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S. Payne-Wardenaar/K. Malhan/MPIA
An illustration shows the Milky Way from above, with green dots indicating the Shiva stars and pink dots representing the Shakti stars.
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Astronomers have used the Gaia space telescope to spy on some of the earliest building blocks of the Milky Way– Two ancient streams of stars called Shakti and Shiva that helped our galaxy grow and evolve more than 12 billion years ago.
The star streams, named after Hindu deities, appear to be the remains of two galaxies that merged with an early version of the Milky Way between 12 and 13 billion years ago, when the first galaxies were forming around The cosmos. The structures are so old that they formed long before the oldest parts of the Milky Way’s iconic spiral arms and central disk.
A study detailing the observations appeared Thursday in The Astrophysical Magazine.
“What is really surprising is that we can detect these ancient structures,” the study’s lead author, Dr. Khyati Malhan, a postdoctoral researcher and Humboldt researcher at Stockholm University in Sweden, said in a statement. “The Milky Way has changed so significantly since these stars were born that we wouldn’t expect to recognize them so clearly as a group, but the unprecedented data we’re getting from Gaia made it possible.”
Observing the stellar wonders of Shakti and Shiva could help astronomers unlock the secrets of the Milky Way’s early days and the evolution of equally massive galaxies across the cosmos, researchers say.
The Gaia space telescope, launched in 2013 by the European Space Agency, began observing the universe the following year. Astronomers have used Gaia observations to discover previously unknown structures in the Milky Way, helping them reconstruct the galaxy’s history. The telescope’s data set has also provided astronomers with the positions, distances and movements of almost 2 billion stars in the galaxy.
In 2022, study co-author Hans-Walter Rix and his colleagues used Gaia to peer into the heart of the Milky Way and discovered the oldest stars ever found in the galaxy during their “galactic archeology“observations. An analysis of data from nearly 6 million stars observed by Gaia and the Sloan Digital Sky Survey revealed two streams that seemed to stand out from the rest.
The study data contained details about the chemical composition of the stars.
“We observed that, for a certain range of metal-poor stars, the stars were crowded around two specific combinations of energy and angular momentum,” Malhan said.
Shakti and Shiva are near the heart of the Milky Way and each stream is estimated to include the mass of about 10 million suns. Here, the ancient stars are all similar in age, orbital path and composition, which helped astronomers determine that both streams were likely threads from an external source that intertwined and became part of the Milky Way.
ESA/Gaia/DPAC/K. malhan
The distribution of the stars Shakti (yellow) and Shiva (blue) can be seen near the heart of the Milky Way.
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.
“The stars there are so old that they lack many of the heavier metallic elements created later in the life of the Universe. These heavy metals are what are forged inside stars and spread through space when they die. “The stars in the heart of our galaxy are metal-poor, so we call this region the ‘poor old heart’ of the Milky Way,” said Rix, director of the department of galaxies and cosmology at the Max Planck Institute for Astronomy in Germany. , it’s a statement.
“Until now, we had only recognized these first fragments that came together to form the ancient heart of the Milky Way. With Shakti and Shiva, we now see the first pieces that appear relatively ancient but situated further away. “These signify the first steps in the growth of our galaxy towards its current size.”
The Milky Way started small and grew in size as it merged with other galaxies, gaining stars and hydrogen to form more stars. Every galaxy has hydrogen gas that helps star birth. As galaxies merge and collide, their clouds of hydrogen gas break up, which can create a frenzy of star birth.
Over time, the Milky Way’s long filaments of gas and dust merged, resulting in the modern spiral structure of today’s galaxy.
Gaia has already helped astronomers determine when the Milky Way experienced different merger events in the past, and future observations could reveal more insights.
“Revealing more about the infancy of our galaxy is one of Gaia’s goals, and it is certainly achieving that,” said Timo Prusti, a Gaia project scientist at the European Space Agency, who was not involved in the study.
“We need to identify the subtle but crucial differences between the stars of 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, such as the Shiva and Shakti currents, we fill in the gaps and paint a more complete picture not only of our current home, but also of our earliest cosmic history.”