The galaxy we live in is totally bent out of shape.
At least, that’s what the latest three-dimensional map of the Milky Way has to say. By pinpointing the locations of more than 2,400 pulsing stars—including some from the outermost edges of our galaxy—scientists have charted out a stellar atlas that might give us one of the most comprehensive portraits of the Milky Way to date.
Their findings, published today in the journal Science, reveal that the spiral galaxy we Earthlings call home isn’t the flat, featureless pancake we often make it out to be. Instead, it seems to be warped into a wave that recalls a beach towel being shaken free of sand.
The new study isn’t the first to ogle the Milky Way’s curves. But getting up close and personal with our galaxy’s warp might give us clues about its history, too—and, in doing so, give us a better sense of place in our neck of the cosmic woods.
“This is important and exciting work,” says Kathryn Johnston, an astronomer studying galactic dynamics at Columbia University who was not involved in the study. “Getting a three-dimensional map is incredibly difficult…so it’s wonderful that [the researchers] have made a global map that really allows you to look across the entire galactic disk.”
For decades, scientists have suspected that the Milky Way suffers from a mild case of the bends. In the 1950s, astronomers tracking our galaxy’s reservoir of hydrogen gas noticed some fraying at its fringes—an observation that appeared to be corroborated by subsequent studies monitoring everything from the distribution of Milky Way’s cosmic dust to the motion of stars skittering across the skies.
But finding definitive proof of the Milky Way’s warp is no simple task. While astronomers have gotten pretty good at snapshotting galaxies in more distant regions of the universe, here on Earth, we don’t exactly have the best vantage point to get an equivalent birds-eye view of the celestial structure that surrounds us. Johnston compares the process to trying to trace the outline of a forest after being dropped into its center.
The installation of an extragalactic telescope might yet be in our (very distant) future. In the meantime, a team of researchers led by Dorota Skowron, an astronomer at the University of Warsaw in Poland, decided to blaze a path through the galactic woodland with something a little more readily available: A trail of stellar breadcrumbs, sprinkled throughout the Milky Way itself.
In particular, the researchers focused on a population called the Cepheids, a class of young stars that can burn up to 100,000 times as bright as the Sun, making them easy to spot from thousands of light-years away. Cepheids can function as cosmic calipers because their luminosity waxes and wanes on a very strict schedule. The brighter a Cepheid is, the longer its cycle—a quirk that makes it possible to calculate a star’s absolute brightness, as seen from a fixed distance. Comparing this figure to a Cepheid’s apparent brightness, as seen from Earth, gives astronomers a way to accurately gauge how far that star is from Earth, and confidently place it on a cosmic map.
As a part of the Optical Gravitational Lensing Experiment (OGLE), Skowron and her colleagues studied more than 1,500 Cepheids throughout the Milky Way—a project that involved close to 154 billion individual astronomical observations taken over the course of six years. They then supplemented their data with measurements on 900 more Cepheids from other star catalogs.
Together, the stars speckled an expanse covering most of the Milky Way. Some were (relatively) near neighbors within 13,000 light-years of Earth; others, however, clung to the galaxy’s very edges, glimmering from more than 100,000 light-years away.
With this stellar skeleton in place, the researchers were able to generate an edge-to-edge, three-dimensional map—the clearest visualization of the Milky Way ever constructed, Skowron says. “This is not an artist’s impression, or a model, or a suspicion,” she says. “It’s a real image of our galaxy.”
What emerged was a familiar picture…with a bit of a twist. It turns out the Milky Way is something of a cosmic contortionist, taking the form of a sideways S with a single peak giving way to a lone valley. (Depending on your perspective, our Solar System is either near the top of a hump or the bottom of a trough; galaxies don’t exactly have a right-side up.)
On either side of the galactic center, the bend begins about 25,000 light-years out, reaching an extreme at the outermost edges another 35,000 light-years away. Some of the Milky Way’s most far-flung stars sit about 5,000 light-years above or below the galactic plane. From center to edge, this averages out close to an 8 percent grade—a slope that would be apparent to the naked eye, Skowron says.
The model also revealed that the edges of the Milky Way are thicker than its center, resulting in a bowtie-like flare when viewed from the side. The reason is a bit paradoxical: As the distance from the galactic center increases, stars and gas get less abundant, relaxing the constraints of gravity on a galaxy’s physique.
These new findings also corroborate those of another paper that also used the Cepheids to map the Milky Way, published in February in Nature Astronomy. The two studies showed similar results, but the newest map remains notable because it leveraged a far larger sample of Cepheids, says Heidi Jo Newberg, an astrophysicist studying the Milky Way’s structure at Rensselaer Polytechnic Institute who was not involved in the study.
Though they’re not very surprising, the findings remain “a big advance,” says Debra Elmegreen, an astronomer and galaxy evolution expert at Vassar College who was not involved in the study. “What’s emerging isn’t vastly different than what we’ve known for a while, but this really sharpens it.”
The disorienting new depiction of the Milky Way might look odd to us Earthlings, but it actually makes our galaxy more commonplace by celestial standards: At least half of all spiral galaxies are warped in some way. What’s behind these twists and turns remains mysterious, but astronomers think there are several possible causes. One theory posits that stars in the inner part of a spiral galaxy can tug on its outer edges as they rotate, distorting the disc. Another suggests that these wrinkles are actually battle scars, left over from collisions with much smaller galaxies that eventually got slurped up by their heftier opponents.
More research is needed to determine whether these scenarios apply to the Milky Way. But in the meantime, it’s useful to know the warp is there at all, Skowron says.
That said, this galactic map quest is far from over. The dust concentrated at the center of our galaxy makes it a lot harder to visualize stars on the far side of the disk, and most of the Cepheids in the study fell on the same side of the Milky Way as our own Solar System. As a result, some quadrants of the new map are spottier than others, Elmegreen says.
That’s where data derived from dust, gas, and other types of stars might become especially important. These measurements might be less exact than those from Cepheids, but they provide complementary sources of information, Newberg says, and when dealing with a behemoth as baffling as the Milky Way, we need all the help we can get. After all, understanding a forest takes more than enumerating its most prominent trees.
“Every tracer [of the shape of the galaxy] will give you a different view,” she says. “They’re all different pieces of the puzzle.”
Considering the vastness of our galaxy, the huge number of Cepheids scientists have characterized remain a mere drop in the celestial bucket. But maybe that’s all the more reason to keep looking. After all, Elmegreen says, “we’ve got the whole sky available.”