A new 3D map of the Milky Way flaunts our galaxy’s warped shape
Universe

A new 3D map of the Milky Way flaunts our galaxy’s warped shape

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 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 appears to have been twisted into a wave like a beach towel being shaken free of sand.

The latest study isn’t the first to look at the curves of the Milky Way. 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.”

Our Milky Way Galaxy is assumed to be a barred spiral galaxy with a central bar-shaped structure made up of stars. Another barred spiral galaxy called NGC1300, imaged by the Hubble Space Telescope, is shown here. Image Credit: NASA, ESA, and The Hubble Heritage Team STScI/AURA

Scientists have believed for decades that the Milky Way has a mild case of the bends. In the 1950s, astronomers studying our galaxy’s reservoir of hydrogen gas discovered some fraying at its borders, which appeared to be confirmed by subsequent studies tracking everything from the distribution of cosmic dust in our galaxy to the speed 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. According to Johnston, the technique is similar to trying to trace the shape of a forest after being thrown into its center.

An extragalactic telescope installation could still 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.

The researchers concentrated on the Cepheids, a type of young star that can burn up to 100,000 times bright than the Sun, making them visible from thousands of light-years away. Cepheids can function as cosmic calipers because their luminosity waxes and wanes on a very strict schedule. The longer a Cepheid’s cycle, the brighter it is—a quirk that allows us to calculate a star’s absolute brightness as seen from a fixed distance. When this figure is compared to the apparent brightness of a Cepheid as seen from Earth, astronomers may accurately calculate its distance to Earth and confidently position it on a cosmic map.

Skowron and her colleagues analyzed more than 1,500 Cepheids across the Milky Way as part of the Optical Gravitational Lensing Experiment (OGLE), a six-year effort that comprised about 154 billion individual astronomical observations. They then supplemented their data with readings from other star catalogs on 900 additional Cepheids.

Together, the stars speckled an expanse covering most of the Milky Way. Some were (relatively) close neighbors, within 13,000 light-years of Earth; others, on the other hand, clung to the galaxy’s fringes, glimmering more than 100,000 light-years away.

With this stellar skeleton in place, the researchers were able to create an edge-to-edge, three-dimensional map of the Milky Way—the clearest visualization of the Milky Way ever produced, according to Skowron. “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 a twist. The Milky Way appears to be a cosmic contortionist, taking the form of a sideways S with a single peak giving way to a single 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 distant stars are located approximately 5,000 light-years above or below the galactic plane. From center to edge, this averages close to an 8% grade—a visible slope to the human eye, according to Skowron.

The Warsaw Telescope at Chile’s Las Campanas Observatory and the Cepheids identified by the OGLE survey. Image credit: K. Ulaczyk / J. Skowron / OGLE / University of Warsaw Astronomical Observatory

The model also indicated that the Milky Way’s edges are thicker than the center, creating 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 findings back up those published in Nature Astronomy, which used the Cepheids to map the Milky Way. 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 in some way warped. What’s causing these twists and turns is unknown, but astronomers believe there are several possibilities. According to one theory, stars at the center of a spiral galaxy might tug on the disc’s outer edges as it rotates, distorting it. Another theory is that these wrinkles represent battle scars from collisions with much smaller galaxies that were later slurped up by their heftier opponents.

More research is required to determine whether these scenarios are applicable to the Milky Way. But, in the meanwhile, knowing the warp occurs is useful, according to Skowron.

Our galaxy’s Cepheid stars, superimposed onto a map of the Milky Way. Image Credit: Plot by J. Skowron / OGLE, Milky Way panorama by Serge Brunier

This galactic map quest, however, 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, Elmegreen claims that some quadrants of the new map are more patchy than others.

Data derived from dust, gas, and other sorts of stars could be especially useful in this situation. These measurements may be less accurate than Cepheid measurements, but they provide complementary sources of information, according to Newberg, and when dealing with a behemoth as perplexing as the Milky Way, we need all the aid we can get. After all, understanding a forest requires more than listing 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.”

Given the vastness of our galaxy, the massive number of Cepheids discovered by scientists remains a small 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.”

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Patrick Jones
Patrick Jones
5 months ago

I love these findings. I will be forever amazed how we can only speculate what our own galaxy looks like because we may never be able to go far enough into the universe to look back at it and take photos.

Michael Brian Bentley
Michael Brian Bentley
5 months ago

some typos in the final copy.

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