ESA's Gaia Mission Discovers 'Shiva' and 'Shakti', Star Streams That May Have Helped Form the Galaxy

New Delhi: In what is being termed a significant breakthrough in the quest to understand the origin of the Milky Way Galaxy that humanity inhabits, the European Space Agency's Gaia project has discovered two ancient star streams that may have helped shape the galaxy in its infancy. Named ‘Shiva’ and Shakti' by the researchers who discovered them, these streams are stated to be so old that they likely predate the formation of the oldest parts of the Milky Way's spiral arms and disc. 

A startling discovery

The European Space Agency's Gaia project, in its own words, is aimed at “building the largest, most precise three-dimensional map of our Galaxy by surveying nearly two billion objects.” A significant part of this mission is to discover the origins of the Milky Way Galaxy and the stars that it contains. This is referred to as ‘galactic archaeology’ and, in 2022, experts making use of the Gaia space telescope discovered the ‘ancient heart’ of the Milky Way Galaxy — a region of space in the centre of the Galaxy which contains the oldest stars. 

According to Hans Walter Rix, a researcher at the Max Planck Institute for Astronomy who was not only involved in the 2022 study but is also a co-author in the recent Shiva and Shakti study, “The stars there are so ancient that they lack many of the heavier metal elements created later in the Universe’s lifetime. These heavy metals are those forged within stars and scattered through space when they die. The stars in our galaxy’s heart are metal-poor, so we dubbed this region the Milky Way’s ‘poor old heart’.” 

Rix noted that, until now, experts have only recognised these early fragments that coalesced to form the Galaxy's ‘heart’ but now, with the discovery of Shiva and Shakti, “we now see the first pieces that seem comparably old but located further out. These signify the first steps of our galaxy's growth towards its present size.”

Khyati Malhan, co-author of the study and a researcher at the Max Planck University underscored the extraordinary nature of the discovery by stating “What’s truly amazing is that we can detect these ancient structures at all. The Milky Way has changed so significantly since these stars were born that we wouldn’t expect to recognise them so clearly as a group – but the unprecedented data we’re getting from Gaia made it possible.”

The data in question allowed the researchers to determine the orbits, content and composition of the individual stars within the Milky Way Galaxy. “When we visualised the orbits of all these stars, two new structures stood out from the rest among stars of a certain chemical composition. We named them Shiva and Shakti," said Malhan. 

Each of these streams, located near the Galaxy's ‘heart’, is said to contain the mass of 10 million Suns, with stars that are between 12 and 13 billion years old, all of them moving in similar orbits with similar compositions.

According to the ESA, the way they are distributed suggests that these streams may have formed as distinct fragments that merged with the Milky Way Galaxy in its infancy. 

These streams, the study notes, while very similar, are not identical. Shakti orbits a little further away from the Galaxy's centre as compared to Shiva and in a more circular orbit. 

As for why any of this matters, Timo Prusti, Project Scientist for Gaia at the ESA said "As we discover surprise parts of our galaxy like the Shiva and Shakti streams, we’re filling the gaps and painting a fuller picture of not only our current home, but our earliest cosmic history.”