The Local Group: Our galactic neighborhood

The Milky Way Galaxy is like a hilltop village, according to astronomer Andrew Fox. “At nighttime you can see torches shining in two nearby villages, the Magellanic Clouds, and a more distant one, Andromeda,” he says. “For a long time, those were the only other villages known. Then, one day, someone put two lenses together to make a telescope, looked round, and saw many tiny villages scattered around the surrounding hills and realized that the maps had to be redrawn. That’s the Local Group — it’s where we live.”

For most of human history, it was inconceivable that anything existed beyond the Milky Way. After all, the Milky Way is our home galaxy, and its billions of stars provided enough of a nighttime show to fascinate stargazers for millennia. Even now, in the era of space-based telescopes and sensitive cameras, backyard and professional astronomers alike can easily devote lifetimes to studying the Milky Way and its planets, stars, and nebulae. 

But a peek over the celestial fence beyond the Milky Way’s tenuous halo yields a rich view of the nearby universe and galaxy evolution in progress. 

The approximately 85 gravitationally bound galaxies near the Milky Way are collectively known as the Local Group. This population of galaxies, spread over roughly 10 million light-years, encompasses not only the Milky Way and several bright galaxies visible to the naked eye, but also many much smaller galaxies that dominate the Local Group by number. By studying the Local Group, astronomers can observe galaxies in their entirety, no longer confined to understanding a galaxy from the inside out.
 
Follow the stream

Astronomers on Earth have front-row seats to the show of galaxy formation in the Local Group. Comparatively puny galaxies often collide with more massive galaxies, their meager stockpiles of gas and dust being absorbed into the larger system of stars. Dwarf irregular galaxies, as their name suggests, are low in mass and lack geometrical structure. They’re visually unimpressive and resemble jumbles of stars — blink and you’ll miss them — with not a spiral arm in sight. Dwarf irregular galaxies in the Local Group can have just a few thousand stars, which makes them downright pint-sized compared with the Milky Way and its hundreds of billions of stars. Even so, astronomers are keen to better understand these featherweights of the cosmos. 

“By studying the extremes of any population, we learn more about the population as a whole,” explains Marla Geha, an astronomer at Yale University whose research focuses on the origin and evolution of dwarf galaxies. 

The largest and best-known dwarf irregular galaxies in the Local Group are the Large and Small Magellanic Clouds, which look like fuzzy patches in the Southern Hemisphere night sky. The Magellanic Clouds feature prominently in myths: Aboriginal people in Australia tell stories of the Magellanic Clouds alternatively as a man and a woman, hunters, or the ashes of rainbow lorikeets (birds). The Large and Small Magellanic Clouds lie some 160,000 and 200,000 light-years away, respectively, distances slightly greater than the diameter of the Milky Way. Given their relative proximity, each Magellanic Cloud holds the distinction of being one of a smattering of galaxies in which scientists can resolve individual stars.