How Two Dim Stars Came Together to Shine Brightly

Brown dwarfs get a bad rap in the stellar world, often labeled as "failed stars" for their inability to sustain nuclear fusion at their cores. The mass of these objects falls between planets and stars, ranging from 13 to 80 times the mass of Jupiter. Because they aren't massive enough to sustain fusion, they are far fainter and cooler than their stellar comrades.

Now, a new finding led by researchers at Caltech shows how these dim bulbs can join together to shine brightly. Searching through archival observations captured by the Zwicky Transient Facility (ZTF) at Caltech's Palomar Observatory, researchers have identified a very tight-knit pair of brown dwarfs in which one is actively siphoning material from the other.

Ultimately, the brown dwarfs are expected to merge to form a new star; alternatively, the brown dwarf gaining the extra mass will ignite to become a star. Either way, a pair of failed stars will have created a brilliant new star.

"The failed stars get a second chance," says Samuel Whitebook, a Caltech graduate student and lead author of a new report on the findings appearing in The Astrophysical Journal Letters. "Brown dwarfs don't have internal engines like stars do, but this result shows they can exhibit very interesting dynamic physics."

Whitebook works with two advisors at Caltech, Tom Prince, the Ira S. Bowen Professor of Physics, Emeritus, and Dimitri Mawet, the David Morrisroe Professor of Astronomy and a senior research scientist at NASA's Jet Propulsion Laboratory, which is managed by Caltech. Both Prince and Mawet are co-authors on the study.

The finding is a first of its kind: Until now, this type of mass transfer between binary objects had only been seen in much heavier objects, such as white dwarfs, which are the dead corpses of stars like our Sun.

The brown dwarf pair, named ZTF J1239+8347 (or ZTF J1239 for short) was spotted after scientists sifted through a database known as ZVAR, or ZTF Variability Archive, which is a collection of all-sky data taken repeatedly by ZTF since 2017. The database, which contains 2 billion objects, reveals how those objects change over time. In the case of ZTF J1239, the object was found to significantly change in brightness every 57 minutes.

A further analysis of the source revealed that it is a pair of dim brown dwarfs orbiting around each other very closely; in fact, the whole system would fit within the distance between the Earth and Moon. The objects, which are roughly 60 to 80 times the mass of Jupiter, lie about 1,000 light-years away in the Ursa Major constellation.

The scientists are not sure how the two faint orbs initially came together; it's possible that a third star could have gravitationally pushed them closer together from separate systems. Once together, the stars would have spiraled closer and closer until one of the brown dwarfs would have puffed up in size due to the gravitational influence of the other, becoming less dense.

"When one star's gravity is overcome by the other's, matter starts flowing from the less dense star to the denser star," Whitebook says. "It's like the matter sloughs off through a nozzle."

This nozzle directs material from one brown dwarf to a fixed spot on the other, which then heats up and glows in blue and ultraviolet light. The rotation of this hot spot, as the two brown dwarfs circle around each other, led to the periodic light curve observed by ZTF.

While other types of stars are known to transfer mass between them, this is a first in the world of brown dwarfs. "These are very exotic objects," Prince says. "We've told some of our colleagues about them, and they didn't believe such a thing exists."

Because the newfound pair is faint and close to Earth, the scientists estimate that there are many more like this out there to be found.

"We expect the Vera Rubin Observatory [a ground-based observatory in Chile] to detect dozens more of these objects," Whitebook says. "We want to find more to understand the population and how common it is. We predict this happens more than you think."

Other telescopes that contributed to the study include the European Space Agency's Gaia mission, the W. M. Keck Observatory in Hawaiʻi, Palomar's 200-inch Hale Telescope, NASA's Wide-field Infrared Survey Explorer (WISE), NASA's Neil Gehrels Swift Telescope, and the Gran Telescopio Canarias in the Canary Islands, Spain. The researchers are planning upcoming observations of ZTF J1239 with NASA's James Webb Space Telescope.

The study titled "A Mass Transferring Brown Dwarf Binary on a 57 Minute Orbit" was funded by NASA and the Heising-Simons Foundation 51 Pegasi b Fellowship. Other Caltech authors include Sam Rose, Anica Ancheta, Antonio Rodriguez (PhD '25, now at Harvard University) and Jerry Xuan (PhD '25, now at UCLA). Additional authors include Kevin Burdge (PhD '21) and Aaron Householder of MIT, Pablo Rodríguez-Gil of Instituto de Astrofísica de Canarias and Universidad de La Laguna in Spain, Ariana Pearson of University of Waterloo, and Sage Santomenna of Pomona College. The paper also acknowledges the contributions of the late Thomas Marsh from the University of Warwick.

ZTF is funded by the National Science Foundation (NSF) and an international collaboration of partners. Additional support comes from the Heising-Simons Foundation and from Caltech. ZTF data are processed and archived by IPAC, an astronomy and data center at Caltech. The ZVAR project, which uses ZTF data, is led by Caltech's Matthew Graham and is funded by NSF.