Supercomputers have moved cosmologists closer to understanding one of nature's baffling peculiarities: that the great communities of stars called galaxies come in two varieties instead of one.
For many years, scientists assumed that galaxies formed about the same time, when the universe itself was but an infant. But if so, wouldn't they all be about the same? Wouldn't the forces that caused billions of stars to cluster together in these great communities have shaped them similarly, ensuring that one galaxy would look pretty much like any other?
Most scientists thought so.
And that is why they were so confused when the great telescopes of the last century revealed a rich mixture of galaxies of various shapes and sizes. Most, like the Earth's Milky Way, were in the shape of giant disks, bulging slightly in the middle but nearly as flat as a pancake when viewed from the edge. But many others were elliptical, looking like giant basketballs of swirling stars.
"It has been a puzzle, why nature makes galaxies of two kinds," Francois Schweizer of the Carnegie Institution said in a presentation during the American Astronomical Society's January meeting in Boston.
For many years, scientists debated among themselves over why, if all galaxies were formed soon after the Big Bang at the birth of the universe, they should seem to be of two very different families. Finally, in the 1970s, two brothers, Alar and Juri Toomre, came up with a startling concept.
The brothers Toomre (Alar is a mathematician and Juri is an astrophysicist) suggested that galaxies are still evolving, and the catalyst in that evolution is one of the most dynamic processes in all of nature. Galaxy formation was not a one-shot event in the earliest period of the universe, the brothers argued, because since the beginning of time galaxies have been colliding with each other, sometimes merging into one, sometimes ripping each other apart, and sometimes passing through each other with only minor consequences.
Initially, the suggestion was met with ridicule by other astronomers, but the Toomre brothers pushed their concept like astronomical evangelicals. And now, with the help of the supercomputer, their theory has become widely accepted.
Over the years, other prominent scientists, including Carnegie's Schweizer, have joined the fold, partly because of growing evidence that the Earth's own Milky Way galaxy is both the product of and a candidate for a collision with another galaxy.
"Andromeda is coming at us at about 300 kilometers per second," Alar Toomre said during a recent interview in Boston. The MIT professor, once ridiculed and now considered a prophet, said the Milky Way is clearly on a collision course with its neighboring galaxy, Andromeda. Furthermore, he said, the two galaxies have collided in the past.
Such a collision, he said, helps explain some of the Milky Way's peculiarities. For example, the galaxy has small groups of stars, called globular clusters, that are outside the disk of stars that make up the bulk of the galaxy. That alone is puzzling, but what has been especially troubling to scientists is that some of those globular clusters are moving in the opposite direction as the rest of the stars in the galaxy.
"Some of them are going backwards," Toomre said with a giggle.
What could explain such a thing?
Simple, Toomre suggested. The Milky Way has been bombarded many times in the past, and not always by great galaxies like Andromeda.
"Some of (the galaxies) were little bits of rubbish early on," Toomre said. Some of these "bits of rubbish" were trapped by the enormous gravitational grip of the Milky Way, but they were traveling at such great speed that they formed clusters with very different orbits around the center of the galactic mass, far removed from the disk inhabited by the other stars, and in some cases, moving in the opposite direction.
Research has revealed that many other disk galaxies have globular clusters, leading Toomre to suspect that the process of galactic mergers has been common throughout the history of the universe.
"My guess is whenever you see globular clusters, you've got to suspect a merger," Toomre said.
The problem for Toomre, as well as many others, is that a guess was about as close as they could come to proving their theories. Galaxies are complex systems made up of billions of stars and enormous amounts of invisible mass in the form of gas and dust and subatomic particles. All of that mass interacts according to complex formulas that in the past made it impossible to simulate galaxy mergers in the laboratory.
But then the age of the supercomputer dawned, giving scientists a tool that is capable of synthesizing the complex mechanics that govern colonies of billions of stars that orbit a center of mass that cannot even be seen.
It has fallen to a young computer wizard named Joshua E. Barnes to give form to the Toomre brothers' theories that could be tested in the laboratory.
Barnes, who is with the Institute for Advance Study at Princeton University, recently fed data into a supercomputer that would simulate the complex forces and masses of two great disk galaxies on a collision course. Then he instructed the computer to reveal what would happen to those two systems if they passed through each other over a time scale equal to a billion years.
As the two systems collided, they initially tore at each other, forming giant arms of stars spiraling around the centers of the two galaxies.
"But it was after the encounter that the real spectacular damage occurred," Barnes said after presenting his findings to the Boston meeting last month. "Stuff on the edge went flying off."
As the "billion years" passed, the galaxies merged into one, but the sum was very different than its parts. Two disk galaxies had, as the Toomre brothers had speculated two decades earlier, become a single elliptical galaxy.
Furthermore, the brothers were able to match the various computer images that emerged during the "billion years" with telescopic images of real galaxies scattered across the heavens. The supercomputer had demonstrated just what the brothers had been arguing, that galaxy evolution is still going on, and that process accounts for part of the rich texture of the universe.
But so much for theory. What about the real world? What about Andromeda and its collision course with the Milky Way?
No one needs to worry just yet, Alar Toomre said.
The two galaxies will not merge for several billion years. But they are locked in each other's gravitational fields, like comets orbiting the solar system, or they would have gone their separate ways billions of years ago as parts of an expanding universe.
'Ghosts in the Night'
Furthermore, space is so vast that when they do reach the same neighborhood, it is doubtful that even two stars will collide.
"They will go through each other like two ghosts in the night," Toomre said.
Life on Earth, if indeed there is life on Earth a few billion years from now, will probably be unaffected, although the nighttime sky should be quite spectacular. The dust of Andromeda will be swept through the dust of the Milky Way, coalescing into millions of new stars. A few nearby stars may burn out, collapsing on themselves and then exploding as supernovas. That will send even more stardust out into the galaxy to form more stars.