Dancing in the Dark

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

This story appears in the December 2011 issue of National Geographic magazine.

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.

For another, the clouds don't look like the disheveled spheroidal dwarf galaxies that closely orbit the Milky Way and other major spirals. More likely, they're previously distant galaxies that have only recently ventured close enough to our galaxy to be perturbed by its gravitational field. Prior to its gambol with the Milky Way, the Large Magellanic Cloud may even have been a classic spiral like the Triangulum galaxy, M33, which looks imposing but is actually not much more massive than the Large Cloud.

In 2006 a team of astronomers using the Hubble Space Telescope measured the motion of the Magellanic Clouds by clocking them against background quasars, which lie billions of light-years beyond and so approximate a static background in a universe where nothing really stands still. These measurements suggest the clouds are pursuing lanky, eccentric orbits that would have brought them into proximity with our galaxy only one time previously since the universe began.

The notion that the clouds have passed our way just once before gains support from the fact that both still contain ample amounts of gas from which to make new stars. Satellites that orbit close to major galaxies eventually forfeit their interstellar gas to the greater galaxy. Unable to fashion new stars, these satellites evolve into celestial retirement communities, inhabited by bald old stars and little else. In time, many of the spheroidal dwarfs circling the Milky Way will likely be cannibalized by it, as others like them have been in the past.

In contrast, astronomers have found that the Small Cloud is still making new stars in bursts, some within just the past few hundred million years. The Large Cloud, for its part, is a star-making factory, full of freshly minted star clusters and the soap-bubble skins shed by hot young stars that have exploded. Conspicuous in the Large Cloud is the glowing red Tarantula Nebula, a gigantic star-forming region that lies 160,000 light-years from Earth but shines so brightly that when observed through a big observatory telescope, its light pours out of the eyepiece like a flashlight beam.

The Tarantula's ruddy glow comes from what astronomers call excited hydrogen gas. The gas is excited by powerful light from giant stars burning furiously enough to deplete their nuclear fuel within only millions of years, rather than the billions of years it takes more modest stars like our sun. They then explode, as supernovae. When a blue giant star in the Tarantula region went supernova, seen on the night of February 23, 1987, it captured the attention of astronomers worldwide. They've been observing its remnants ever since.

The Milky Way and the Magellanic Clouds appear destined to see a lot more of one another in the eons ahead. Will their dynamic dance eventually result in a merger? Or will the clouds just come and go, spending their careers as a quiet, composed couple that comes downtown for a star-making binge every couple of billion years? Nobody will live long enough to witness the fate of these galaxies, but sooner or later scientists should be able to learn their dance steps, and to catch faint echoes of the music.