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The Milky Way Is Warped

Astronomers have used 1339 “standard” stars to map the real shape of our home galaxy. They found the Milky Way’s disc of stars becomes increasingly “warped” and twisted like an “S” the further away the stars are from the galaxy’s centre. “We usually think of spiral galaxies as being quite flat like Andromeda,” says Prof Richard de Grijs of Macquarie University, who co-authored the paper published in Nature Astronomy (https://goo.gl/67EoXQ).

The researchers were able to determine our galaxy’s S-shaped appearance after they developed the first accurate three-dimensional picture of the Milky Way to its far outer regions.

“It is notoriously difficult to determine distances from the Sun to parts of the Milky Way’s outer gas disc without having a clear idea of what that disc actually looks like,” says Xiaodian Chen of the Chinese Academy of Sciences in Beijing, who was lead author of the paper. “However, we recently published a new catalogue of well-behaved variable stars known as classical Cepheids, for which very accurate distances can be determined with an error of only 3–5%.”

Classical Cepheids are young stars that are four to 20 times as massive as our Sun and up to 100,000 times as bright. Such high stellar masses imply that they live fast and die young, burning through their fuel very quickly, sometimes in only a few million years.

These Cepheids show day- to month-long pulsations, which are observed as changes in their brightness. Combined with a Cepheid’s observed brightness, its pulsation period can be used to obtain a highly accurate distance.

“Somewhat to our surprise, we found that in 3D our collection of 1339 Cepheid stars and the Milky Way’s gas disc follow each other closely. This offers new insights into the formation of our home galaxy,” says de Grijs. “Perhaps more importantly, in the Milky Way’s outer regions, we found that the S-like stellar disc is warped in a progressively twisted spiral pattern.”

Astronomers have observed a dozen other galaxies that showed similar patterns. Combining their results with these earlier observations, the researchers concluded that the Milky Way’s warped spiral pattern is most likely caused by torque from the galaxy’s massive inner disc.

“Most of the matter in the Milky Way is dark matter,” de Grijs says. “But we have absolutely no idea what dark matter is or even where it is. Our research showing the shape of the Milky Way could help us determine how dark matter is distributed around our galaxy.”