Sign up for CNN’s Wonder Theory science newsletter. Explore the universe with news about fascinating discoveries, scientific developments and more.
Astronomers have used the Gaia Space Telescope to spy on some of the Milky Way galaxy’s first building blocks: two ancient star streams called Shakti and Shiva that helped our home galaxy grow and evolve more than 12 billion years ago.
The star streams, named after Hindu gods, appear to be the remnants of two galaxies that merged with an early version of the Milky Way between 12 and 13 billion years ago, when the first galaxies formed in 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 was published Thursday in The Astrophysical Journal.
“What’s really amazing is that we can detect these ancient structures at all,” says lead study author Dr. Khyati Malhan, postdoctoral scholar and Humboldt Research Fellow at Stockholm University in Sweden, said in a statement. “The Milky Way has changed so dramatically since the birth of these stars that we wouldn’t expect to recognize them so clearly as a group – but the unprecedented data we’re getting from Gaia has made this possible.”
Observing the stellar wonders of Shakti and Shiva could help astronomers unlock the secrets of the earliest days of the Milky Way Galaxy and the evolution of similarly massive galaxies across the cosmos, the researchers said.
‘The first steps of the growth of our galaxy
The Gaia Space Telescope, launched in 2013 by the European Space Agency, began observing the universe the following year. Astronomers have used Gaia’s observations to discover previously unknown structures in the Milky Way, allowing them to trace the history of the Milky Way. The telescope’s dataset has also provided astronomers with the positions, distances and movements almost 2 billion stars in the Milky Way.
In 2022, 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 Milky Way during their “galactic archaeology” observations. An analysis of data from nearly six million stars observed by Gaia and the Sloan Digital Sky Survey revealed two streams that seemed to stand out from the rest.
The research data contains details about the chemical composition of the stars.
“We observed that for a certain range of metal-poor stars, the stars are clustered around two specific combinations of energy and angular momentum,” Malhan says.
Shakti and Shiva are located near the heart of the Milky Way, and each stream is estimated to include about 10 million suns. Here, the old 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.
The researchers compared the discovery of Shiva and Shakti to finding the first 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 that formed later in the life of the universe. These heavy metals are metals that are forged in stars and are scattered through space as they die. The stars in the heart of our Milky Way are metal-poor, so we have named this region the ‘poor old heart’ of the Milky Way,” said Rix, director of the galaxies and cosmology department at the Max Planck Institute for Astronomy in Germany, in a declaration.
“Until now, we had only recognized these very early fragments that came together to form the ancient heart of the Milky Way. At Shakti and Shiva we now see the first pieces that appear relatively old but are further away. These represent the first steps in the growth of our galaxy to its current size.”
Tracking Galactic Evolution
The Milky Way started out small and grew larger as it merged with other galaxies, creating both stars and hydrogen to form more stars. Every galaxy has hydrogen gas that helps in the birth of stars. As galaxies merge and collide, their hydrogen gas clouds are disrupted, which can cause a frenzy of star birth.
Over time, the Milky Way’s long filaments of gas and dust coalesced, resulting in the modern spiral structure of today’s galaxy.
Gaia has already helped astronomers determine when the Milky Way has experienced various merger events in the past, and future observations could unlock more insights.
“Revealing more about the infancy of our galaxy is one of Gaia’s goals, and it is certainly achieving that goal,” said Timo Prusti, project scientist for Gaia at the European Space Agency, who was not involved in the study.
“We need to spot the subtle but crucial differences between stars in the Milky Way to understand how our galaxy formed and evolved. This requires incredibly accurate data – and thanks to Gaia, we now have that data. As we discover surprising parts of our Galaxy, such as the Shiva and Shakti currents, we are filling in the gaps and painting a fuller picture of not only our current home, but also our earliest cosmic history.”
For more CNN news and newsletters, create an account at CNN.com