A sugar cube sits on a saucer next to a cup of hot tea. Drop it in, and it dissolves within minutes. But if you lower it slowly, the sugar melts gradually, releasing its sweetness into the drink. Something similar is happening in two newly discovered stellar pairs, except the sugar is a brown dwarf and the tea is a star.
A brown dwarf is an object that never became a full-fledged star: it lacked the mass to ignite thermonuclear fusion in its core. Such 'failed stars' usually either remain solitary or, if they venture too close to a real star, quickly plunge into it and are destroyed. But in the systems ZTF J0440+2325 and ZTF J1444+4820, the story is different. There, a brown dwarf orbits a red dwarf (a small, dim star) so closely that a full circle takes just 87 and 67 minutes. Yet the material from the brown dwarf's surface doesn't fall all at once; instead, it flows onto the star in a thin stream, like syrup dripping from a spoon.
This is the first time astronomers have directly observed such a stable mass transfer from a substellar object to a main-sequence star. Previously, this had only been predicted theoretically. The discovery changes our understanding of the fate of such systems: instead of rapid engulfment, it's a 'slow feast' that could last billions of years.
These two pairs are living laboratories for studying how material flows between objects of different natures. Such systems will help us better understand what happens when a star and its small companion get too close, and why some pairs merge while others don't.
An orbital period of 67 minutes is shorter than a football match. In that time, the brown dwarf completes a full orbit around its star.