May 18, 2024

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Two ancient stellar streams that helped build the early Milky Way Galaxy have been found

Two ancient stellar streams that helped build the early Milky Way Galaxy have been found

S. Payne-Wardenar/K. Malhan/MPIA

Illustration depicting the Milky Way from above, with green dots indicating the Shiva stars and pink dots indicating the Shakti stars.

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Astronomers used the Gaia space telescope to spy on some of the galaxy's first building blocks Milky Way: Two ancient streams of stars called Shakti and Shiva helped our home galaxy grow and evolve more than 12 billion years ago.

These star streams, named after Hindu deities, appear to be the remains of two galaxies that merged with an early version of the Milky Way 12 to 13 billion years ago when the first galaxies were forming across the universe. The structures are so ancient that they formed before the oldest parts of the spiral arms and central disk of the Milky Way.

A study detailing the observations appeared Thursday in Astrophysical Journal.

“What is truly amazing is that we can discover these ancient structures at all,” lead study author Dr. Khayati Malhan, a postdoctoral researcher and Humboldt Research Fellow at Stockholm University in Sweden, said in a statement. “The Milky Way has changed so significantly since the birth of these stars that we did not expect to clearly recognize them as a group – but the unprecedented data we obtained from Gaia makes that possible.”

According to the researchers, observing the stellar wonders of Shakti and Shiva could help astronomers unravel the secrets of the early days of the Milky Way and the evolution of similar massive galaxies throughout the universe.

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The Gaia space telescope, launched by the European Space Agency in 2013, began observing the universe the following year. Astronomers used Gaia observations to discover previously unknown structures in the Milky Way, helping them piece together the history of the galaxy. The telescope's data set also provided astronomers with positions, distances, and motions Nearly 2 billion stars in the galaxy.

In 2022, study co-author Hans-Walter Rex and his colleagues used Gaia to look into the heart of the Milky Way and discovered the oldest stars ever found in the galaxy during “Galactic archeology“Notes. Analysis of data from nearly 6 million stars observed by Gaia and the Sloan Digital Sky Survey has revealed two streams that appear distinct from the rest.

The survey data included details about the chemical composition of the stars.

“We observed that for a particular population of metal-poor stars, the stars were crowded around two specific bands of energy and angular momentum,” Malhan said.

Shakti and Shiva are located near the heart of the Milky Way, and it is estimated that the mass of each stream is equivalent to about 10 million suns. Here, all the ancient stars are similar in age, orbital path, and composition, which helped astronomers determine that both streams were likely filaments from an external source that wove together and became part of the Milky Way.


The distribution of the stars Shakti (yellow) and Shiva (blue) can be seen near the heart of the Milky Way.

The researchers compared the discovery of Shiva and Shakti to finding the initial traces of an ancient settlement that eventually grew into a large, modern city.

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“The stars there are so old that they lack many of the heavy metal elements that were formed later in the universe. These heavy metals are the ones that form inside stars and spread out into space when they die. “These heavy metals are the ones that form inside stars and spread out into space when they die,” said Rex, director of the Max Planck Institute for the Department of Galaxies and Cosmology in Germany. In a statement: “The stars in the heart of our galaxy are poor in metals, so we called this region the ‘poor old heart’ of the Milky Way.”

“So far, we have only recognized these very early pieces that came together to form the ancient core of the Milky Way. With Shakti and Shiva, we are now seeing the first pieces that appear relatively ancient but exist farther away. This indicates the first steps of our galaxy's growth toward its current size.”

The Milky Way started small and then grew in size as it merged with other galaxies, gaining stars as well as hydrogen to create more stars. Every galaxy contains hydrogen gas, which helps in the birth of stars. When galaxies merge and collide, their clouds of hydrogen gas are disrupted, which can lead to a star birth frenzy.

Over time, long filaments of gas and dust collected in the Milky Way, giving rise to the modern spiral structure of the galaxy today.

Gaia has already helped astronomers determine when the Milky Way experienced various merger events in the past, and future observations could open up more insights.

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“Revealing more about the origins of our galaxy is one of Gaia’s goals, and it certainly achieves that,” said Timo Prosti, a Gaia project scientist at the European Space Agency, who was not involved in the study.

“We need to identify the subtle, crucial differences between stars in the Milky Way to understand how our galaxy formed and evolved. This requires incredibly precise data — and now, thanks to Gaia, we have that data. As we discover surprising parts of our galaxy like the Shiva and Shakti streams, we are filling in the gaps We paint a fuller picture not only of our current habitat, but of our early cosmic history.