The Mystery of Intermediate-Mass Black Holes

Stellar-mass black holes, such as the famous Cygnus X-1, form when massive stars collapse at the end of their lives. At the other extreme, supermassive black holes reside

at the centers of large galaxies, including Sagittarius A* at the heart of the Milky Way. Both types are well-known and widely accepted by astronomers.

Figure 1. Star Ejected at a Million MPH – Uncovering a Hidden Black Hole.

However, there is a puzzling gap in between: intermediate-mass black holes (IMBHs). Believed to represent a crucial evolutionary step between small stellar black holes and the supermassive giants that anchor galaxies, IMBHs have remained elusive. While several candidates have been suggested, none have been confirmed, leaving their existence as one of the biggest unanswered questions in black hole research. Figure 1 shows Star Ejected at a Million MPH – Uncovering a Hidden Black Hole.

Astronomers suspect that globular clusters—dense collections of stars tightly packed together—are promising places to look for IMBHs. These environments could support two main pathways for IMBH formation: one involves rapid, repeated star mergers that create a massive object which eventually collapses, while the other is a slower process where smaller black holes merge over time to form a larger one.

Early Clues from Hubble Observations

In the early 2000s, astronomers used the Hubble Space Telescope (HST) to observe the globular cluster M15, utilizing its ultra-high spatial resolution Space Telescope Imaging Spectrograph (STIS) to derive its line-of-sight velocity dispersion curve. Through N-body numerical simulations, researchers suggested that M15 could host an intermediate-mass black hole (IMBH) with a mass between 1,700 and 3,200 solar masses.

However, this conclusion faced skepticism. The HST velocity dispersion curve was measured at a projected radius of 0.5 arcseconds from the cluster’s center (approximately 5,000 AU at the distance of M15), a region where thousands of compact stars might exist, complicating the interpretation and leaving open the possibility that the observed velocity distribution could be caused by the stars, not an IMBH.

Follow-up studies using pulsar timing also provided hints of IMBHs in globular clusters, but because these pulsars were located farther from the cluster centers, they couldn't definitively rule out the presence of a dense cluster of stellar remnants instead of an actual black hole.

The Search for a Better Method

As a result, finding an effective method to detect IMBHs as close as possible to the centers of globular clusters has become the crucial missing piece of evidence (see Figure 1, which illustrates the measurement principles).

“To address this challenge, we propose a systematic search for hypervelocity or high-velocity stars accelerated by the gravitational slingshot effect (the Hills mechanism),” says Jifeng Liu, Director of the National Astronomical Observatories at the Chinese Academy of Sciences and lead author of the study. “In a cluster hosting an IMBH, a tight binary system orbiting nearby could be disrupted, concentrating the black hole’s mass into a much smaller region.”

“By tracing nearly a thousand high-velocity stars (with velocities exceeding 400 km/s) and over a hundred globular clusters using data from Gaia, LAMOST, SDSS, and other spectroscopic surveys, we have discovered that J0731+3717 was ejected from M15—marking the first time a high-velocity star from a globular cluster has been identified—at a velocity of nearly 550 km/s (over 1 million miles per hour) approximately 20 million years ago,” says Yang Huang, researcher at the School of Astronomy and Space Science, University of the Chinese Academy of Sciences and first author of the study.

Chemical Clues and Explosive Evidence

To eject a star at such an extreme velocity from a globular cluster, a tight binary system must have passed within one astronomical unit (AU) of an IMBH with a mass of several thousand solar masses.

“The strong tidal forces of the IMBH would have torn the binary apart—capturing one star while flinging the other away at high speed,” explains Xiaobo Dong, researcher at the Yunnan Observatories, Chinese Academy of Sciences, and lead author of the study.

Final Proof and Future Prospects

This unique discovery confines a few thousand solar masses within just a few AUS, effectively ruling out the possibility that the mass is composed of thousands of neutron stars or stellar-mass black holes. The only plausible explanation is a single black hole, thereby confirming the existence of an IMBH in M15. By identifying the first-ever high-velocity star ejected by an IMBH in a globular cluster, this study completes the final missing link in the evidence chain for IMBHs.

Source: SciTECHDaily

Cite this article:

Priyadharshini S (2025), A Star Was Ejected at A Million MPH – Unveiling A Hidden Black Hole, AnaTechMaz, pp.341