Giant galaxies such as the Milky Way and its neighbor Andromeda originated long ago after smaller galaxies crashed together and grew larger. Observing this process in action, however, is difficult because it requires detecting collisions between dwarf galaxies near the edge of the observable universe, where we see galaxies as they appeared more than 10 billion years ago. Now astronomers have uncovered evidence of a similar collision much closer to home—a mere 2.6 million light-years from Earth—in a small galaxy named IC 10, allowing them to watch a dwarf–dwarf smashup in detail.



David Nidever, an astronomer at the University of Michigan, calls IC 10 one of the most intriguing galaxies in the heavens. "It's the only starburst galaxy in our Local Group of galaxies," he says. A starburst is a galaxy spawning stars at a rapid rate.

IC 10 emits only a few percent as much light as the Milky Way. Unfortunately, astronomers have taken the better part of a century to unravel the nature of this unusual neighbor, because our galaxy tries to block the view. IC 10 lies behind the Milky Way's dust in the W-shaped northern constellation Cassiopeia.



In his 1936 book, The Realm of the Nebulae, astronomer Edwin Hubble suggested that IC 10 might belong to the Local Group, a gathering of nearby galaxies that now encompasses more than six dozen known members. But for decades astronomers failed to pin down the galaxy's distance; some estimates were four times greater than others.



Finally, in the 1990s astronomers detected the galaxy's Cepheids, the gold standard for measuring galaxy distances, and proved Hubble right. A Cepheid is a yellow supergiant star that periodically brightens and dims as it expands and contracts. The longer a Cepheid takes to pulsate, the larger and more luminous it is; therefore, the pulsation period reveals the star's luminosity, and comparing this luminosity with the Cepheid's apparent brightness yields the distance to the star—and to its host galaxy.



Meanwhile other astronomers were finding a surprising number of short-lived blue supergiants in IC 10, which indicated the galaxy was experiencing a starburst. But the galaxy had few red supergiants—stars that take some time to arise. Their scarcity meant the starburst started less than 10 million years ago. Such starbursts are mysterious. "It's not clear why they all of a sudden have this great burst of star formation," Nidever says.



Now he may have found the secret to IC 10's star-making success: a collision with another small galaxy. "It was actually a serendipitous discovery," Nidever says. He was using the Green Bank Telescope in West Virginia to study radio waves from hydrogen gas shed by the Large and Small Magellanic Clouds, the two brightest galaxies that orbit the Milky Way. IC 10 lies in the part of the sky he was observing, and he detected a strand of hydrogen gas extending at least 60,000 light-years from the galaxy. This gaseous strand weighs about a million times more than the sun.



As Nidever's team will report in a future issue of The Astrophysical Journal Letters, the gaseous strand likely arose from a companion galaxy that dove into its neighbor. Gas clouds in one galaxy hit those in the other, compressing the gas until it gave birth to a rash of new stars. "This is what everyone would like the answer to be, because it's very simple and it's straightforward," says Philip Massey, an astronomer at Lowell Observatory who has studied IC 10 for decades and calls the new discovery "very important." But he asks, "If there was an interaction, then where's the companion galaxy?"



To find IC 10's companion galaxy, Nidever and his colleagues are using optical telescopes to hunt for stars in the newfound gaseous strand. Nidever suspects that IC 10 and its companion resemble a smaller version of the Magellanic Clouds, which are also prolific star creators. They orbit the Milky Way and probably each other, too; IC 10 orbits the Andromeda Galaxy but lies more than four times farther from Andromeda than the Magellanic Clouds do from our galaxy.



Andromeda and the Milky Way have stripped most of their other satellite galaxies of gas, the raw material for creating stars. The Magellanic Clouds, however, still possess gas, probably because they are swinging by us for the first time. "I wonder whether IC 10 might also be on one of its first passages around Andromeda," Nidever says. That may be why the small galaxy has been able to retain its gas until now, giving astronomers the chance to study the nearest starburst—as well as witness the process that helped construct the giant galaxy we call home.