ÐÓ°ÉÔ­´´

THIS is cannibalism but not as you know it, Jim. Our Universe is big enough for billions of galaxies, star assemblies, planets and clouds of gas and dust to dance harmlessly around each other – most of the time. Only in crowded galaxy clusters does this dance become more intimate, and it can turn deadly.

We’re talking galactic cannibalism – the tendency of large galaxies to gobble up their lesser neighbours. When a large galaxy approaches a smaller one, its gravitational pull rips the tiddler apart. Over a few million years, the attacker reduces the smaller galaxy to pieces, which it then devours.

So what happens to a galaxy as it gets eaten? In the vastness of intergalactic space, head-on collisions are extremely rare. But even a close encounter can dramatically alter the orbits of the stars and planets in the victim galaxy as they come under the influence of the cannibal galaxy’s gravitational system. In addition, the interstellar clouds of dust and gas from the two galaxies collide with explosive effect. The collisions lead to a rise in temperature, increasing the pressure in the cannibal galaxy and leading to the formation of new stars and high-speed jets of gas and dust.

How do you spot past episodes of galactic cannibalism? If you know what to look for they are very distinctive, since the new stars tend to form in a halo around the cannibal galaxy. These young stars are easily identifiable because they look blue. A team headed by Eric Peng at Johns Hopkins University in Maryland recently used this blue signature to identify the remains of a dwarf galaxy that had been swallowed by the massive galaxy Centaurus A several million years ago (Science News, 14 December 2002).

Does galactic cannibalism look as spectacular as it sounds? Yes it does – take a look at some recently released pictures from the Hubble Space Telescope ().

How common is galactic cannibalism? Not as common as it used to be. The Universe in its early days was much smaller than it is now, and embryonic star systems regularly consumed each other. Indeed, it was this primordial cannibalism that drove galactic evolution.

Does our own galaxy, the Milky Way, show any cannibalistic tendencies? Yes: we are currently feasting on our immediate galactic neighbours, the Large and Small Magellanic Clouds, which we will have fully digested in about 100 million years.

We are also in danger of being cannibalised ourselves. Andromeda, a massive galaxy, is heading our way at 300 kilometres a second. Andromeda’s enormous gravitational pull will tear apart the Milky Way’s spiral arms and shred us inside-out. There’s no point in worrying about this: it won’t take place for another two to three billion years, by which time our Sun will have long since burnt out, leaving the Earth frigid and lifeless.