Bent star disk: The Milky Way is not completely flat, but clearly deformed, as a study revealed. For example, the outer regions of our galaxy in large arcs tend to move up and down from the main plane of the star disc. For the first time astronomers have been able to determine the size and shape of these "bumps" using variable stars – and they have also received the first indications for their cause.
It seems almost paradoxical: precisely because the Milky Way is our cosmic home, we know that much less well than, for example, our neighboring Galaxy Andromeda. Because our position in the middle of this cluster of stars makes it impossible for us to see the Milky Way as a whole. It just seems clear that our galaxy is a spiral rod, whose spiral star arms concentrate in a flat main plane. At the same time, however, the Milky Way is a dynamic system characterized by internal changes and external influences.
Variable stars as a position indicator
There has also been evidence for some time that the Milky Way can be less flat than previously thought, especially in the starry region. But how our galaxy differs from the flat form was previously only roughly estimated. In addition, it is unclear whether the shape of the gas and star disc matches in the Melkweg. Because it is necessary to measure the distance of stars and other objects in the outer area of the Milky Way as precisely as possible.
Now Xiaodian Chen from the National Astronomical Observatory in China and his team has succeeded. For their study they had evaluated the positions of variable stars, the so-called Cepheids. These young, massive stars exhibit regular fluctuations in brightness, the tact of which is directly linked to their brightness. This stroke, in combination with its apparent brightness, thus reveals quite a lot of the distance. By using 1,339 Cepheids as position markers, the researchers have now determined the three-dimensional shape of the Milky Way and the outer regions more accurately than before.
Curved in an S-shape
The result: the galaxy and the star disc of the Milky Way are relatively congruent, but both are remarkably curved. "We usually think that spiral galaxies are quite flat, such as Andromeda, which you can see through a telescope," says co-author Richard de Grijs of Macquarie University in Sydney. But this is not the case with the Milky Way. Instead, some parts of the galactic disk are up, others bulge out.
© Xiaodian Chen
The Milky Way is therefore slightly s-shaped from the side. "In the outer regions of the galaxy we also discovered that the s-shaped star disk was deformed in an increasingly curved spiral pattern," says De Grijs. He and his colleagues suspect that this distortion is the result of interactions with the rotation movement of the Milky Way Center. Evidence for this is provided by observations of comparable deformation patterns in a dozen other spiral galaxies.
Important basis for further research
"These results provide us with a new basic map for the study of star movements in the galaxy," says Chen's colleague Licai Deng. For without a clear idea of what the Milky Way looks like, it would be very difficult to accurately determine the distances from the Sun to the outer regions of the galaxy. Moreover, the new findings can help to further decipher the history of the Milky Way. (Nature Astronomy, 2019; doi: 10.1038 / s41550-018-0686-7)
Source: Chinese Academy of Sciences