Madrid 2 (European Press)

Astronomers have discovered a new type of binary star that has been speculated to exist for 50 years, confirming how a rare type of star in the universe forms and evolves.

The new star class, described in Monthly Notices of the Royal Astronomical Society, was discovered by postdoctoral fellow at Harvard-Smithsonian Center for Astrophysics Karim El-Badri using the Shin Telescope at the Lake Observatory in California and data from various astronomical studies.

“We have observed the first physical evidence of a new group of transition binary stars,” Al-Badri says in a statement. “That’s exciting; it’s an evolutionary link missing in binary models of star formation that we’ve been looking for.”

When a star dies, there is a 97% chance that it will turn into a white dwarf, a small, dense body that shrinks and becomes faint after burning all of its fuel.

But in rare cases, the star can turn into an extremely low-mass white dwarf (ELM). At less than a third of the mass of the Sun, these stars present a conundrum: If the calculations of stellar evolution are correct, all ELM white dwarfs would appear more than 13.8 billion years old, older than the age of the universe itself and, therefore, physically impossible.

“The universe simply isn’t old enough to produce these stars by natural evolution,” says El-Badri, a fellow at the Institute for Theory and Computing at the Harvard-Smithsonian Center for Astrophysics.

Over the years, astronomers have come to the conclusion that the only way to form an ELM white dwarf is with the help of a binary partner. The gravity of a nearby companion star rapidly (at least, in less than 13.8 billion years) can devour a star until it becomes an ELM white dwarf.

But the evidence for this picture is not infallible. Astronomers have observed massive regular stars like our Sun that accumulate to become white dwarfs, which are called catastrophic variables. They have also observed ELM white dwarfs with normal white dwarf companions. However, they did not observe the transitional phase of evolution, or the intermediate transition: when the star loses most of its mass and shrinks nearly to become an ELM white dwarf.

Al-Badri often compares stellar astronomy to zoology in the nineteenth century. “You go out into the woods and find an organism. Describe its size, how much it weighs, and then move on to another organism,” he explains. “You see all these different types of objects and you need to reconstruct how they are related.”

In 2020, Al-Badri decides to return to the forest in search of the star scientists have long hinted at: the white dwarf before ELM (also known as the Evolving Cataclysmic Variable).

Using new data from Gaia, the space observatory launched by the European Space Agency and the Zwicky Transient Facility at the California Institute of Technology, Al-Badri narrowed down a billion stars to 50 potential candidates.

The astronomer emphasizes the importance of general data from astronomical studies to his work. “If it weren’t for projects like the Zwicky Transient Facility and Gaia, which represent a huge amount of behind-the-scenes work for hundreds of people, this work wouldn’t be possible,” he says.

Then Badri followed up with detailed notes for 21 of the stars. The selection strategy worked. “100 percent of the candidates were by the ELMs we were looking for,” he says. “It was more puffy than the ELMs. It was also egg-shaped because the gravity of the other star distorted its spherical shape.”

“We found the evolutionary link between two classes of binary stars, cataclysmic variants and ELM white dwarfs, and found an appropriate number of them,” Al-Badri adds.

Thirteen of the stars showed signs that they were still losing mass to their companion, while eight of the stars seemed to lose no mass. All of them were also much hotter in temperature than the previously observed catastrophic variables.

Al-Badri plans to continue studying white dwarfs before ELM and may track 29 other candidate stars he discovered earlier.