These Stars are Already Merging, but Their Future Will Be Catastrophic

Close-orbiting binaries are a ticking time bomb. Over time they spiral ever closer to each other until they merge in a cataclysmic explosion such as a supernova. But in the middle of their story, things can get interesting. Some stars collapse into a white dwarf before merging with their partner, others edge so close to each other that their surfaces touch for a time, becoming contact binaries before finally colliding. But one newly discovered binary system will have a wild ride before its final demise.

The system is known as SSN 7, and it is a spectroscopic binary in the Small Magellanic Cloud. Spectroscopic means the two stars are so close to each other and so far away that we can’t resolve them as individual stars. Instead, we know they are binary by observing the redshift and blueshift of their spectral lines. From the spectral line data, astronomers can calculate their mutual orbits, and thus their masses.

The larger star is about 55 solar masses, and the smaller one is about 32 solar masses. Interestingly, the smaller one is the “primary” star, meaning that it’s the brighter of the two. This suggests that the smaller star is already feeding off the larger one in the early stage of merging. They orbit each other every three days, and their gravitational centers are only about 40 solar radii apart. They orbit so closely that they have to be a contact binary.

Based on their orbits, the two stars will eventually merge in about 18 billion years. But given their masses, these stars won’t live long enough to merge. Stars above about 20 solar masses become supernovae before collapsing to become black holes. The larger one will likely become a black hole in about 700,000 years, and the smaller one about 200,000 years after that. This system will experience two supernovae within the next million years, only to merge as black holes billions of years later.

What makes this system particularly useful to astronomers is that it is squarely in the middle ground of binary systems. Most massive stars are part of close binary systems. We see lots of them as stable binaries not in the process of merging, and we have observed lots of merging stellar-mass black holes through gravitational wave astronomy. Until this system, we haven’t observed a merging binary system that will become a black hole merger. It gives us an excellent view of the elder years of these systems, which will help astronomers better understand their evolution.

Reference: Rickard, M. J., and D. Pauli. “A low-metallicity massive contact binary undergoing slow Case A mass transfer: A detailed spectroscopic and orbital analysis of SSN 7 in NGC 346 in the SMC.” arXiv preprint arXiv:2304.13720 (2023).