MIGHTY MAGELLANIC CLOUDS: Gas (pink stream) spills out of the Magellanic Clouds (two white blobs at lower right) into a ribbon that wraps around most of the Milky Way, whose disk runs horizontally across the middle of this image. Image: David Nidever et al., NRAO/AUI/NSF and Meilinger, Leiden-Argentine-Bonn Survey, Parkes Observatory, Westerbork Observatory, Arecibo Observatory
Our galaxy, the Milky Way, is more than just a giant barred spiral harboring hundreds of billions of stars. It's also the hub of a gargantuan empire that stretches over more than a million light-years and rules some two dozen lesser galaxies, which revolve around it the way moons orbit a giant planet.
Of all our galaxy's many satellites, none compares with the Magellanic Clouds, which look like fragments of glowing mist torn from the Milky Way. Bold and beautiful, both galaxies are by far the liveliest and most lustrous of the Milky Way's retinue. In the last seven years astronomers have discovered just how rare this galactic duo is—and how lucky we are to have the pair close by. Moreover, new observations from the Hubble Space Telescope have given us the best clue yet to their path through space, which holds the key to their unique nature.
The Magellan galaxies, named for the Portuguese explorer Ferdinand Magellan and best seen from the Southern Hemisphere, turn out to be unusual in several ways. For their size, they possess a large amount of star-making gas and dust: more than a billion suns’ worth. Indeed, of the several dozen galaxies that belong to the so-called Local Group in our neighborhood of the cosmos, it's the Large Magellanic Cloud—not the much mightier Andromeda Galaxy or Milky Way—that boasts the greatest known stellar nursery. Named the Tarantula Nebula, it spans 700 light-years. If it were as close as the Milky Way's well-known Orion Nebula, the Tarantula Nebula would loom 55 times larger than the moon.
Surprisingly, three recent studies, each by different astronomical teams, show that smaller galaxies rarely flourish close to a giant one; yet the Large and Small Magellanic Clouds sit only 160,000 and 200,000 light-years from Earth, respectively, which is nearer than most of the Milky Way's other galactic satellites.
Astronomer Phil James at Liverpool John Moores University in England, co-author of one investigation, recounts what happened as he and a student were imaging distant galaxies: "One night, when we were both up at the telescope, it occurred to us, ‘What would it look like if someone were in one of those galaxies and looking back at us? What would they see of our galaxy?’" They'd see the Milky Way, plus its two brightest companions: The Large Magellanic Cloud would look a tenth as bright as the Milky Way and the Small Magellanic Cloud a fourth as bright as that. James says, "As we went through taking hundreds of these galaxy images, we weren't seeing that very often." In fact, only a few percent of giant galaxies like our own host a close-in pair of star-forming satellites as luminous as the Magellanic Clouds.
Andromeda certainly doesn't. It's the nearest giant galaxy to our own, 2.5 million light-years away, and the largest member of the Local Group. The Milky Way ranks number two, and all its satellites except the Magellanic Clouds have run out of gas—literally. Without gas, galaxies can't create new stars. Known as dwarf spheroidals, these gas-poor galaxies are so ghostly and diffuse that the first one found—the Sculptor dwarf in 1938—was initially mistaken for a fingerprint or some other defect on a photographic plate.
The Magellanic Clouds’ flamboyance is astonishing because the first rule of survival for modest galaxies is to steer clear of the big ones. Through their great gravitational pull, giants like the Milky Way and Andromeda rob other galaxies of gas and thereby thwart star birth. Just as the moon's gravity pulls hardest on the Earth's near side and lifts the sea, so a giant galaxy's tide tears gas out of another galaxy via what astronomers call tidal stripping. And when a small galaxy dodges through a giant galaxy's halo, halo gas hits the lesser galaxy's gas, knocking it out in a process called ram pressure stripping. That's why the typical satellite galaxy is dead, devoid of gas and bright young stars.