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Berkeley, California

THE small galaxy never stood a chance. It was on course for one of the deadliest objects around-the mighty Milky Way, home to the Sun and hundreds of billions of other stars. Attracted by the Milky Way鈥檚 great gravity, the small galaxy drew nearer. From its perspective, the Milky Way was a beautiful, even haunting sight: a bright pinwheel in space, 130 000 light years across, its spiral arms speckled with blue, yellow and red stars. But surrounding that pinwheel, and unseen to any eye, lay a lethal web of dark matter into which the small galaxy had just strayed.

Unaware of the danger, the small galaxy was drawn still closer. By pulling harder on the small galaxy鈥檚 near side than on its far side, the Milky Way began to rip it apart, spraying its stars and star clusters into the giant鈥檚 vast halo.

Meanwhile, on the other side of the Milky Way, three astronomers in Cambridge-Rodrigo Ibata, Gerard Gilmore, and Mike Irwin-had noticed something peculiar. The team was investigating the constellation of Sagittarius, in the direction of the Milky Way鈥檚 centre, when they detected a group of stars that stood out from the Galaxy鈥檚 central metropolis. Unlike the other stars, whose velocities were completely random, the unusual stars were all travelling in the same direction, and at the same speed. What鈥檚 more, they all lay the same distance from Earth-about three times farther away than the Galactic centre.

The year was 1994, and the Cambridge team was witnessing the death of an entire galaxy-at the hands of the Galaxy we call home.

Caught in the act

The idea that our Galaxy could be a celestial cannibal was first proposed back in the 1970s. But the demise of the Sagittarius galaxy was the first time the Milky Way had been caught in the act. And in recent months, still more evidence has been gathered to prove that our Galaxy has mercilessly torn dozens of others to pieces, acquiring their stars to augment its already enormous power.

The Milky Way is huge. Popular books often incorrectly call it average. But it is actually a giant, far larger and brighter than most other galaxies. Together with the even larger Andromeda galaxy, the Milky Way rules the 30 or so galaxies that make up the Local Group. Most Local Group members revolve around either Andromeda or the Milky Way, just as the Moon orbits the Earth. The Milky Way鈥檚 empire holds at least 10 satellite galaxies-11 if you count the wreck in Sagittarius.

Although most galaxies are found in groups or clusters, astronomers once believed that galaxies rarely interacted with one another. But in 1972, Alar Toomre of the Massachusetts Institute of Technology and his brother Juri, now at the University of Colorado, ran computer models which suggested that several peculiar galaxies owed their oddness to collisions. In the Toomre brothers鈥� most famous example-the two Antennae galaxies in the constellation of Corvus-the gravitational pull from each of the colliding galaxies had ripped out a tail of stars from the other.

Six years later, Leonard Searle of Carnegie Observatories in Pasadena, California, and Robert Zinn, now at Yale University, put forward a new model for how the Milky Way formed. The researchers were particularly interested in the Galaxy鈥檚 stellar halo, a diffuse, roughly spherical collection of old stars that envelops the Galaxy鈥檚 flat disc. The previous idea was that the entire Milky Way formed from a single ball of collapsing gas. The first stars were born throughout the spherical cloud as it began to collapse, the theory stated, and then the gas settled into a swirling disc, gradually forming the rest of the stars. Those that were formed earlier remained in the halo.

But Searle and Zinn argued that the outer part of the Galaxy鈥檚 stellar halo originated elsewhere, in small galaxies that crashed into the Milky Way over its life. They developed their theory after observing ancient star clusters called globulars, most of which belong to the halo.

According to the earlier theory, the halo formed so quickly that all globular clusters arose almost simultaneously. But Searle and Zinn discovered that some globular clusters in the outer halo were younger than those in the inner halo. They concluded that many of the outer globulars had been born in other galaxies and had been created more recently than those produced by the Milky Way itself.

Because it contradicted the earlier theory of how the Galaxy formed, the Searle and Zinn model was highly controversial. In the late 1980s and 1990s, though, astronomers confirmed that some globular clusters were indeed billions of years younger than others.

Galactic victim

But the strongest evidence came with the discovery of the Sagittarius dwarf spheroidal, as the Milky Way鈥檚 latest victim is known. It seems that this former galaxy surrendered several globular clusters to its conqueror-of the Milky Way鈥檚 147 known globulars, four share the velocity and distance of the Sagittarius ex-galaxy.

More evidence for the Milky Way鈥檚 galactic appetite comes from individual stars in the halo. In 1994, George Preston of Carnegie Observatories, Timothy Beers of Michigan State University and Stephen Shectman, also of Carnegie, discovered a mysterious group of halo stars. Halo stars were all supposed to be ancient-10 to 15 billion years old. They stood out from more modern stars because they contained only small amounts of the heavy elements-like oxygen, calcium and iron-that stars create. This was supposed to be because all halo stars were born before other stars had a chance to enrich the Galaxy with these elements.

Preston鈥檚 team, however, discovered more than a hundred stars that broke this pattern. Like other halo stars, these were deficient in heavy elements. But to the surprise of the astronomers, they were also young-only a few billion years in age. Preston and his colleagues proposed that these stars were born in another galaxy, one much smaller than the Milky Way.

A small galaxy has only small amounts of the heavy elements-first, because few stars exist to create the elements, and second, because even if stars eject newly minted material, it can escape the galaxy鈥檚 weak gravitational grasp and avoid being recycled into new generations of stars. So a small galaxy can have stars that are both young and deficient in heavy elements-just like those discovered by Preston鈥檚 team. The Milky Way, the researchers reasoned, must have grabbed these stars for itself billions of years ago.

Chemical brews

In the past couple of months, astronomers have reported further evidence for foreign material in the Milky Way. Most elements heavier than helium are created in supernovae. But there are two main varieties of supernovae, and each casts off a different chemical brew. Most supernovae occur when massive stars explode. These eject large quantities of oxygen, magnesium, silicon and other elements. Massive stars die fast, so they began showering the Galaxy with these elements almost as soon as it was born.

The other type of supernova is exploding white dwarfs, small stars that spew forth huge quantities of another element, iron. Because they live longer than massive stars, white dwarfs began exploding only after most of the halo had formed. So stars in our Galaxy鈥檚 halo received much more oxygen, magnesium and silicon than they did iron. Halo stars therefore tend to have much higher oxygen-to-iron, magnesium-to-iron and silicon-to-iron ratios than the Sun, as astronomers discovered during the 1970s and 1980s.

But several halo stars now appear to break this pattern. Jeremy King of the Space Telescope Science Institute in Baltimore studied HD 134439 and HD 134440, two halo stars travelling together in the constellation of Libra. Last month he reported that both stars have far lower magnesium-to-iron and silicon-to-iron ratios than other halo stars. His observations actually confirmed findings published by California astronomer Ruth Peterson back in 1981, before such ratios were known to be unusual, and King says both stars may have come from beyond the Galaxy.

And this month, a team led by Bruce Carney of the University of North Carolina at Chapel Hill will report odd ratios of the elements in another halo star, BD +80掳 245, which lies in the northern constellation of Camelopardalis. Furthermore, the orbits of this star and the two that King studied all take them far from the Milky Way鈥檚 centre, suggesting they did indeed come from another galaxy.

Also this month, Jeffery Brown of Washington State University and his colleagues will publish a paper reporting that two globular clusters, Ruprecht 106 and Palomar 12, violate the abundance pattern of elements for halo stars in a similar way, suggesting that they too come from outside the Galaxy.

Several years earlier, Douglas Lin of the University of California at Santa Cruz and Harvey Richer of the University of British Columbia looked at these two clusters鈥� relatively young ages as well as Ruprecht 106鈥檚 velocity and position and proposed that both clusters came from the Magellanic Clouds, the two largest galaxies to revolve around the Milky Way. Though the Magellanic Clouds are still intact, and make a beautiful sight from the southern hemisphere, they could easily have lost some outlying stars to the voracious Milky Way.

Meanwhile, several astronomers have discovered streams of halo stars that orbit the Galaxy backwards. Most stars, including the Sun, revolve around the Galaxy clockwise, as viewed from north of the Galaxy鈥檚 plane, but these groups of halo stars revolve anticlockwise. For example, Steven Majewski of the University of Virginia found such backward stars while observing stars that lie far above the Milky Way鈥檚 plane. The backward stars, he says, may be the remnants of galaxies the Milky Way has captured and annihilated.

But these galactic victims may have been small potatoes compared with what many now believe was the Milky Way鈥檚 greatest feast-not a small galaxy, like Sagittarius, but a substantial one about the size of the Large Magellanic Cloud.

A galaxy this big would have been a grand acquisition for the ancient Milky Way, but because it happened so long ago, evidence of this great takeover exists only in the pattern of stars in the Galaxy. This evidence was discovered in the 1980s by Gerard Gilmore, now at the University of Cambridge, and Neill Reid, now at Caltech, who were able to distinguish a stellar population called the thick disc, so-called because its stars typically lie farther above or below the Galactic plane than the rest of the stars in the Milky Way鈥檚 disc.

The Sun is a thin disc star, as are most of its neighbours, but about 4 per cent of the stars close to the Sun belong to the thick disc. The most famous likely thick disc member is Arcturus, an orange giant that is the fourth brightest star in the night sky. Thick disc stars are old and have velocities and heavy element abundances that distinguish them from both the thin disc and the halo.

Although the thick disc鈥檚 origin is unclear, one theory, proposed by several astronomers in the 1980s, holds that it resulted from the merger of our Galaxy with a galaxy having roughly 10 per cent of its mass-about the mass of the Large Magellanic Cloud. This collision may have puffed up the disc of our Galaxy, casting stars from the thin disc into a thick disc. The collision may also have sparked a wave of star formation, and the smaller galaxy surely contributed its own stars to the Milky Way. So a thick disc star like Arcturus may have come from another galaxy.

The Large and Small Magellanic Clouds themselves face the same grim fate. Billions of years from now, they will spiral into the Milky Way, contributing huge numbers of stars and large amounts of gas. The gas will ignite spectacular fireworks when it collides with the Milky Way鈥檚 gas, because such collisions trigger bursts of star formation. Already the Milky Way has torn out a trail of gas from the Magellanic Clouds. This Magellanic Stream, discovered in 1973, stretches across 300 000 light years, more than twice the diameter of the Milky Way鈥檚 disc.

The Large and Small Magellanic Clouds are 160 000 and 190 000 light years away respectively, and each harbours billions of stars (see Diagram). At greater distances lie the eight other galaxies that orbit the Milky Way, but their stars number only in the millions. Like the Sagittarius ex-galaxy, these galaxies are called dwarf spheroidals. They are extremely dim, diffuse and delicate, because their stars are well spread out. They have little or no gas-the raw material for forming stars-so they have little chance of giving birth to more. The faintest dwarf spheroidals emit less light than the single brightest star in the Milky Way.

In recent years, astronomers have noticed that the closer these galaxies are to the Milky Way, the older their stars tend to be-another sign of the Milky Way鈥檚 power. The two nearest dwarf spheroidals are the Ursa Minor and Draco galaxies, about 220 000 light years away. They consist entirely of ancient stars, suggesting that the Milky Way stole their gas long ago and prevented further star formation.

By contrast, some of the more distant dwarf spheroidals, such as Fornax and Leo I, were able to form stars as recently as a few billion years ago. For example, in 1993, a team led by Korean astronomer Myung Gyoon Lee found that Leo I, the most distant galaxy in the Milky Way鈥檚 empire, has stars that are only 3 billion years old.

The Milky Way is interesting not just because it is our home galaxy. It also serves as a touchstone for giant galaxies elsewhere. Indeed, like the Milky Way, other giant galaxies have probably conquered lesser entities. The great Andromeda Galaxy, just 2.4 million light years away, governs an empire of colonies that resembles the Milky Way鈥檚. Over Andromeda鈥檚 life, the galaxy may well have dined on a few of its neighbours.

Consuming beauty

One of the most picturesque galaxies in the heavens shows explicit evidence of a hearty appetite. The Whirlpool Galaxy, or M51, was the first in which astronomers detected spiral structure, back in 1845. At the edge of its disc, a lesser galaxy is being drawn into the vortex. That lesser galaxy鈥檚 gravity may even be stirring up the Whirlpool and contributing to its beauty.

So the Milky Way is not the only cannibal in the cosmos. Furthermore, it is a gentle cannibal. Although it tears apart entire galaxies, the individual stars of those galaxies do not suffer. The Sagittarius dwarf spheroidal probably has hundreds of millions of stars, but none will collide with the Milky Way鈥檚 stars, because they are all so widely separated. Instead, the stars will keep on shining, and any planets will keep on circling them. It鈥檚 just that the stars and planets will salute a new galactic leader.

Recently acquired residents may actually relish the chance to abandon a faint, dying galaxy in favour of a bright, vibrant one. Perhaps, posted around the dark frontiers of our Galaxy, there are signs that read: 鈥淲elcome to the Milky Way. It鈥檚 one killer of a galaxy.鈥�