The Cosmic Window Series: Chapter 1 - Top 5 Biggest Black Holes in the Universe

 

Introduction

Figure 1: Artist's illustration of Sagittarius A*, the supermassive black hole at the center of the Milky Way. The rendering highlights the surrounding accretion disk of hot gas, gravitational light-bending effects, and energetic flaring hotspots recently observed by the James Webb Space Telescope. Credit: Illustration - NASA, ESA, CSA, Ralf Crawford (STScI)

The Cosmic Window is a series dedicated to the systematic exploration of astrophysical phenomena and cosmic objects - bridging the frontier of observational astronomy with accessible scientific inquiry, one chapter at a time.

Imagine a void so massive that an entire Solar System would vanish inside it without a trace - a place where time itself slows to a crawl and even light, the fastest thing in the universe, cannot escape. These are not science fiction. They exist, they are real, and some of them have been growing for over 13 billion years.

Black holes form when gravity wins its ultimate battle. When a sufficiently massive star exhausts its nuclear fuel, no outward pressure can resist the inward crush. The star implodes, forging a singularity - a point of extreme density - wrapped in an event horizon, the invisible boundary beyond which nothing returns. Supermassive black holes grow further by consuming gas, dust, and entire stars across cosmic time. Our own Milky Way's central black hole, Sagittarius A*, weighs roughly 4 million solar masses - colossal by human reckoning, yet reduced to an insignificant footnote by the five monsters below. Brace yourself: the numbers that follow challenge the limits of imagination.

1. TON 618:  ~66 Billion Solar Masses

Figure 2: Circled here is the supermassive black hole TON 618, whose light has traveled over 10 billion years across cosmic time to reach Earth. Credit: SDSS

Located ~10.4 billion light-years away in Canes Venatici, TON 618 is a hyperluminous quasar - its central black hole consuming surrounding matter so voraciously that it outshines approximately 140 trillion Suns combined. Its mass of ~66 billion solar masses gives it a Schwarzschild radius of roughly 1,300 AU, wider than our entire Solar System. Placed at the Sun's position, its event horizon would engulf every planet out to well beyond Neptune.

Specialty: The most massive black hole with a confirmed direct mass measurement (via reverberation mapping). Its luminosity alone makes it visible from nearly the edge of the observable universe - a beacon from a time when the cosmos was just 3 billion years old.

2. Holm 15A*: ~40 Billion Solar Masses

At the heart of elliptical galaxy Holm 15A, ~700 million light-years away in the Abell 85 cluster, sits a black hole of ~40 billion solar masses. The surrounding stellar core is strikingly faint and diffuse - a so-called core deficit - caused by gravitational slingshot events as two massive black holes spiralled inward over billions of years, ejecting stars outward as they merged.

Specialty: The largest directly measured black hole in the local universe, and its ultra-depleted stellar core serves as fossilised evidence of a colossal black hole merger - making it a unique laboratory for studying the final parsec problem in astrophysics.

3. Abell 1201 BCG: ~32.7 Billion Solar Masses

~2.4 billion light-years away, the brightest cluster galaxy of Abell 1201 hides a black hole of ~32.7 billion solar masses - detected not by watching stars orbit, but by observing a distant background galaxy whose light was bent and stretched into a luminous arc as it passed this galaxy: strong gravitational lensing.

Specialty: The first ultramassive black hole of this scale discovered purely through gravitational lensing (Nightingale et al., 2023). This technique does not require resolving individual stellar orbits, meaning dozens of similarly massive, previously invisible black holes may now be within reach of discovery across the sky.

4. NGC 4889: ~21 Billion Solar Masses

The dominant galaxy of the Coma Cluster, ~320 million light-years away, hosts a black hole with an estimated mass of roughly 21 billion solar masses under its highest-orbital-model scenario (the observational range spans 6–37 billion M☉). Its event horizon extends approximately 130 billion kilometres - about ten times the Sun-to-Pluto distance. Notably, despite this immense scale, NGC 4889's black hole appears largely dormant today, no longer actively accreting.

Specialty: An early benchmark in ultramassive black hole science; its quiescent state offers a rare window into the "quiet phase" of these giants after billions of years of feeding - a glimpse of what TON 618 may one day become.

5. NGC 1600: ~17 Billion Solar Masses

Located ~200 million light-years away in the constellation Eridanus, NGC 1600 hosts a black hole of ~17 billion solar masses - yet it sits in a sparse, unremarkable galaxy group, not the rich cluster environment typically associated with black holes of this scale.

Specialty: Its existence fundamentally challenged models of black hole growth, suggesting ultramassive black holes may be far more common - quietly lurking in ordinary environments all across the cosmos, waiting to be found.

Comparison at a Glance

Black Hole	Est. Mass	Distance	Host Environment	Mass vs. Our Sun
TON 618	~66 billion M☉	10.4 billion ly	Quasar host	66,000,000,000×
Holm 15A*	~40 billion M☉	~700 million ly	Rich galaxy cluster	40,000,000,000×
Abell 1201 BCG	~32.7 billion M☉	~2.4 billion ly	BCG elliptical	32,700,000,000×
NGC 4889	~6–37 billion M☉†	~320 million ly	Rich galaxy cluster	up to 37,000,000,000×
NGC 1600	~17 billion M☉	~200 million ly	Sparse galaxy group	17,000,000,000×
Sgr A* (Milky Way)	~4 million M☉	~26,000 ly	Spiral galaxy	4,000,000×

M☉ = solar masses. ly = light-years.

†NGC 4889 has a wide observational uncertainty; ~21 billion M☉ reflects the tangentially-biased orbital model (McConnell et al., 2011).

Video 1: Sizing Up the Universe’s Biggest Black Holes: A visual scale comparison tracking black holes from 100,000 solar masses up to TON 618’s staggering 66 billion solar masses, set against the orbital scale of our own Solar System. Credit: NASA Animation Sizes Up the Biggest Black Holes via NASA Goddard

Conclusion

These five black holes stand at the outermost frontier of known mass - each billions of times heavier than our Sun, making even our galaxy's own central black hole look negligible. Yet this ranking is almost certainly provisional. Gravitational lensing surveys, the Event Horizon Telescope, and upcoming missions such as LISA (Laser Interferometer Space Antenna) are uncovering candidates at an accelerating pace. This list will be updated - and likely overturned - as new discoveries arrive almost daily. The true upper limit of black hole mass remains one of the most tantalising open questions in all of science. The Cosmic Window series will continue to chart these frontiers - turning next to other extraordinary classes of objects that populate our universe.