When “Failed Stars” Get a Second Chance: The Strange Lives of Pink, Beige, and Maroon Dwarfs

Brown dwarfs are often described as “failed stars” because they never reach high enough temperatures in their cores to sustain hydrogen fusion. But Luisi and collaborators show that the boundary between brown dwarfs and true stars becomes surprisingly complicated once these objects start gaining mass from a companion. Their study uses MESA computer models to explore what happens when an initially ordinary brown dwarf is pushed above the hydrogen burning limit (HBL). The authors find that the object’s fate depends not just on its final mass, but critically on how cool and compressed, or degenerate, its core has become by the time mass transfer occurs. This leads to a taxonomy of three possible outcomes: beige, maroon, and pink dwarfs, each representing a distinct evolutionary path.

Simulation Approach

To explore these outcomes, the authors design a sequence of numerical experiments beginning with a 0.065 solar-mass brown dwarf. In the first phase, the brown dwarf cools for varying lengths of time, which allows its core to reach different levels of degeneracy. The second phase adds mass either quickly or slowly to mimic different kinds of binary interactions. Finally, the third phase evolves the object without additional mass for up to 100 billion years. This setup allows the authors to compare many possible combinations of “when” and “how much” mass is added, providing the foundation for their classification of stellar fates.

The Three Evolutionary Paths

The simulations naturally separate into three categories. Beige dwarfs are objects whose cores remain too degenerate to restart convection even after gaining mass above the hydrogen burning limit; they continue cooling like massive brown dwarfs. Maroon dwarfs gain their mass early in life, before they have cooled significantly, and thus behave just like ordinary low-mass stars. Pink dwarfs represent the most intriguing case: they land in a state where nuclear energy production exceeds surface cooling, causing the core to heat up until the object ultimately reaches the main sequence. Despite having stellar masses, pink dwarfs spend an extended period appearing underluminous, which makes them potentially distinguishable from normal stars of the same mass.

Frozen Cores and Luminosity Plateaus

A key feature of pink dwarfs is the appearance of luminosity plateaus, long periods during which the surface brightness barely changes. This happens because the star’s core becomes “frozen,” meaning convection temporarily shuts down. Nuclear reactions continue to generate energy, but because convection cannot transport this energy outward, the core must first reheat until it regains a uniform entropy structure. Only then can convection restart and allow the object’s surface luminosity to climb toward its final value. The duration of this plateau depends on how cold and degenerate the core was at the moment mass transfer ended.

Observational Implications

These unusual evolutionary paths invite the possibility of finding beige or pink dwarfs in real stellar populations. Both types end up with masses above the hydrogen burning limit but with luminosities far below what normal stars of similar mass would exhibit. This means they would stand out in a mass–luminosity diagram as objects lying well below standard stellar isochrones. Identifying them, however, requires accurate masses and luminosities, which are easiest to obtain for binary systems. Because mass transfer from an evolved giant star is a likely formation channel, many beige and pink dwarfs should be companions to white dwarfs, making them promising targets for astrometric surveys such as Gaia.

Conclusion

Luisi and collaborators show that brown dwarfs are far more diverse than the simple label “failed stars” suggests. Depending on when and how they gain mass, they may evolve into maroon dwarfs that behave like ordinary stars, beige dwarfs that remain permanently underluminous, or pink dwarfs that undergo long-lived frozen-core phases before joining the main sequence. Detecting even a single clear example of these exotic objects would provide valuable tests of theories of binary mass transfer and the physics of partially degenerate stellar interiors.

Source: Luisi