Astronomers track brightness changes with graphs called light curves. Supernovae, for instance, produce smooth, bell-shaped curves. This signal traced something closer to a triangle.

"I remember first looking at this and thinking, stars don't make triangular waveforms like this," Burdge recalls.

At first, the team suspected a "black widow binary" — a rapidly spinning neutron star, the dense remnant of a massive star, stripping material from a smaller companion. But the data didn't fit. Black widow binaries show a pronounced wobble as the light companion whips around the heavy neutron star, and this system showed almost none.

"We weren't seeing that whipping back and forth here," Burdge says. "It didn't make any sense. We couldn't explain what this was."

The smaller wobble pointed to a system where both objects have relatively low mass. Graduate student Aaron Householder confirmed it with follow-up telescope observations. "When we measured that wobble, we found we were not seeing a black widow," Householder says. "This was a low-mass star that's orbited by a brown dwarf."

What's the mechanism behind the feeding?

The process is called accretion — one astronomical object drawing gas or other material from another. Black holes and neutron stars usually do this via a swirling disk, because they are physically tiny.

Here, the object receiving matter is a star with a large surface, so the material hits it head-on.

"The difference here is, the thing absorbing matter is not a tiny black hole but a star, which is relatively big in size," Burdge explains. "So matter just pummels directly onto the surface, at very high speeds, like an asteroid hitting the Moon."

Computer simulations backed this up. The researchers modeled particles leaving the brown dwarf under the gravity of both objects, using the system's measured properties and standard orbital equations. The particles fell straight onto the star.

"When we track those test particles, we see they indeed fall right onto the surface of the star," Householder says.

The feeding rate — about 40 million dump trucks' worth of matter every year, or 1.3 trillion one-pound burritos per second — sounds enormous. But the brown dwarf holds so much mass that the loss represents only a tiny fraction of its total, which is why the star could keep feeding for hundreds of thousands of years, potentially billions.

The accretion also explains the triangular signal. Material striking the stellar surface creates an intensely bright hotspot — a persistent fireball that rotates in and out of view every 87 minutes. "The peak of the triangle signal is when you're looking right at the fireball," Burdge explains.

What comes next?

The researchers, who include collaborators from Caltech, the University of Hawaii, Spanish institutions and the Harvard-Smithsonian Center for Astrophysics, now plan to search for similar systems. Finding more could reveal how common this slow form of stellar cannibalism is and what ultimately happens to objects trapped in such tight orbits.

"It's inspiring a lot of new searches on our part," Householder says. "I think we're going to learn a lot about a different kind of way that planets and brown dwarfs interact with their host stars."

The findings are preliminary in one sense: this is a single system, and the mass-transfer rate is an estimate based on modeling. The National Science Foundation partially supported the research.