For the first time astronomers have detected stars in an enormous stream of gas shed by the Magellanic Clouds, the two brightest galaxies that orbit our own.
Sought for decades, the newfound stars are young, which means they formed recently, while the Magellanic gas collided with gas in the Milky Way. The newborn stars offer insight into processes that occurred in the ancient universe, when small, gas-rich galaxies smashed together to give rise to giants like the Milky Way. "This is the one and only galaxy interaction we can model in very much detail," says Dana Casetti-Dinescu, an astronomer at Southern Connecticut State University, who notes that other collisions of gas clouds between galaxies are farther away and thus harder to observe. "For more distant systems that interact, we don't have the wealth of information."
Some two dozen galaxies revolve around our own but only the Magellanic Clouds shine so brightly that stargazers can see the pair with the naked eye. What really sets these two apart is their vigor: Unlike all other Milky Way satellites, the Magellanic Clouds abound with gas, the raw material galaxies use to create new stars.
The Magellanic Clouds are certainly nearby: The Large Magellanic Cloud is just 160,000 light-years from Earth, whereas the Small Magellanic Cloud is 200,000 light-years distant and 75,000 light-years away from its partner. As the two galaxies orbit the Milky Way, they probably orbit each another, too.
A closer look at the Magellanic Clouds reveals more details. In the early 1970s radio astronomers discovered a long stream of gas that trails behind the two galaxies in their orbit around us. This gas, named the Magellanic Stream, consists mostly of neutral hydrogen atoms, which broadcast radio waves that are 21 centimeters long. A shorter gaseous component leads the Magellanic Clouds and is therefore called the Leading Arm. From the tip of the Leading aArm to the far end of the Magellanic Stream, this gaseous strand is at least 200 degrees long and stretches across more than half a million light-years of space.
Just as the moon lifts the terrestrial seas, the Large Magellanic Cloud's gravitational pull has torn most of this gas out of the Small Magelleanic Cloud, whose grasp on its contents is less secure. Stars should also have spilled out of the Magellanic Clouds. Although both stars and gas exist between the Magellanic Clouds, no one has ever found any stars in either the Magellanic Stream or the Leading Arm.
Until now. Casetti-Dinescu and her colleagues used the 6.5-meter Walter Baade telescope at Las Campanas Observatory in Chile to uncover six luminous blue stars in the Leading Arm. "They are formed in situ," she says. "They have to be, because they're too young—they don't have enough time to travel from the Clouds to their current location in their lifetime." Five of the six stars are about 60,000 light-years from the Milky Way's center, near the periphery of our galaxy's disk of stars.