The white dwarf WDJ165335.21-100116.33 is the cooled core of a star, about the size of Earth but with the mass of the Sun. On its surface, there is an unusual hot spot that appears and disappears. Astronomers long puzzled over why the star's brightness changes every 80 hours. New observations with the VLT telescope using the FORS2 instrument have revealed that the star has a weak magnetic field that strengthens and weakens over time.

The star's magnetic field acts like an invisible hand, compressing the gas in its atmosphere. When the field is stronger, the gas heats up and glows more brightly: this is how the hot spot is born. At the same time, the star's overall brightness drops because the spot blocks some of the light. It's like how a cloud's shadow makes the day darker, while the cloud itself glows from within.

The scientists compared this star with other similar white dwarfs. They found that in all of them, brightness and glow intensity change in opposite phases, like a seesaw: when one parameter rises, the other falls. This means that all such stars operate by the same mechanism — a magnetic spot on the surface creates a hot region above the star that shines like a tiny sun.

Now the star has been officially reclassified: it is no longer simply 'non-magnetic' but belongs to the class of weakly magnetic white dwarfs. This is the first time the magnetic field of such a star has been measured directly. This suggests that magnetic fields may be present in many more white dwarfs than previously thought.