Milky Way’s Halo Yields Clues to its Evolution


A team of astronomers succeeded in decoding the earliest phases of the evolutionary history of the Milky Way. The scientists, from the Argelander Institute for Astronomy at Bonn University and the Max-Planck Institute for Radio Astronomy in Bonn, found that the early Galaxy went from smooth to clumpy in just a few hundred million years.

The researchers looked at the spherical groups of stars known as globular clusters that lie in the halo of the Milky Way, outside the more familiar spiral arms where the Sun is found. They each contain hundreds of thousands of stars and are thought to have formed at the same time as the ‘proto-Galaxy’ that eventually evolved into the Milky Way.

Globular star clusters can be thought of as fossils from the earliest period of the history of the Galaxy and the astronomers found that they left a hint of the conditions under which they formed. The stars of the clusters condensed out of a cloud of molecular gas, not all of which was used up in their formation. The residual gas was expelled by the radiation and winds coming from the freshly hatched population of stars.

“Due to this ejection of gas, the globular clusters expanded and thereby lost the stars that formed at their boundaries. This means that the present shape of the clusters was directly influenced by what happened in the early days of their existence”, explains Michael Marks, PhD student of Professor Kroupa and lead author on the new paper.

The clusters were also shaped by the forming Milky Way and the Bonn scientists calculated exactly how the proto-Galaxy affected its smaller neighbours. Their results show that the gravitational forces exerted on the star clusters by the proto-Milky Way appear to increase with the metal content of their member stars (in astronomy ‘metals’ in stars are elements heavier than helium).

“The amount of iron in a star is therefore an age indicator. The more recently a star cluster was born, the higher the proportion of heavy elements it contains”, adds Marks. But since the globular clusters are more or less the same age, these age differences can't be large. In order to explain the variation in the forces exerted on different globular clusters, the structure of the Milky Way had to change rapidly within a short time.

The giant gas cloud from which the Milky Way formed had to evolve from an overall smooth structure into a clumpy object in less than a few hundred million years in order to increase the strength of the forces significantly. This timespan corresponds to the astronomically short duration in which the proto-galaxy-sized gas cloud collapsed under its own gravity. In parallel, the globular clusters formed successively within the collapsing cloud. The material from which the somewhat younger globular clusters formed and which according to the results of this investigation felt stronger attractive forces, was previously enriched with heavy elements by fast-evolving stars in the older clusters.

“In this picture we can elegantly combine the observational and theoretical results and understand why later forming, more metal-rich clusters experienced stronger force fields. On the back of this work, for the first time we have a detailed insight into the earliest evolutionary history of our Galaxy," Prof. Pavel Kroupa.

Image at top of page shows 47 Tucanae –what astronomers refer to as a "globular cluster." Globular clusters are compact regions with anywhere from ten thousand to several million stars. 47 Tucanae is an ancient cluster of several million stars located about 15,000 light years from Earth. The stars in 47 Tucanae are an estimated 10-12 billion years old, placing them among the oldest in our galaxy. Because the stars in clusters like 47 Tucanae are about the same age, they make perfect laboratories for the study of stellar life cycles.

The Daily Galaxy via Royal Astronomical Society and

Image credit: Southern African Large Telescope (SALT) project


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