Four white dwarfs long hidden by brighter stars have been identified in the vicinity of the Sun. These dead stars are located less than 65 light-years from Earth, a short distance on the scale of the galaxy.
A white dwarf is the compact core left behind by a star comparable to the Sun after the end of its life. It concentrates a large amount of matter in a volume close to that of Earth. Despite its density, its low luminosity makes it difficult to observe.
The four objects belong to binary systems. They orbit red dwarfs, which are smaller stars but bright enough to mask their companion. Until now, astronomers mainly observed the visible stars of these pairs.
The first indications came from unusual movements of the red dwarfs. Their speed varied slightly under the gravitational effect of an invisible object. These oscillations provided indirect evidence of the presence of a compact companion.
The researchers then used the Hubble Space Telescope in the ultraviolet. This part of the spectrum makes it possible to better distinguish a hot white dwarf from the visible light emitted by its red companion. The observations confirmed the nature of the four objects.
One of the systems is located about 25 light-years away. Its white dwarf is now among the closest known to the Sun. This proximity provides an opportunity to study the evolution of star pairs with precision.
The discovery also shows that the census of our galactic neighborhood remains incomplete. White dwarfs are particularly easy to miss when they orbit active or luminous stars. Other systems could therefore be hiding.
Astronomers will now compare these pairs with models of stellar evolution. Their objective is in particular to understand how the two stars exchanged matter and how their orbits evolved. This information will help to better estimate the population of nearby white dwarfs.