The international team, including experts from the University of Hertfordshire, the UK, Spain, and China, made the discovery while examining stars near Earth. They identified a pair of stars-gravitationally bound together-consisting of a white dwarf and an ultracool subdwarf. The pair is believed to have originated from the Milky Way's halo, a distant and sparsely populated region surrounding the galaxy.

The white dwarf, named VVV1256-62A, is about half the mass of our Sun and has cooled over billions of years, placing it near the bottom of the white dwarf cooling sequence. White dwarfs form at the end of a star's life cycle, cooling and reddening over time as they lack the nuclear fusion that powers other stars.

Its companion, VVV1256-62B, is a low-metallicity subdwarf, meaning it contains very few elements heavier than hydrogen and helium. Such low metallicity is an indicator of the star's advanced age, as heavier elements were scarce during the Milky Way's early formation. This star is particularly noteworthy as it lies at the boundary between stellar and substellar objects, offering valuable insights into metal-poor ultracool atmospheres.

The binary system has a highly eccentric orbit, leading to significant variations in the distance between the two stars. Typically residing in the Milky Way's halo, the stars' orbit occasionally brings them into the Milky Way's plane, the galaxy's central region where most of its mass is concentrated.

The ultracool subdwarf was initially identified by Leigh Smith, a University of Hertfordshire PhD student, due to its large motion. It was later confirmed as an ultracool subdwarf by former University of Hertfordshire PhD student Zenghua Zhang. The discovery of its white dwarf companion and the binary nature of the system was made by current University of Hertfordshire PhD student Sayan Baig, in collaboration with Zenghua Zhang, now a faculty member at Nanjing University.

These findings were made possible with the support of the Science Technology and Research Council, funding from the University of Hertfordshire, and the European Space Agency's Gaia mission. Multiple telescopes, including the Gemini South telescope, the Very Large Telescope of the European Southern Observatory, and the Dark Energy Camera Plane Survey of the Blanco telescope at the Cerro Tololo Inter-American Observatory in Chile, played a crucial role in confirming the discoveries.

"These fascinating discoveries open a window into the farthest reaches of our galaxy. Understanding the link between the halo and the plane of the Milky Way is a step towards understanding how the galaxy was formed. While the massive, eccentric orbit of these stars has yet to be explained, it could be related to the existence of an inner halo or to a past merger of the Milky Way with another galaxy," said Professor Hugh Jones, Professor of Astronomy at the University of Hertfordshire. "I'm immensely proud of the current and former University of Hertfordshire students who have been involved in this is phenomenal discovery."

You can read more about the discovery online via the European Space Agency's webpage about the star system here.

Research Report:Primeval very low-mass stars and brown dwarfs

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Stellar Chemistry, The Universe And All Within It