When galaxies collide, fireworks ensue. No stars actually bump into each other, but the merger of two large galaxies usually triggers major bursts of star formation in both galaxies as interstellar gas clouds collide and collapse under the strain of the changing gravitational forces.
But that's not the end of the show, as Robert Irion explains in the cover story of the July 2006 issue of Sky & Telescope magazine. If both galaxies harbor supermassive black holes, the two monsters will lock each other into a tight gravitational embrace and settle into a mutual orbit. As the black holes' gravity scatters stars into the far reaches of the galaxy, the systems draw ever closer. Eventually the galaxies and the black holes will merge into one.
Gas falling toward the monster black hole heats up and generates a torrent of radiation streaming out of the central region of the newly merged galaxy. A quasar is born sometime during or after this process. Powerful winds blowing off the black hole's accretion disk sweep gas out of the galaxy, often leaving behind a "red and dead" elliptical galaxy.
These large-scale processes unfold over hundreds of millions of years. Since astronomers cannot learn about galaxy mergers by watching them evolve in real time, they gain insight into these violent events by simulating them on computers. They write complex programs that include all the relevant physics, then feed them into distributed networks of computers that crunch the numbers. The computers churn out animations such as those shown on this page.
After August 1, 2006, Robert Irion's article "A Quasar in Every Galaxy?" will be available for download as a PDF file from the S&T Magazine Archive.