Asteroseismology Reveals Surprising Secrets of Stars Orbiting Hidden Black Holes

Astronomers are learning to “listen” to stars to uncover the secrets of black holes that do not actively feed on matter. A new study of two such systems, known as Gaia BH2 and Gaia BH3, uses asteroseismology – the study of stellar vibrations – to probe the history and nature of their red giant companion stars. The results are surprising, sometimes puzzling, and highly informative for anyone curious about how stars and black holes evolve together.

What Makes Gaia BH2 and BH3 Special?

Gaia BH2 and Gaia BH3 are binary systems. Each contains:

1. A dormant black hole that is not shining in X rays like typical “active” black holes
2. A red giant star orbiting the black hole

These pairs are rare and extremely valuable for astrophysics. Because the black holes are quiet, the companion stars become the main clue to the system’s past. By studying the red giants in detail, astronomers can infer how the black holes formed, how mass and angular momentum were exchanged, and how the systems evolved over billions of years.

Listening to a Red Giant: Asteroseismology in Gaia BH2

How Starquakes Reveal a Star’s Life Story

Like musical instruments, stars vibrate. These vibrations slightly change their brightness in regular patterns. Space telescopes can record tiny variations and, using asteroseismology, scientists can extract key properties of a star, such as:

1. Mass
2. Radius
3. Age
4. Internal structure

In Gaia BH2, researchers clearly detected such asteroseismic signals in the red giant companion.

What the Vibrations Say about Gaia BH2

From the observed oscillations, astronomers determined:

1. The star’s mass is about 1.2 times that of the Sun
2. Its age is around 5 billion years

This relatively modest mass and middle age fit within current models of stellar evolution, but the system becomes more intriguing when rotation is considered.

A Red Giant Spinning Faster than Expected

The red giant in Gaia BH2 rotates with a photometric period of roughly 398 days, which is rapid for a star in this evolutionary stage. This fast spin suggests a dynamic and possibly violent history, such as:

1. Tidal spin up due to the black hole’s gravity
2. A past merger with another star
3. Accretion of material that transferred extra angular momentum

In plain terms, the star in Gaia BH2 does not look like a quietly aging red giant. Instead, it carries signs of strong past interactions, making the system a valuable test case for models of how black holes and companion stars exchange energy and matter.

Gaia BH3: The Mysterious Silence of a Red Giant

In Gaia3, the situation is very different. Astronomers expected to detect asteroseismic oscillations in its red giant companion, based on its known properties. Yet no such signal appeared in the data.

Why the Missing Vibrations Are a Big Deal

The lack of detectable oscillations is not just an observational curiosity. It raises serious questions:

1. Are the currently quoted stellar parameters for the Gaia BH3 companion wrong or incomplete?
2. Do the usual scaling relations used in asteroseismology fail for very metal poor stars like this one?
3. Is there some physical process in this system that suppresses or alters the oscillations?

Because asteroseismology is widely used to measure stellar properties across the Galaxy, any limitation in its applicability has far reaching consequences. Gaia BH3 may thus be pointing to gaps in our understanding of how metal poor red giants behave, especially when paired with black holes.

What These Systems Teach Us about Black Hole Binaries

The contrasting behavior of Gaia BH2 and BH3 has several important implications.

Different Paths for Similar Looking Systems

1. Gaia BH2 shows clear oscillations, a well constrained mass and age, and unusually fast rotation. This hints at a complex history involving tidal forces, mass transfer, or mergers.
2. Gaia BH3 lacks detectable oscillations, defying expectations and placing stress on standard stellar models.

These differences suggest that not all black hole red giant binaries the same evolutionary route. Some experience strong interactions that spin up the star, while others may evolve more quietly or under physical conditions that alter the star’s oscillation properties.

Testing and Refining Models of Black Hole Formation

Because both systems contain dormant black holes, they are crucial for understanding how black holes form without necessarily producing bright X ray outbursts. Key questions include:

1. How much mass did the progenitor stars lose before collapsing into black holes?
2. How did the orbits change during supernova events or mass transfer phases?
3. How does metallicity influence both stellar evolution and black hole formation?

The puzzling nature of Gaia BH3 in particular may provoke revisions of models that link stellar parameters, metallicity, and oscillation behavior in stars paired with black holes.

Looking Ahead: The Role of Future Space Missions

The new findings are not the end of the story. They are the beginning of a more detailed investigation.

What Future Observations Can Reveal

Continued monitoring with missions such as TESS, and possibly future dedicated asteroseismology satellites, will help to:

1. Refine the oscillation frequencies and amplitudes in Gaia BH2
2. Search more sensitively for hidden or subtle oscillations in Gaia BH3
3. Better constrain the masses, radii, and ages of both red giants
4. Clarify the role of chemical composition in shaping oscillation properties

Impro datasets may reveal whether Gaia BH3 is truly silent or whether present analyses simply lack the sensitivity or the correct assumptions to detect its starquakes.

Broader Impact on Stellar and Black Hole Physics

The deeper lesson from Gaia BH2 and BH3 is that stars in black hole binaries carry a fossil record of their system’s past:

1. Rotation rates encode past tidal interactions and mergers
2. Oscillation patterns trace internal structure and evolution
3. Discrepancies between expectation and observation expose gaps in theory

By systematically studying such binaries, astronomers can refine models of:

• Stellar evolution from main sequence to red giant stages
• Black hole birth from massive stars
• Binary interactions including mass transfer, tidal locking, and mergers

Conclusion: Starquakes as a New Window on Quiet Black Holes

The research on Gaia BH2 and BH3 demonstrates how asteroseismology can act as a precise diagnostic tool even when black holes themselves are quiet. In Gaia BH2, clear vibrations and rapid rotation reveal a dynamic and possibly tumultuous past. In Gaia BH3, the unexpected absence of oscillations flags an important puzzle, especially for very metal poor stars.

Together, these systems push the boundaries of our knowledge about how stars and black holes live and evolve together. For science enthusiasts and the general public, they offer a compelling story in which listening to stellar vibrations can uncover invisible black holes, challenge existing theories, and guide the next generation space missions that will continue to map the hidden structure of our universe.

The research was published in The Astronomical Journal on November 13, 2025.