An international team of astronomers has used 1,339 classical Cepheid stars — pulsating variable stars each up to 100,000 brighter than our Sun — to map the real shape of our Milky Way Galaxy. The team has found the Milky Way’s stellar disk becomes increasingly warped and twisted the further away the stars are from the Galaxy’s center.
From a great distance, our Milky Way Galaxy would look like a thin disk of stars that orbits once every few hundred million years around its central region, where hundreds of billions of stars provide the gravitational ‘glue’ to hold it all together. But this pull of gravity is much weaker in the Galaxy’s far outer disk. There, the hydrogen atoms making up most of the Milky Way’s gas disk are no longer confined to a thin plane, instead they give the disk an S-like, or warped, appearance. Image credit: Xiaodian Chen.
For the past five decades there have been indications that the hydrogen clouds in the Milky Way Galaxy are warped.
The new map, produced by astronomers from Macquarie University in Australia and the Chinese Academy of Sciences, shows that the warped Milky Way disk also contains young stars.
It confirms that the warped spiral pattern is caused by torque from the spinning of the Milky Way’s massive inner disk of stars.
“We usually think of spiral galaxies as being quite flat, like Andromeda which you can easily see through a telescope,” said Macquarie University’s Professor Richard de Grijs, co-author of the study.
Professor de Grijs and colleagues built their map using data for 1,339 classical Cepheid stars from NASA’s Wide-field Infrared Survey Explorer (WISE).
“It is notoriously difficult to determine distances from the Sun to parts of the Milky Way’s outer gas disk without having a clear idea of what that disk actually looks like,” said Chinese Academy of Sciences astronomer Dr. Xiaodian Chen, lead author of the study.
“This research provides a crucial updated map for studies of our Galaxy’s stellar motions and the origins of the Milky Way’s disk,” said co-author Dr. Licai Deng, also from the Chinese Academy of Sciences.
Classical Cepheids are young stars that are some 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.
They 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 1,339 Cepheid stars and the Milky Way’s gas disk follow each other closely,” Professor de Grijs said.
“This offers new insights into the formation of our home Galaxy.”
“Perhaps more importantly, in the Milky Way’s outer regions, we found that the S-like stellar disk is warped in a progressively twisted spiral pattern.”
The results were published online this week in the journal Nature Astronomy.
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Xiaodian Chen et al. An intuitive 3D map of the Galactic warp’s precession traced by classical Cepheids. Nature Astronomy, published online February 4, 2019; doi: 10.1038/s41550-018-0686-7
