Hidden stars found in Earth's cosmic backyard

(CN) — Four dead stars have been hiding in Earth’s cosmic backyard, unnoticed for decades despite lying within just 65 light-years of the sun.

Astronomers say the newly confirmed white dwarfs were concealed by larger, brighter red dwarf companions, making each system appear to contain only a single star.

The study, published Monday in the Monthly Notices of the Royal Astronomical Society, found that one hidden star, just 25 light-years away, is now the ninth-closest known white dwarf to the sun.

Researchers first identified the systems by detecting a subtle radial wobble in the visible red dwarfs, a back-and-forth motion that often signals an unseen companion. They then used ultraviolet observations from the Hubble Space Telescope to investigate further.

“Through our research, we confirmed the existence of four new stars right on our back doorstep. Our research shows that there is still so much we don’t know about what’s in our local neighborhood of space,” wrote lead author Mairi O’Brien, a research fellow at the University of Warwick, in an interview. “Astronomers are able to observe other galaxies that are almost as old as the age of our Universe with the James Webb Space Telescope, and yet we are still discovering new stars so much closer to home.”

Astronomers have surveyed nearby stars for decades, but systems like these have proven especially difficult to identify because the brighter red dwarfs overwhelm the white dwarfs in visible light.

“The red dwarfs in our binary systems burn brightly in visible light, which is the light our eyes can see. The white dwarfs, however, are twice as hot as their red dwarf companions and are brighter in the ultraviolet,” O’Brien said.

That meant researchers had to look beyond visible light, using precise ultraviolet spectroscopy from the Hubble Space Telescope to distinguish hidden white dwarfs from ultraviolet flares commonly produced by red dwarfs.

“The spectroscopy helped us confirm the white dwarfs were really there,” O’Brien said.

One newly confirmed system, G 203-47, stood out for another reason. Although its hidden white dwarf orbits its companion every 14.9 days, the red dwarf rotates only once every 100 days or more, far slower than expected for this type of binary system.

“The systems we have observed are called post-common envelope binaries. These binaries exist in two populations: ones with fast orbits and ones with slow orbits. G 203-47 is unusual because it sits in between these two populations,” O’Brien said. “We found that even though the white dwarf orbits its companion every 15 days, it rotates much more slowly, likely taking over 100 days to complete one spin. This finding places G 203-47 at the extreme low end of the long-period population. Finding extreme systems like this one helps us to understand how binary stars evolve.”

The discoveries also matched long-standing predictions that four to five closely orbiting white dwarf-red dwarf pairs should exist within about 65 light-years of Earth. Researchers identified four, supporting those population models.

The team believes more hidden white dwarfs remain undiscovered, as only about 30% of nearby red dwarfs have been systematically searched for white dwarf companions.

“A data release from the Gaia satellite is scheduled for the end of 2026. Gaia covers the whole sky, meaning many new red dwarfs will be surveyed,” O’Brien said.

Researchers say the new Gaia data could reveal nearby red dwarfs showing the characteristic wobble caused by an unseen white dwarf companion. Any promising candidates would then need to be observed with the Hubble Space Telescope to confirm whether a hidden white dwarf is present.

“The only telescope that can currently provide high-quality ultraviolet spectra to confirm these nearby candidate white dwarfs is the Hubble Space Telescope. We would need to follow up the new candidates with Hubble to confirm them,” O’Brien said.