The Milky Way Is Not a Flat Disk—It's Actually 'Twisted' and 'Warped,' Study Finds

Our galaxy the Milky Way is often depicted as a vast disk of stars and gas that is relatively flat and stable in shape.

However, a study published in the journal Nature Astronomy has challenged this view, suggesting that this disk becomes increasingly "warped" and "twisted" the farther away you move from the galactic center.

Researchers from Macquarie University, Australia, and the Chinese Academy of Sciences made their findings after creating a new 3-D map of the Milky Way—a spiral galaxy—which allowed them to better estimate its shape.

"We usually think of spiral galaxies as being quite flat, like Andromeda which you can easily see through a telescope," Richard de Grijs, a co-author of the paper from Macquarie, said in a statement.

However, trying to create a picture of the galaxy from inside it is very difficult. Imagine attempting to work out the shape of the entire United States while standing somewhere in Kansas, and you get the idea.

To create their map, the team harnessed the unique characteristics of a class of large pulsating stars known as Classical Cepheids, which are 20 times as massive as our Sun and up to 100,000 times as bright.

"In a previous study, we had compiled a catalog of more than 50,000 variable stars observed with an infrared space observatory, WISE," de Grijs told Newsweek. "Among those 50,000-plus variables, we had included more than 1,300 young, massive stars known as Cepheid variables."

By calculating the length of their pulsation periods, the astronomers were able to accurately determine how far away they were. The distances of the stars were then used as markers to help map out the rest of the galaxy, even its distant outer regions.

"Cepheids show regular brightness variations because their outer layers pulsate in and out, and so it's quite straightforward to measure their pulsation periods," de Grijs said. "There are very well-established 'period-luminosity relations' for Cepheids, so once you know a star's pulsation period, you know how bright it should be.

"You can then measure the actual brightness as observed, and, in principle, the difference between observed and expected brightness is a direct measure of the distance to the object," he said. "The resulting distances had uncertainties of less than 3-5 percent. That's pretty much as good as it gets in astronomy."

The resulting resource—which de Grijs described as "the most accurate 3D map of our Milky Way"—enabled the team to conclude that the galaxy's stellar disk is warped like an "S-shape" and features a "progressively twisted" spiral pattern.

"Warping of spiral galaxies is well known, but it is usually only seen in the gas layer (hydrogen atoms,) because this tends to extend much farther out than the stars," de Grijs said. "In our paper, we found that the Cepheids also follow this warped structure—a new result—but importantly, that the warped stellar disk is also twisted."

The team said their findings could have significant implications for our understanding of the Milky Way, especially when it comes to determining the origin of its disk and the motions of its stars, among other problems.

"By having a much better 3D representation of the Milky Way (MW) out to large radii, we can now set constraints on the gravitational forcing of the outer disk and possibly determine the distribution of all matter in the MW—not just the matter we can see (stars, gas, dust,) but also of the dark matter which is supposed to make up most of the mass of our galaxy," de Grijs said.

"This is a timely result as in the next few years we expect a much better picture in 3D of the MW provided by the European Space Agency's Gaia satellite, so our result will serve as a benchmark to compare the Gaia results against," he said.

This article was updated to include comments from Richard de Grijs.

191749
An artist's impression of the warped and twisted Milky Way disk. Chen Xiaodian