Astronomers have observed for the first time a low-mass star slowly consuming a brown dwarf in a binary system 300 light-years away, a process that could last billions of years. The discovery, published in Nature Astronomy, reveals a new form of stellar interaction.
The star, with a mass 85 times that of Jupiter, gradually pulls material from its companion, a brown dwarf 25 Jupiter masses in size, without fully engulfing it. This slow and constant consumption, described as a "third path" between planetary engulfment and black widow binaries, was detected via a triangular light curve recorded by the Zwicky Transient Facility (ZTF). ZTF J0440+2325 is located in the Milky Way.
Triangular Light Curve Defies Expectations
The discovery began with an anomaly: a triangular light curve detected by the ZTF, which typically tracks rapid brightness changes. Stars don’t produce triangular waveforms like this,
said Kevin Burdge, an MIT astrophysicist and lead author of the study.
After years of puzzlement, the team reanalyzed the data, pinpointing the system’s location 300 light-years away in the Milky Way. Using multiple telescopes, they measured the system’s oscillation and confirmed it was not a black widow but a low-mass star-brown dwarf pair. The mass was too low for a neutron star, said graduate student Aaron Householder, a co-author. This was a star of low mass orbiting a brown dwarf. The system’s unique characteristics led researchers to rule out denser objects like black holes or neutron stars.
Material Falls Directly onto Star’s Surface
Unlike typical accretion processes around black holes or neutron stars, which form accretion disks, the material from the brown dwarf fell directly onto the red dwarf’s surface. The difference here is that the object absorbing the material isn’t a tiny black hole but a star, which is relatively large,
Burdge said. So the material just slams into the surface at high speed, like an asteroid hitting the Moon.
This direct impact generates a persistent hot spot, where the hot spot’s cyclical visibility creates the triangular light pattern.
Simulations showed that the brown dwarf loses about 1/100,000 of Earth’s mass annually—equivalent to 40 million dump trucks of material. This gradual transfer creates a persistent hot spot on the star, which enters and exits the observer’s view as the brown dwarf orbits. It’s like a continuous fireball on one object, said Burdge. As it orbits, that hot spot comes in and out of view, creating the triangular light pattern. This process could endure for billions of years, with the brown dwarf shedding a minimal fraction of its mass each year.
New Model for Stellar Interactions
The findings challenge traditional models of stellar evolution, where planets or brown dwarfs are either fully consumed or remain unchanged. When we think of stars interacting with planets or brown dwarfs, the image is always that the star eventually swallows the other object,
Burdge said. But here, we’ve found an alternative: the star can eat it slowly over billions of years.
This discovery opens new avenues for understanding how low-mass objects interact with stars.
The team hopes the discovery will inspire new searches for similar systems. This is opening up a new way to think about how planets and brown dwarfs interact with their host stars, Householder said. The study, funded in part by the National Science Foundation, was published in Nature Astronomy on October 5, 2026.