An international team of astronomers has identified four previously undiscovered white dwarfs in the Sun’s vicinity. White dwarfs are the dense cores of dead stars that lacked sufficient mass to go supernova. In several billion years, the Sun will eventually transform into a red dwarf. The study was published in the journal Monthly Notices of the Royal Astronomical Society.
These four dead stars possess something the Sun does not: a red dwarf companion. All four objects existed as binary systems within 65 light-years of Earth, and the veteran Hubble Space Telescope managed to distinguish the small white dwarfs from their brighter, larger partners.
“Since white dwarfs are twice as hot as their red dwarf companions, their light shines brightest in a different part of the electromagnetic spectrum. Red dwarfs are more luminous in visible light—the kind we can see. But white dwarfs glow brighter in the ultraviolet range,” explained lead author Mary O’Brien from the University of Warwick.
Typically, when observing the universe in ultraviolet light, a white dwarf would dominate, outshining its companions. The challenge with binary systems containing red dwarfs is that red dwarfs flare up in the ultraviolet spectrum, creating significant confusion for astronomers. Many red dwarfs emit false signals that mimic those of white dwarfs.
“This meant we couldn’t rely on just any broadband ultraviolet data. We needed high-precision spectroscopy—splitting light into its component colors—using the Hubble Space Telescope to separate the white dwarf’s signal from the red dwarf’s,” O’Brien noted.
One particularly puzzling system is G 203-47, located 25 light-years away and currently the ninth closest white dwarf to the Sun. What makes it unusual is that the two objects are not tidally locked—they do not always face each other with the same side.
Gravitational forces typically induce tidal locking, like what happens between Earth and the Moon. However, in the case of G 203-47, despite their mutual gravitational pull, the red dwarf rotates at a pace exceeding 100 days, while the white dwarf spins in just 14.9 days.
The fact that these two stars are not tidally locked, despite gravity’s influence, suggests they may have followed a different evolutionary path to reach their current state compared to other pairs.
We know about the strong gravitational attraction not only from theory but also from measured oscillations in the red dwarf. The white dwarf’s gravity shakes its companion, and it was precisely this effect that drew astronomers’ attention to these objects and their potential as a search method for others.
“We aim to uncover more candidates using data from the Gaia satellite later this year. Gaia’s data will reveal whether any nearby red dwarfs exhibit the characteristic oscillations caused by a hidden white dwarf companion,” O’Brien said. “Currently, the only telescope capable of providing the ultraviolet spectroscopy needed to confirm these candidates is Hubble, and we’re eager to continue investigating some nearby possibilities.”
Theoretical estimates had predicted the existence of four to five pairs of red and white dwarfs within 65 light-years of the Sun, so this discovery aligns perfectly with those predictions. It also highlights that surprises still await us in the Sun’s neighborhood.
“The stars we’re observing are white dwarfs—dead stars that have exhausted their fuel. In billions of years, the Sun itself will become a white dwarf. By studying these stellar remnants, we’re getting a glimpse into the future of our own Sun and the Solar System,” O’Brien concluded.
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