How big can black holes get, and is there one at the center of our galaxy?

When we say big, do we mean mass or width?

A solar mass means the mass of our Sun. It is a unit for comparing objects, not a count of actual Suns trapped inside. Saying that Sagittarius A* has four million solar masses describes its mass; it does not say that its horizon is four million times as wide as the Sun.

For a simple, non-spinning black-hole model, horizon size grows in direct proportion to mass. Double the mass and you double its horizon radius. The EHT's 2019 explanation connects mass with the scale of the shadow. Spin and viewing geometry matter when moving beyond that simple comparison.

NASA's anatomy guide distinguishes the horizon from the shadow produced by captured and bent light. The shadow is larger than the horizon. The glowing gas around it is larger still in extent and has no single universal outer edge. Always check which of these a diagram measures.

How massive are the black holes we have found?

NASA's classification puts stellar-mass black holes in a broad range from a few to hundreds of solar masses. The category boundaries are approximate. They describe populations and possible growth through mergers; they are not fixed limits on what gravity permits.

For a concrete observed system, the LIGO-Virgo-KAGRA collaboration's 2025 account of GW231123 inferred component masses of about 137 and 103 Suns. These estimates come from matching a measured wave signal to merger models. The collaboration also discusses limits of those models. This is a dated example, not a claim about a current record.

Supermassive black holes occupy a different scale: hundreds of thousands to billions of solar masses, with some estimates reaching tens of billions. A NASA animation published in May 2023 used more than 60 billion solar masses for TON 618. That illustrative estimate is not a measured universal upper limit.

Black holes grow by gaining matter and merging. The origins of the giant population remain uncertain, as NASA explains. The largest measured example would tell us what has been found, not automatically what can ever form. Questions about available matter, growth history and the time allowed belong in any account of an astrophysical upper limit.

How small could a black hole be?

Separate a black hole's mass from its small physical extent. A stellar-mass object can have a compact horizon while remaining very heavy. The lower edge of the observed stellar population is also a measurement problem: a heavy neutron star can resemble a light black hole in some observations.

In GW190814, observed in August 2019, one merging object had about 2.6 solar masses. The collaboration could not establish whether it was a black hole or an unusually heavy neutron star. Quoting that number as the mass of a definitively identified smallest black hole would discard the uncertainty.

Primordial black holes are a different proposal. Early-universe theories allow formation without a star collapsing first, potentially at far smaller masses. They have not been confirmed. NASA also describes atom-sized possibilities as hypothetical, not an observed collection of tiny black holes.

Do not turn the familiar star-collapse range into a universal minimum. Different formation proposals ask different questions. Nor can an unconfirmed proposal supply evidence that a tiny object exists. A formation model, an observed mass range and a fundamental limit are different claims. This answer supplies no settled smallest size.

What do Sagittarius A* and the EHT images tell us about size?

Sagittarius A* is the supermassive black hole at the center of the Milky Way. The EHT's May 12, 2022 release gives its mass as about four million Suns and explains the earlier evidence from orbiting stars. The image supplied a separate view of gas and the shadow near the hole.

The first EHT image, released April 10, 2019, was M87*. That release estimated about 6.5 billion solar masses. M87* and Sagittarius A* are different objects, and the releases are different milestones. They should not be combined into a story in which the 2019 picture showed our galaxy's center.

The EHT gathers radio observations and reconstructs an image from measurements made at separated telescopes. The bright ring is radiation from surrounding gas, affected by gravity. The dark center is the shadow. It is not a view of material behind the horizon, and its edge is not simply the horizon drawn in ink.

Distance also matters when comparing pictures. A more distant object can look smaller despite being physically larger. Two images presented at the same width therefore need their labels and scales. A mass comparison, a horizon diagram and a reconstructed radio image answer different size questions.

Do black holes spin, and how can we tell?

Black holes can carry angular momentum. In Einstein's theory, spin drags nearby space-time into rotation, an effect called frame dragging. It changes the closest stable circular orbits: motion in the same direction as the spin can remain stable closer in, while motion against it has its stable boundary farther out.

Christopher Reynolds's review of spin measurements explains why this matters for the gas disk's inner region and its radiation. The stable circular orbit is not the event horizon. Actual gas also feels pressure and magnetic forces, so treating the disk's inner edge as a clean orbit marker requires assumptions.

NASA's X-ray explanation shows one measurement approach: study a feature associated with iron in the disk's spectrum. Motion and gravity distort it. Researchers compare models of that distortion, while testing alternatives such as obscuring gas. They infer spin rather than observing a painted surface rotate.

Merger waves offer another approach. LIGO's GW190814 account explains that spin influences the signal less strongly than mass and can be harder to measure. Agreement with a model is informative, but its assumptions and uncertainties remain part of the result. Spin estimates can illuminate formation and growth without uniquely revealing the whole history.

What is the strongest objection, and how can you check this?

The strongest objection is that our measurements depend on models. That is true, especially for spin. The answer is to test the assumptions and seek observations that constrain different parts of the explanation. Uncertainty in a spin estimate does not make a star-orbit mass measurement and a gas image interchangeable or erase the evidence each provides.

Check a black-hole claim by writing down the object, the quantity measured and the method. Is the number a mass, horizon width, shadow width or apparent size on the sky? Does the report identify a black hole, or leave a neutron-star alternative open? For spin, ask which gas or wave model was fitted and what limits the authors discuss.

Then check the publication date before repeating a ranking. For the broader account of horizons and how black holes are detected, read what a black hole is. A useful comparison keeps observations, interpretations and hypothetical extremes visibly separate.