Image: ESA
Named after a divine couple from Hindu philosophy, Shakti and Shiva are ancient star streams that scientists say could fill in some of the blanks on how the Milky Way formed.
The European Space Agency’s (ESA) Gaia space telescope has just made another discovery that may explain the Milky Way’s ‘galactic archaeology’ – or how the galaxy was formed billions of years ago.
The telescope spotted two star streams, called Shakti and Shiva. Each stream contains the mass of about 10m suns, with stars of between 12bn and 13bn years in age all moving in very similar orbits with similar compositions.
The way the stars are distributed leads scientists to believe that they may have formed as distinct fragments that merged with the Milky Way early in its composition. In fact, Shakti and Shiva may be so ancient that they may have formed before even the oldest parts of our present-day galaxy’s spiral arms and disc.
Not identical
Although they are similar, the two streams aren’t identical. Shakti stars orbit a little bit further from the Milky Way’s centre and in more circular orbits than Shiva stars. The streams are named after a divine couple from Hindu philosophy who unite to create the universe (or macrocosm).
“What’s truly amazing is that we can detect these ancient structures at all,” said Dr Khyati Malhan, who led the research.
“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,” said Malhan, who is based at the Max Planck Institute for Astronomy (MPIA) in Heidelberg, Germany.
Gaia is doing stellar work
Since its launch in 2013, Gaia’s views of space have contributed massively to scientists’ understanding of how the Milky Way works.
“Using Gaia observations, the researchers were able to determine the orbits of individual stars in the Milky Way, along with their content and composition. When we visualised the orbits of all these stars, two new structures stood out from the rest among stars of a certain chemical composition,” said Malhan.
“Revealing more about our galaxy’s infancy is one of Gaia’s goals, and it’s certainly achieving it,” said Timo Prusti, project scientist for Gaia at ESA. “We need to pinpoint the subtle yet 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 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.”
The resulting paper from the Shakti and Shiva discoveries has been published in The Astrophysical Journal. Gaia carries out regular surveys of the Milky Way, and scientists are hoping the next dataset will yield yet more answers.
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