It's a galaxy-eat-galaxy universe out there. Big ones eat little ones, then the big ones "merge" with other big ones. Mergers start out as collisions. Two spiral galaxies sail into and through each other, pulling long trails of stars behind them, then come back and do it again. Along the way, they compress each other's gases to trigger the formation of stars, hot stars brighter than a welder's torch. Finally each is caught by the other and they merge. The two galaxies may have been the breathtaking pinwheels of astronomy picture books before the merger, but afterward all we see is a blob of light known as an elliptical galaxy. The "Local Group," namely our own and some 25 other galaxies, someday will be one.
It could happen here. The Andromeda galaxy and we in the Milky Way galaxy are heading for each other at 300,000 miles per hour. And no one, no one, is doing anything to stop it. The excuse is that the collision, if there is one, won't happen for another three or four billion years. And unlike head-on automobile crashes, this one would take a few hundred million years.
These galactic tidbits came to light at the annual meeting of the American Association for the Advancement of Science, convening in Philadelphia where it was founded 150 years earlier. Five thousand scientists and journalists came to hear 850 speakers on the hard, soft and social sciences.
Looking for a really big story, I naturally headed for "Galaxies in Collision." Bradley Whitmore of the Space Telescope Science Institute in Baltimore brought us up to speed. We've known for a generation or more that violent processes are the rule rather than the exception around the universe. Astronomers had not thought specifically about galaxies colliding, however, because they believed such events to be rare, although there had been clues to the contrary, Whitmore said. During the 1950s Fritz Zwicky at CalTech systematically photographed interacting galaxies, noting especially the wispy tails he guessed consisted of stars. In 1966 Halton Arp published his Atlas of Peculiar Galaxies, containing lots of images of what looked like colliding galaxies, and suggested that colliding galaxies could form stars in bursts. In 1983 a satellite that looks at the sky at infrared wavelengths found that the most star formation was occurring where galaxies were colliding. (Stars just beginning their lives are often obscured at optical wavelengths. They glow brightly in the infrared.)
Theoretical astronomers have been working on colliding galaxies since the 1940s. Not until the advent of supercomputers, however, could scientists follow the complex interactions between stars, gas clouds and dark matter. Nowadays they can produce "movies" of galaxies colliding. Chris Mihos of Case Western Reserve University is one practitioner. He uses several hundred thousand "superstars," each representing 100,000 stars, across thousands of light-years of space to produce his animations of collisions. (Mihos is quick to point out that the resolution in his simulations is only a tenth as good as that of actual images made by the Hubble Space Telescope.) He said it is no coincidence that our galaxy is "barred"--the center looks more like a wide, straight line than a circle--or that the disk is thick. We've been in collisions before.
In classical astronomy, the globular clusters (spherical bunches of, typically, 100,000 stars) that orbit our galaxy were thought of as being unique remnants of the beginnings of the galaxy. But the models and observation both find colliding galaxies causing bursts of star formation in brand-new globular clusters as well as in galaxies proper. At least some galaxies have three populations of globular clusters. In a galaxy known as NGC 7252, there are globular clusters made up of stars 15 billion years old, 500 million years old and only 10 million to 20 million years old. The last two presumably are the work of collisions. Some of the youngest stars are so hot that they will burn out in 10 million years. (The sun has been going strong for 4.5 billion years and is good for another 4.5 billion or so.)
Rosemary F. G. Wyse of Johns Hopkins University reported on our space invader, the dwarf galaxy known as Sagittarius. Three astronomers had been studying small squares of sky, measuring the motion of stars close to the Milky Way's galactic center, when they noticed that some stars were not moving like all the others. These mavericks were all moving together, and were part of a dwarf galaxy only one-fifth the size of our own, and with only one one-thousandth the mass. (Much of the mass has to be dark matter. The visible stars would not produce a gravitational field strong enough to hold Sagittarius together against the tremendous pull of our galaxy.)
If current estimates of its age are right, then Sagittarius may have passed through our galaxy ten times. It is also possible, however, that Sagittarius hit the Large Magellanic Cloud (a satellite galaxy visible to the naked eye in the Southern Hemisphere) not so long ago, astronomically speaking. It may have made only two passes through our galaxy since. Astronomers are now searching its orbit for streams of stars ripped out of Sagittarius by our own galaxy.
As to us colliding with the Andromeda galaxy, the watchword is relax. Mihos said we cannot yet tell whether it will be a true collision or a near miss. Watch this space for breaking news.
If we do collide, most people may never notice. Stars are so far apart that the Solar System might slide through such an event with no perturbation whatever. Mihos offered this analogy: if the sun were a marble on a step of Philadelphia's City Hall, then the nearest star would be in the Caribbean. The Solar System might be pulled out into a tidal tail, but while the night sky would look very different, our tight little family would be together as always.