That is approximately the size the three available dimensions of space have grown - outward in all directions at once - since the Big Bang occurred about 14.7 billion years ago.

It also presumes an indefinite period when space expanded briefly at several hundred-thousand times the speed of light.

Along with our own Milky Way, hundreds of billions of galaxies occupy this universe, but most of them are separated by millions of light-years and most are moving apart from one another at an increasingly rapid rate, courtesy of the mysterious force known as dark energy.

Despite the vast expanse, over the eons some galactic collisions have occurred - although the events resemble intersecting clouds more than colliding worlds.

Even more amazing, sometimes whole clusters of galaxies can pull together, producing energy on a scale mere mortals cannot fathom.

Using the European Space Agency's XMM-Newton observatory, the world's most powerful orbiting X-ray telescope, an international team of astronomers has attempted to unravel some of the complex interactions that can occur during these galactic pile-ups, when clusters containing hundreds of galaxies, trillions of stars, and seemingly endless stretches of gas, dust and another mysterious substance called dark matter interact and merge.

The team has compiled images and data that reveal a process producing gigantic shock waves that can compress interstellar gas so tightly its temperature can rise to many millions of degrees.

Galactic clusters and super-clusters are the largest features in the universe. Some of them measure 6 million light-years across.

They have be come the benchmarks by which astronomers can estimate the distribution of matter in the cosmos. Moreover, by observing many clusters, astronomers can hypothesize what the universe is made of, how it began and how it might end.

Astronomers have determined that only 5 percent of the mass in galactic clusters comprises stars and galaxies. The rest is gas, some of which is so hot - 10-million to 100-million degrees - it cannot be seen in visible light. Only an instrument that can detect X-rays can see it.

The temperatures are the key. It turns out the collisions of galactic clusters are the most energetic events since the Big Bang. Their tremendous gravity is the force that heats the interstellar gas up to 10,000 times the temperature of the sun's surface.

This great heat allows astronomers to determine if the clusters are, in fact, in the process of colliding, because only through tremendous kinetic energy can such temper atures be generated.

Using the XMM-Newton, the astronomy team, led by Elena Belsole of Britain's University of Bristol, measured the origins and energy of X-rays from three galactic clusters.

They also pin-pointed the distribution of the gas in the clusters, determined the gas temperature and, by combining the data, mapped the temperature structure of the cluster gas.

"Thanks to observations obtained with XMM-Newton ... we are now able to describe fully the gas in galaxy clusters," Belsole said.

"From the temperature, we calculate that clusters can collide at velocities greater than (1,200 miles per second)."

Even at that blinding speed, galactic collisions are no events for the impatient. One of the collisions observed will take up to 2 billion years to complete. At that point, however, the gas temperature within the merger could reach 70 million degrees Celsius.

Such temperatures would fry any living t hing in the galactic neighborhood, but they are not the only threat. Generally, when galaxies or galaxy clusters collide, the larger players wreak havoc on the smaller ones.

As an example, in colliding cluster A3921, which is located about 1.2 billion light-years from Earth, the smaller group of galaxies has been almost completely destroyed by the encounter.

Hundreds of millions of stars and billions upon billions of planets basically ripped apart by heat, gravity and enormous shockwaves of turbulence.

"This research shows the violent manner by which the largest structures in the universe form," Belsole said, "and that the formation has happened in the recent past. The process is still taking place today."

That is an understatement. At some future date, Belsole explained, several billion years from now, the local group of galaxies containing the Milky Way will be torn apart by a merger with the nearby Virgo clus ter.

What astronomers observe today at a safe distance our far descendants, if they survive until then, must either endure or resolve.

In the Stars is a series by UPI examining new discoveries about the cosmos. E-mail: [email protected]

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