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\t\t\t\t\t\t21\/03\/2024<\/span> ESA\u2019s Gaia space telescope has further disentangled the history of our galaxy, discovering two surprising streams of stars that formed and wove together over 12 billion years ago.<\/p>\n<\/div>\n The two streams, named Shakti and Shiva, helped form the infant Milky Way. Both are so ancient they likely formed before even the oldest parts of our present-day galaxy\u2019s spiral arms and disc.<\/p>\n \u201cWhat\u2019s truly amazing is that we can detect these ancient structures at all,\u201d says Khyati Malhan of the Max Planck Institute for Astronomy (MPIA) in Heidelberg, Germany, who led the research. \u201cThe Milky Way has changed so significantly since these stars were born that we wouldn\u2019t expect to recognise them so clearly as a group \u2013 but the unprecedented data we\u2019re getting from Gaia made it possible.\u201d<\/p>\n<\/p><\/div>\n Using Gaia observations, the researchers were able to determine the orbits of individual stars in the Milky Way, along with their content and composition. \u201cWhen we visualised the orbits of all these stars, two new structures stood out from the rest among stars of a certain chemical composition,\u201d adds Khyati. \u201cWe named them Shakti and Shiva.\u201d<\/p>\n<\/p><\/div>\n Each stream contains the mass of about 10 million Suns, with stars of 12 to 13 billion years in age all moving in very similar orbits with similar compositions. The way they\u2019re distributed suggests that they may have formed as distinct fragments that merged with the Milky Way early in its life.<\/p>\n Both streams lie towards the Milky Way\u2019s heart. Gaia explored this part of the Milky Way in 2022 using a kind of \u2018galactic archaeology\u2019; this showed the region to be filled with the oldest stars in the entire galaxy, all born before the disc of the Milky Way had even properly formed.<\/p>\n<\/p><\/div>\n \u201cThe stars there are so ancient that they lack many of the heavier metal elements created later in the Universe\u2019s lifetime. These heavy metals are those forged within stars and scattered through space when they die. The stars in our galaxy\u2019s heart are metal-poor, so we dubbed this region the Milky Way\u2019s \u2018poor old heart\u2019,\u201d says co-author Hans-Walter Rix, also of MPIA and the lead \u2018galactic archaeologist\u2019 from the 2022 work.<\/p>\n \u201cUntil now, we had only recognised these very early fragments that came together to form the Milky Way\u2019s ancient heart. With Shakti and Shiva, 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.\u201d<\/p>\n<\/p><\/div>\n While very similar, the two streams are not identical. Shakti stars orbit a little further from the Milky Way\u2019s centre and in more circular orbits than Shiva stars. Fittingly, the streams are named after a divine couple from Hindu philosophy who unite to create the Universe (or macrocosm).<\/p>\n<\/p><\/div>\n Some 12 billion years ago, the Milky Way looked very different to the orderly spiral we see today. We think that our galaxy formed as multiple long, irregular filaments of gas and dust coalesced, all forming stars and wrapping together to spark the birth of our galaxy as we know it. It seems that Shaki and Shiva are two of these components \u2013 and future Gaia data releases may reveal more.<\/p>\n Khyati and Hans-Walter also built a dynamical map of other known components that have played a role in our galaxy\u2019s formation and were discovered using Gaia data. These include Gaia-Sausage-Enceladus, LMS1\/Wukong, Arjuna\/Sequoia\/I\u2019itoi, and Pontus. These star groups all form part of the Milky Way\u2019s complex family tree, something that Gaia has worked to build over the past decade.<\/p>\n \u201cRevealing more about our galaxy\u2019s infancy is one of Gaia\u2019s goals, and it\u2019s certainly achieving it,\u201d says Timo Prusti, Project Scientist for Gaia at ESA. \u201cWe 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 \u2013 and now, thanks to Gaia, we have that data. As we discover surprise parts of our galaxy like the Shiva and Shakti streams, we\u2019re filling the gaps and painting a fuller picture of not only our current home, but our earliest cosmic history.\u201d<\/p>\n<\/p><\/div>\n ‘Shiva and Shakti: Presumed Proto-Galactic Fragments in the Inner Milky Way’ by K. Malhan and H.-W. Rix (2023) is published in\u00a0The Astrophysical Journal.<\/i> DOI: 10.3847\/1538-4357\/ad1885<\/p>\n ESA\u2019s Gaia space telescope\u202fwas launched on 19 December 2013 and has been surveying the skies since 2014. In this time, the mission has flipped our understanding of the Milky Way on its head, unveiling its shape and structure and revealing how mergers have affected the stars that call our galaxy home. Read more about some of the telescope’s key achievements.<\/p>\n \nFor more information, please contact:<\/b>
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\n\t\t\t\t\t\t\t\t<\/figcaption><\/figure>\nTruly ancient fragments <\/h2>\n
\n\t\t\t\t\t\t\t\t<\/figcaption><\/figure>\nA complex family tree <\/h2>\n
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Email:\u202fmedia@esa.int<\/p>\n<\/p><\/div>\n