Small star 300 light-years away is slowly eating a brown dwarf on an 87-minute orbit

Black widow pulsars were what the team was originally looking for. In those systems a neutron star feeds on a lighter companion, and the mismatch in mass puts a wobble in the light signature ^[8]. The ZTF J0440+2325 signal had none. "We weren't seeing that whipping back and forth here," Burdge said. "It didn't make any sense." ^[9]

Burdge's alternative was a pair of objects with similar masses. "If you have less mass in the system overall, things can gently orbit each other without whipping back and forth," he said. "That was the idea. But we never had any proof. And this weird triangle just sat for years." ^[10]

The follow-up worked by elimination. When the team went back to the source, they placed it inside the Milky Way, pointed several telescopes at it and found a wobble smaller than a black widow's ^[11]. "The wobble was too small in amplitude to be anything else," Aaron Householder of MIT said ^[12]. The missing large wobble excluded a neutron star. The small one that did appear fixed the pair as a low-mass star and a brown dwarf ^[12].

The masses show just how light the pair is. The star is about 85 Jupiter masses and the brown dwarf about 25 ^[14]. Brown dwarfs top out near 80 Jupiter masses, below which hydrogen fusion never starts ^[4], so the star clears that line by roughly 5 Jupiter masses ^[16]. The companion has about 0.29 of the star's mass ^[17]. The two circle each other every 87 minutes, close enough that the whole system would fit inside the sun ^[13].

Of all the figures, the lifetime is the softest. Space.com gives hundreds of thousands to billions of years ^[3], a spread of roughly four orders of magnitude, from about 10^5 to 10^9 years or more ^[18]. Burdge's own summary takes the top of that range: "Instead of swallowing the thing up, the star can gradually eat it for billions of years," he said ^[5]. The feeding rate appears only as the equivalent of 5.2 trillion quarter-pounder burgers a second ^[19]. The article does not convert that into a mass-loss rate, and it does not say what model sets the lifetime.

Space.com contrasts the process with black holes stripping stars. Black holes have radii of 9 to 300 kilometres, and the article says the size gap between them and this star means the stripping proceeds differently ^[15].

This one system doesn't tell you how common such pairs are. It turned up during a search for something else ^[8]. Space.com's report that it is the first of its kind ^[1] reflects what has been searched as much as what exists. With the lifetime uncertain by roughly four orders of magnitude ^[18], the number of similar pairs the galaxy should hold is just as uncertain.