So how can the Magellanic Clouds shine, radiant and defiant—such stunning sights in the southern sky? Why are they there? A series of studies from the Hubble Space Telescope is bringing the answer into focus.
Seven years ago, Nitya Kallivayalil, an astronomer now at Yale University, and her colleagues reported Hubble observations that tracked the motion of the Magellanic Clouds through space and began to illuminate how the Clouds have prospered while their fellow satellites atrophied. And her latest Hubble data, published in February, follow the galaxies over a longer period of time, yielding a more precise path for the pair, and strengthen her startling discovery: The Magellanic Clouds are speeding through space so fast they are likely passing us for the first time. "I was initially very surprised," she says. But her discovery actually explains how the two galaxies have retained their youthful glow. Until recently they've avoided the Milky Way and its gas-grabbing tactics.
Before Kallivayalil's work and before it became so clear that getting close to a big galaxy was bad for a smaller galaxy’s health, astronomers thought the Magellanic Clouds revolved around our galaxy every one billion or two billion years. When the Clouds came closest, the thinking went, the Milky Way's gravity stirred up their gas, triggering the birth of new stars. The Hubble work, however, suggests that the Clouds' orbit around us is much more enormous. Indeed, Kallivayalil's new work rules out orbital periods less than four billion years. The two galaxies came closest to us just 200 million years ago; thus, for most of their lives, the Milky Way has had little effect on them.
Instead, the Clouds owe much of their splendor to each other. The Small Magellanic Cloud has probably been dancing around its partner every few billion years, sometimes getting closer, sometimes getting farther. And 200 million to 500 million years ago, the Small Magellanic Cloud skirted past its larger sibling. "I think the Small Magellanic Cloud smashed right through the Large Magellanic Cloud," says astronomer Gurtina Besla of Columbia University, who has computed their past paths. She says the collision compressed gas and spawned many of the young stars we see in the Clouds today.
But this fertility dance has come at a price. Besla says tides from the Large Magellanic Cloud extract stars and gas from its smaller sibling. In 2011 Knut Olsen, an astronomer at the National Optical Astronomy Observatory, and his colleagues reported the Large Magellanic Cloud has stars that aren't moving around the galaxy with its other stars. "I hadn't expected anything like this at all," Olsen says. The wayward stars' chemical composition matches that of the Small Magellanic Cloud, implying that the larger galaxy likely snatched millions of stars from its smaller mate.
Gas is also spilling out of the two galaxies. Since the early 1970s astronomers have known that a long stream of hydrogen gas stretches away from the pair. The old thinking: The Milky Way's tide ripped out the gas. The new thinking: The Clouds themselves are to blame. In 2009 astronomer David Nidever of the University of Virginia discovered that this stream is even longer than had been thought, stretching over more than half a million light-years, so it encircles most of the Milky Way. Nidever suspects that bright stars in the Large Magellanic Cloud blow out gas through their winds and supernova explosions. Besla thinks, however, the gas stream flows instead from the Small Magellanic Cloud, torn out by the tide of its partner, which has 10 times more mass. Either way, the Magellanic Stream traces the pair’s path through space, revealing that their orbit around us is polar, like a terrestrial satellite sailing over the North and South poles. The Milky Way will grab the lost gas to prolong its own star-making career.