Astronomers have uncovered an exciting new discovery in the binary system G3425, located about 5,800 light-years away. The system contains a black hole that appears to be a rare “missing link” stellar-mass black hole, which offers valuable insight into the mysteries of black hole formation and binary star systems.
The visible star in G3425 is a red giant—a star that has exhausted its hydrogen fuel, causing its outer layers to expand. This is the fate awaiting our sun in around 5 billion years, when it will swell up to nearly the orbit of Mars, engulfing the inner planets, including Earth. However, unlike our sun, this red giant has a unique companion: a black hole. Weighing in between 3.1 and 4.4 times the mass of our sun, this newly discovered black hole is one of the lightest black holes ever detected. It falls within the “mass gap,” a range of black hole masses between 3 and 5 solar masses that astronomers had previously found difficult to detect.
A team led by Wang Song from the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC) made the discovery using data from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) and the Gaia space telescope. By observing the gravitational pull that the black hole exerts on its red giant companion, the team was able to confirm the presence of the otherwise invisible black hole.
The Mass Gap Puzzle
Black holes come in a wide range of sizes. The largest, supermassive black holes, can have millions or even billions of times the mass of our sun and are found at the centers of galaxies. Smaller black holes, known as stellar-mass black holes, are usually formed from the remnants of massive stars that collapse under their own gravity after exhausting their fuel. These typically have masses ranging from 5 to 25 times that of the sun.
The mass gap refers to the lack of black holes between 3 and 5 solar masses. According to theories of black hole formation, stars that have lost most of their mass in a supernova explosion should still be able to form black holes in this mass range. However, very few black holes in this range have been detected, leading scientists to wonder if some unknown process prevents their formation.
One possibility is that black holes in the 3 to 5 solar mass range are more likely to be disrupted during a supernova explosion. The intense “kick” that black holes receive during the explosion may send them flying away from their companion stars, making them harder to detect. This theory could explain why so few mass-gap black holes have been found, as they may not remain in binary systems where their presence can be inferred through their impact on a companion star.
The Puzzling Orbit of G3425
Another mystery surrounding the G3425 system is the black hole’s wide, circular orbit. The two objects in the binary system orbit each other with a period of approximately 880 Earth days, in a nearly perfect circle. This is unusual, as most binary systems have elliptical orbits, especially when one of the objects is a black hole. The team has not yet been able to explain how such a binary system, particularly one involving a low-mass black hole, could form under standard evolutionary models.
“The most surprising thing is the wide circular orbit of the binary,” Song explained. “The formation of such a system, especially involving a low-mass black hole, presents a significant challenge to current theories of binary evolution and supernova explosion.”
Challenging Current Models of Stellar Evolution
The discovery of this system presents a challenge to current models of how binary systems evolve and how black holes form. However, it also provides new opportunities for understanding the processes involved in the formation of black holes and binary stars.
This discovery demonstrates that even low-mass black holes, which are difficult to detect, can exist in binary systems and survive supernova explosions. Additionally, it highlights the potential for discovering other hidden black holes through their gravitational effects on nearby stars. This opens the door for further exploration of binary systems and may help solve the mystery of the “missing link” black holes.