Scientists say Hubble lets them watch a galaxy being formed 11 billion light-years away, star clusters gather

September 05, 1996|By Frank D. Roylance | Frank D. Roylance,SUN STAFF

WASHINGTON D.C. -- Astronomers using the Hubble Space Telescope have discovered what they say is the first galaxy ever observed under construction.

Peering nearly to the edge of the visible universe, Hubble found 18 gigantic star clusters packed within a space just 2 million light-years across, and apparently on the verge of merging to form one brand-new galaxy.

The construction site is 11 billion light-years from Earth. That means it shows the star clusters as they existed when their light began the trip toward Earth 11 billion years ago.

At that time, the universe was 15 percent of its present age.

"If this discovery is representative of what the universe is like at that distance, it means these baby galaxies would have grown through a series of collisions and mergers into the giant galaxies we see around us today," said astronomer Dr. Rogier A. Windhorst of the Arizona State University.

If all galaxies began this way, these galactic building blocks should be visible at similar distances in almost any direction astronomers choose to look.

At NASA headquarters yesterday, Windhorst said more observations are being planned to search for more.

Windhorst is a co-author of the report on the discovery, which appears in today's issue of the journal Nature.

The article's lead author is Sam Pascarelle, a Ph.D candidate at Arizona State. Other co-authors include astronomers Stephen Odewahn of ASU and William Keel of the University of Alabama at Tuscaloosa.

Galaxies are the largest assemblages of stars in the universe -- billions held in tight formations by the bonds of gravity.

First recognized in the 1920s, they have since been found to exist in a variety of sizes and shapes. These include the beautiful spiral galaxies similar to the Milky Way, of which our solar system is a tiny part.

Scientists have long debated how galaxies might have evolved, and answering the question has been one of the Hubble Space Telescope's top priorities.

One theory suggested that galaxies, like stars, formed from the collapse of much larger structures -- vast clouds of hydrogen gas created soon after the Big Bang.

Another proposed that galaxies were built up from much smaller objects -- the collision and mergers of stars and star clusters that formed first.

The latest Hubble findings seem to support the latter theory.

"A year ago, we had no direct observations of how anything was being born," said Dr. Bruce Margon, former chairman of astronomy at the University of Washington, who was not involved in the Hubble findings.

While not conclusive, Hubble's discovery "is the first page in an otherwise blank book," he said.

The Hubble discovery supports the idea that matter was uniformly distributed across the universe after the Big Bang, which is widely accepted to have marked the beginning of all time and space. During the first 500 million years, gravity gradually drew that matter into stars and then large clumps of stars.

One billion to 2 billion years later, those clumps of stars would have grown into clusters of about a billion stars each -- like those revealed in the new Hubble images.

After 2 billion to 4 billion years, the clusters would have merged to form larger, irregular galaxies. Those would later evolve into the spiral and elliptical galaxies seen today in relatively nearby regions of space.

The Earth's solar system lies about two-thirds of the way out from the center of the Milky Way galaxy, on one of its spiral arms, called the Orion arm.

Astronomers suggest that some of the 140 globular star clusters still visible within the arms of the Milky Way may be leftover building blocks that never were drawn into the galaxy's center.

Scientists trying to understand galactic evolution have long been limited by the inability of their telescopes to see objects distant enough to reveal galaxies in their earliest forms.

With Hubble's repaired optics, however, scientists have been pushing back the limits of their vision.

Windhorst and his colleagues found the star clusters by using an optical filter precisely tuned to detect the ultraviolet radiation emitted by hydrogen in newborn stars at extreme distances.

The clusters have a bluish color, indicating that they are filled with young stars and glowing gases. Each is concentrated in a space just 2,000 light-years across.

"That's not very big," Odewahn said. "Our own galaxy is 100,000 light-years across." A light-year is the distance light travels in one year, about 5.9 trillion miles.

At least four of the 18 clusters also show double structures at their centers that suggest they had only recently formed from the merger of two smaller clusters.

"We've caught them in the act of falling together," Keel said.

Hubble's pictures of the star clusters are available to computer users on the World Wide Web, at www.nasa.gov or www.stsci.edu.

Pub Date: 9/05/96

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