Milky Way is Embedded in a Huge Halo of Stars –“That Travel Together Like Flocks of Birds”

 

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A recent analysis of data for millions of stars from the Gaia space mission show that many stars in the halo that surrounds the Milky Way travel in groups..  Astronomers report their discovery today in the international journal Astronomy & Astrophysics.


The Milky Way has likely formed in part from the merging of many smaller systems. How exactly that happened, is still a puzzle. Astronomers from the University of Groningen in the Netherlands and UC Riverside in the US, studied the motions of stars in the so-called Galactic halo, which are more pristine and spend most of their time outside of the disk-like structure that gives the Milky Way its name. It is thought that these halo stars are the stars that joined the Milky Way onboard small galaxies.

The image below shows the Milky Way disk embedded in a roundish halo of stars. The stars (in purple) are from a computer simulation of the remains from a merger with a small galaxy. The arrows indicate the motion of these stars that are now part of the halo. Larger arrows indicate faster motion. The astronomers suspect that tens to hundreds of such flows of stars are crisscrossing the Milky Way.

 

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The team of astronomers were bewildered of the behavior of halo stars that spend most of the time in the outskirts of the Milky Way. Surprisingly more than 70% of those stars appear to be moving in the opposite sense than the vast majority of stars in the Milky Way. Such a high fraction is unexpected in current models. "One may compare stars from the outer halo with commuters that drive the wrong way. We do not yet quite understand why," said Helmi.

These discoveries were made using halo stars that, in their journey through the Milky Way, are by chance currently close to the Sun. In the future, Gaia will provide us with data from stars from all over the Milky Way. "With such data we will get many new insights on how the Milky Way formed and be able to reconstruct its genealogy tree," said Helmi.

The Daily Galaxy via Netherlands Research School for Astronomy

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