<p>The Magellanic Clouds—two gauzy patches of light (at far right)—share the sky above the Patagonian Andes with a streaking comet and the luminous band of the Milky Way.</p>

The Magellanic Clouds—two gauzy patches of light (at far right)—share the sky above the Patagonian Andes with a streaking comet and the luminous band of the Milky Way.

Dancing in the Dark

Satellite galaxies of the Milky Way usually perish in its grasp. Why are two thriving?

Looming near the mighty sweep of the southern Milky Way, the Large and Small Magellanic Clouds resemble detached pieces of our galaxy. Astronomers once assumed they had always orbited the Milky Way at approximately their current distances, like the other, lesser satellite galaxies in the Milky Way's gravitational thrall. But new evidence suggests that the Magellanic Clouds have instead spent most of their careers farther away and are currently experiencing a rare close encounter with our galaxy. If so, we may be witnessing the onset of an intergalactic pas de trois—a dance of the sort that can shatter the composure of galaxies, forging billions of new stars and planets while flinging others into the depths of space.

With the clarity of hindsight, astronomers can now make sense of conspicuous clues that suggested a more regal status for the clouds all along.

For one thing, the clouds are much brighter than our galaxy's other satellites—bright enough to have captured the attention of naked-eye observers like Ferdinand Magellan's chronicler, Antonio Pigafetta, who remarked upon the "many small stars congregated together." They're bright because they're close by and contain lots of stars. The Milky Way's known satellites harbor up to ten million stars each. The Small Magellanic Cloud holds some three billion stars, and the Large Cloud perhaps 30 billion.

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