Image: Early universe
Tom Theuns  /  Max Planck Institute for Astroph
In this computer model of the early universe, gravity arranges matter in thin filaments. High-density regions, shown in yellow, undergo collapse and ignite bursts of star formation. These proto-galaxies stream along the filaments and meet at nodes, causing a buildup of galaxies. Low-density areas are blue.
By Senior Science Writer
updated 4/13/2004 5:43:50 PM ET 2004-04-13T21:43:50

Star formation across the cosmos peaked more recently than astronomers had thought, according to a new study, but the universe continues to grow gradually dimmer.

Early on, the universe was crowded and chaotic as it grew from the confines of a theoretical Big Bang. Stars were generally more massive than today. Massive stars die quickly; they can explode after just tens of millions of years.

Those primordial explosions rapidly seeded nascent galaxies with new and heavier elements, which led to subsequent generations of stars that were less massive but which multiplied like crazy. Meanwhile, free gas in the universe was drawn along gravitational filaments toward nodes, something like water drops gathering on a spider web. More coalescing gas meant more stars.

Scientists don't know exactly how it all played out. But the pace became feverish at some point.

According to previous estimates, the activity topped out about 8 billion years ago. The new study shows the peak in starbirth was roughly 5 billion years ago, or just prior to the formation of our own sun, which is now middle-aged at 4.6 billion years. The overall stellar birth rate has declined drastically since.

Pinning down the peak of star formation and understanding the rate of decline helps astronomers better grasp the overall evolution of the universe and galaxies within it.

Seeing the future
Stars develop when gas collapses under its own weight. The gas is generally part of a galaxy, and galaxies are thought to have grown over time by pulling in surrounding gas (an act that compresses the gas and triggers star formation), as well as by merging with other galaxies, which can also compresses gas. Both processes continue today but are less intense.

Alan Heavens, a University of Edinburgh astronomer and co-researcher in the new study, explained why activity is ramping down.

"The universe is expanding, and the structures within it are evolving," Heavens said via e-mail. "Matter is continuously falling into galaxies and clusters of galaxies. The matter falling in later is generally falling into larger-mass systems, and the result is the gas gets hotter."

Hot gas tries to expand, rather than contract. "At some point a lot of the gas is too hot to collapse under its own gravity and form more stars," Heavens said.

Star formation won't stop anytime soon, but significant change looms for the patient.

"The night sky will gradually dim over billions of years," Heavens said. "But new generations of stars will still be formed, from material thrown out by stars when they die, for much longer. The time scale is very uncertain, but it could be thousands of billions of years" before star formation ceases.

100,000 galaxies analyzed
To a stargazer without a telescope, the night sky is made up almost entirely of stars in our own galaxy. The fuzzy swath called the Milky Way, which is the central region of the galaxy, "will get dimmer, and a bit redder," Heavens said, but he added that the change would be difficult to discern with the naked eye by any humans who might be around.

The study was done by analyzing nearly 100,000 galaxies observed as part of the Sloan Digital Sky Survey. Heavens and Raul Jimenez of the University of Pennsylvania created a computer program to more quickly analyze each galaxy's light spectrum, which reveals the average age of the stars in it.

"If we want to understand how structure in the universe formed and evolved, then we need to understand the history of the stars," Jimenez said. "Fortunately we can read the history of the stars. By analyzing all of the light coming out of a particular galaxy — that is, the entire spectrum of visible light — we can effectively see the entire 'fossil record' of that galaxy at one glance."

Dim and dimmer
The study was detailed in the April 8 issue of the journal Nature. It also revealed that galaxies with more mass formed their stars earlier than less massive galaxies, suggesting that star formation rates are dependent on galactic bulk.

"The mass-dependence of the star-formation history explains why previous surveys showed a much earlier date for star formation, since those studies were only able to examine more massive galaxies," Jimenez said. The relatively new Sloan survey, which is drawn on by many research teams, is the most comprehensive of its kind.

Declining star formation isn't the only dramatic cosmic change in store. The universe is expanding at an accelerating pace, and other theorists have shown that galaxies will eventually recede from each other at the speed of light.

The result is that some galaxies are already too far away for us to ever communicate with, and eventually the light from one galaxy will no longer reach most others, so that even with powerful telescopes, nearly all galaxies beyond our local group will be out of reach to astronomers of the very distant future.

And that's if things go well.

In the most extreme theoretical scenario, the expansion accelerates to the point that everything is torn apart in a Big Rip, which begins by shredding galaxies and stars and ends with the demise of planets and even atoms.

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