Researchers have discovered that an unusual pair of ten-billion-year-old stars are visiting from the farthest corners of the galaxy. They formed in the early days of the Milky Way.

The international team of experts from the UK, Spain and China, including the University of Hertfordshire, were studying stars near Earth when they discovered the binary star system – two stars bound together by gravity. To their surprise, they found that the pair had migrated from the Milky Way’s halo into our local stellar neighborhood.

The work appeared in Monthly Notices of the Royal Astronomical Society as the 8th article in a series entitled “Primeval very low-mass stars and brown dwarfs.”

The pair consists of a white dwarf and an ultracool subdwarf with very large motions, suggesting that they spend most of their time far away from the visible stars in our galaxy.

The team calculated its age based on the mass of the white dwarf (called VVV1256-62A) and the time it took to cool. This particular star has about half the mass of our Sun and is at the bottom of the white dwarf cooling phase, meaning it took billions of years to cool.

A white dwarf is formed at the end of the evolution of a normal star. At the beginning of their life, these stars are very hot, but without the active fusion that keeps other stars alive, they gradually cool down and turn red.

The other star (designated VVV1256-62B) is a low-metallicity subdwarf, meaning it doesn’t contain many elements heavier than hydrogen and helium. Although it’s harder to determine ages so directly, low metallicities themselves are an indication of old age, since there were very few heavier elements in the early stages of the Milky Way when it was forming.

If a star lacks these heavier elements, it can give us clues about the Milky Way’s very distant past. This subdwarf is particularly interesting because it lies on the border between stellar and substellar objects, making it an age benchmark for studying metal-poor ultracool atmospheres.

This binary star system is also interesting because it has a highly eccentric orbit, meaning that the distance between the two stars varies considerably. For this reason, the stars are usually found in the halo of the Milky Way, but their orbit also takes them into the Milky Way plane, the central region of the galaxy where most of its mass is located.

The ultracool subdwarf component of this binary system was originally identified based on its large motion by PhD student Leigh Smith of the University of Hertfordshire and confirmed as an ultracool subdwarf by former PhD student Zenghua Zhang of the University of Hertfordshire.

Its white dwarf companion and its binary nature were then discovered in parallel by Sayan Baig, a current PhD student at the University of Hertfordshire, and Zenghua Zhang, who now holds a faculty position at Nanjing University.

Telescopes around the world were used to confirm these discoveries, including the Gemini South Telescope, one half of the International Gemini Observatory, which is supported in part by the U.S. National Science Foundation (NSF).

Also involved were 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.

Professor Hugh Jones, Professor of Astronomy at the University of Hertfordshire, said: “These fascinating discoveries open a window into the farthest reaches of our galaxy.”

“Understanding the connection between the halo and the plane of the Milky Way is a step toward understanding the formation of the galaxy. While the massive, eccentric orbit of these stars has not yet been explained, it could be related to the existence of an inner halo or a previous merger of the Milky Way with another galaxy.

“I am immensely proud of the current and former students of the University of Hertfordshire who were involved in this phenomenal discovery.”