Many astronomers think that quasars eventually settle down and become radio galaxies--larger, elliptical structures that continue to emit strongly in the radio region of the spectrum, but are no longer as bright as their quasar precursors. Eventually these radio galaxies might evolve into galaxies.
Because of their brightness and distance, quasars are used by cosmologists as beacons to estimate distances on grand scales and to study faraway gas clouds, which absorb some of the quasar's radiation. The characteristics of this absorption can reveal a lot about the mass, density and composition of the intervening gas. These gas clouds may be the stuff from which galaxies are condensing.
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