Chances are, you’ve never met Brent Tully, and yet he knows exactly where you live. Better than you do, in fact — and probably better than anybody else in the world. Working at the University of Hawaii’s Institute for Astronomy, he has spent decades researching the locations and distributions of galaxies across deep space. Oh sure, if you just want your location in a city, your phone’s GPS can do that. But if you want to find your address in the universe as a whole, Tully is your go-to guy.
In terms of what he does and why he does it, Tully falls in with a long line of cartographers who’ve helped to make sense of the world and our place in it. But rather than filling in the terra incognita of our planet, he’s plotting the lay of the cosmic land, sketching oceans of empty space and the shorelines of vast superclusters of galaxies.
Tully teased some of his latest findings two years ago at a conference in Marseille, France, held partly in honor of his 70th birthday. Then last September, he and his colleagues published a paper in Nature that unveiled their completed masterwork, a map of a single cosmic structure at least 500 million light-years in diameter. It contains about 100 quadrillion times the mass of the sun, equivalent to 100,000 Milky Ways. If you put the size of the structure in miles, it would be a 3 followed by 21 zeroes. It’s big.
Tully and company propose calling this greatest-known cosmic feature “Laniakea,” from the native Hawaiian words meaning “immeasurable heaven.” The name is evocative yet strangely ironic. Laniakea is measurable, and with this latest discovery, researchers have come closer than ever to defining humanity’s true place within the heavens.
Putting Stars in Their Places
The philosophical inspiration behind Tully’s cosmic cartography comes from a multitude of mapmakers, but the scientific inspiration can be traced back to a single person: German astronomer and mathematician Friedrich Bessel.
In 1838, Bessel observed the apparent back-and-forth movement of the star 61 Cygni that results from our shifting perspective as Earth orbits the sun, an effect known as parallax. The movement was tiny — just 1/100,000th of a degree. (For comparison, the full moon spans half a degree.) But it was enough for Bessel to deduce correctly that 61 Cygni is about 10 light-years away. For the first time in history, humans knew the stars not just as points on a sky chart but as objects with defined locations in three-dimensional space.
Bessel’s approach was far too limited to reach beyond our immediate galactic neighborhood, much less to the cosmic scales that Tully reckons, but it provided the crucial first step. Parallax made it possible to quantify the distances to the closest Cepheid variables, stars whose brightness varies regularly over time. That period of variation is directly related to their true luminosity, so if you measure the period, you’ll know the luminosity. Compare that with the apparent brightness and you know the distance, even at larger scales where parallax measurements are impossible.
Starting in 1914, American astronomer Harlow Shapley exploited the Cepheids to derive the overall shape of the Milky Way, and to show that we are in the outskirts of our galaxy, far from the crowded core of stars at the center. A decade later, Shapley’s rival Edwin Hubble pushed the same technique much further and measured the distance to what was then known as the Andromeda Nebula, showing that it is actually a full-fledged galaxy far outside our own. Hubble then discovered the expansion of the universe, cementing his scientific fame and forever eclipsing Shapley.