New research shows how the host galaxy's gravity affects distant pairs of stars, bringing the black holes into extreme and rare collisions accompanied by powerful gravitational waves
In a groundbreaking study, researchers reveal a hypothesis for how pairs of binary stars, previously thought to evolve quietly when they are far apart, are profoundly affected by the gravity of the host galaxy. This galactic interaction may change their orbits, thus leading the black holes or even distant stars to violent collisions.
The detection of gravitational waves generated by black hole collisions has raised an intriguing question: How do black holes get so close together that they collide? Researchers at the Max Planck Institute for Astrophysics (MPA) offer an answer: that some black holes began their journey as massive stars that orbited each other at enormous distances - 1,000 to 10,000 times the distance between the Earth and the Sun. After completing their life cycle and collapsing into black holes, the gravitational forces of the host galaxy may gradually distort their orbits. This process can bring the black holes closer together and lead to their merging.
Many of the stars in the universe are not alone. Unlike our sun, many of them have a partner, and together they form a binary system. The distance between the couple in this system plays a central role in their development. Stars in very close orbits tend to exchange mass, leading to complex dynamical interactions.
In massive stars, these interactions can lead to the formation of binary black hole pairs that may merge following energy loss through gravitational waves. In contrast, the previously accepted hypothesis was that distant binary pairs evolve uneventfully, behaving as single stars that form binary black holes too far away to ever merge.
Understanding the evolution of distant binary stars
New research, published in the journal The Astrophysical Journal Letters Led by Jakob Stegmann, a research fellow at the Max Planck Institute for Astrophysics, it offers new insights into distant binary systems in the context of their galactic environment.
Distant binary systems – where the distance between the stars exceeds 1,000 times the distance between Earth and the Sun – are susceptible to perturbations from the host galaxy's gravitational forces and close encounters with passing stars. The study reveals that these galactic influences can lead to dramatic and unexpected interactions, affecting both stars and their compacted remnants.
The interactions are caused by the very low binding energy that holds pairs of distant binary black holes together. As a result, the host galaxy's gravity can distort their orbit and make it increasingly elliptical. In these orbits, the black holes stay far apart most of the time, but pass close together once in each orbit.
Effects on low-mass stars
This mechanism may also be relevant to the evolution of distant low-mass binary stars. A recent study by MPIA showed that about 10% of all low-mass stars have a distant partner.
The results of the study represent an important advance in understanding the interactions of distant binary systems with their host galaxy. The study opens the door to understanding the processes that led to the most energetic events in the universe.
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