Our galaxy, the Milky Way, is more than just a giant 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 to the Magellanic Clouds—a pair of bold and beautiful galaxies far more lively and lustrous than any other in the Milky Way's retinue. Visible in the southern sky as fragments of glowing mist, this galactic duo poses something of a mystery to astronomers. The Milky Way's gravity has robbed all its other satellites of the gas they need to make stars, so how have the Large and Small Magellanic Clouds remained so bright and bountiful, filled with brilliant young stars and the gas and dust that create them?
Evidence is mounting that the Magellanic Clouds are so healthy because until recently they have avoided the Milky Way and its gas-grabbing tactics. Seven years ago Nitya Kallivayalil, an astronomer now at Yale University, and her colleagues reported Hubble Space Telescope observations showing that their orbit was much more enormous than previously thought. Prior to Kallivayalil's work, astronomers thought the Magellanic Clouds orbited our galaxy every one or two billion years. Now it seems they require at least four billion years to fully revolve around it—and possibly much longer. Her latest Hubble data, published in February, yield a more precise path for the pair and strengthen her initial discovery, suggesting the Magellanic Clouds are most likely passing us for the first time, which would explain their youthful glow.
Yet even the clouds' splendor will someday fade. Gurtina Besla, an astronomer at Columbia University, says tides from the Large Magellanic Cloud extract stars and gas from its smaller sibling, whose fate is bleak: “I think the Small Magellanic Cloud is on its way to becoming a dwarf spheroidal”—a ghostly, gas-poor object like the Milky Way's other satellites. Fortunately, that will take a long time. By venturing past the Milky Way only now, both galaxies still bear abundant gas to forge brilliant new stars that will deck southern skies for eons to come.