The UniverseExplainer
What Is a Black Hole, Really?
Not really holes and not cosmic vacuum cleaners, black holes are places where matter is packed so tightly that not even light can climb back out.
THE QUESTION LIBRARY THE UNIVERSE
How big can black holes get, and is there one at the center of our galaxy?
Mass, horizon size and a glowing image tell different parts of the story; spin adds another clue.
THE SHORT ANSWER
Black holes span masses from a few Suns to billions, and our galaxy has one at its center: Sagittarius A*, with about four million solar masses. Mass, horizon width and the shadow seen against glowing gas are different measurements. Spin changes motion nearby and is inferred from gas or merger signals. Tiny primordial black holes remain hypothetical. We do not have a settled universal smallest or largest size.
- Sagittarius A* has about four million times the Sun’s mass.
- The EHT images show glowing gas and a shadow, not a view inside the horizon.
- Stellar-mass categories do not establish universal minimum or maximum masses.
- Spin affects stable orbits and must be inferred through measured signals and models.
About this picture
Original mass diagram on a logarithmic scale. The 10-Sun point is illustrative; Sagittarius A* is about four million Sun masses. M87* uses the Event Horizon Telescope’s 2019 estimate of about 6.5 billion. Equal circles mark mass, not horizon widths or image sizes.
WHERE THE EVIDENCE STANDS
How sure are we?
- A supermassive black hole of about four million solar masses sits at our galaxy’s center.The EHT2022 release connects Sagittarius A*’s gas image with the previous star-orbit evidence and approximate mass.
- An EHT black-hole image shows the interior of its event horizon.The EHT releases and NASA anatomy guide identify surrounding emission and a shadow produced by capture and bending of light.
- The universal smallest and largest black-hole masses remain unresolved.NASA describes approximate categories and uncertain formation histories, while LIGO’s low-mass example leaves the object’s identity open.
- Tiny primordial black holes have been confirmed.NASA distinguishes the early-universe formation proposal from established detections; no observed atom-sized example is claimed.
- Black-hole spin estimates depend on how measured gas or wave signals are modeled.Reynolds reviews disk assumptions; NASA contrasts spectral explanations; LIGO discusses spin’s imprint on merger signals.
WHAT WE DON’T KNOW YET
We do not know how the giant population first formed or whether primordial black holes exist. Some low-mass objects remain hard to classify. The exact spin inferred for an object can change when measurements or models improve.
WHAT WOULD CHANGE THIS ANSWER
A clear tiny-black-hole detection would change the observed size range. Better tests could settle ambiguous light objects and sharpen spin estimates. A universal mass limit would need a supported physical argument, not a current record alone.
Black holes range from objects with a few times the Sun's mass to giants with billions of times its mass. Yes, one sits at the center of our galaxy. It is called Sagittarius A*, and it has about four million Suns' worth of mass. That is enormous, but it is far less massive than some black holes in other galaxies.
The word big needs care. Mass tells you how much an object contributes to gravity. The event horizon is its boundary of no return. Its shadow is the dark region seen against light from nearby gas. These are related, but they are not the same measurement. A gas disk can extend well beyond the horizon.
The image released in 2019 showed the shadow and bright gas around M87*, in another galaxy. The 2022 image showed Sagittarius A*. Neither lets us see inside a black hole. Nor does a picture's size on your screen tell you how large the object is in space.
Small black holes need a separate explanation. The low end of the observed mass range can be hard to distinguish from very dense stellar remnants called neutron stars. Much tinier black holes appear in theories about the early universe. Those primordial black holes have not been confirmed. An atom-sized horizon is a hypothetical possibility, not a photographed example.
Black holes can also spin. Here spin means angular momentum, the physical quantity associated with rotation. It changes motion near the hole and helps researchers interpret gas and wave signals. A black hole is not a solid ball with a marked spot to watch turning. Scientists infer its spin from effects on its surroundings or a merger signal.
THE LONG ANSWER
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God Plays Dice answers big questions in plain words, with sources and one of six honest labels.
Words worth knowing (6)
- Solar mass
- A unit equal to the mass of our Sun.
- Event horizon
- The boundary beyond which light cannot escape to a distant observer.
- Black-hole shadow
- A dark region against surrounding light, produced by light capture and gravity bending its paths; larger than the horizon.
- Angular momentum
- The physical quantity associated with rotation; a black hole’s angular momentum is called its spin.
- Frame dragging
- The change in nearby motion caused by a rotating mass dragging local space-time around with it.
- Primordial black hole
- A proposed black hole formed in the early universe rather than from a star; none has been confirmed.
Sources & further reading
(10) · Checked Oct 2, 2026
- Types of Black Holes ↗NASA ScienceApproximate mass categories, growth and uncertain origins; primordial objects remain unconfirmed.
- Anatomy of a Black Hole ↗NASA ScienceHorizon, surrounding gas and the larger light-capture shadow are different structures.
- NASA Animation Sizes Up the Universe’s Biggest Black Holes ↗NASA (May 1, 2023)Dated mass-scale illustration, including TON 618 above 60 billion solar masses; not a universal cap or current ranking.
- Astronomers Capture First Image of a Black Hole ↗Event Horizon Telescope Collaboration (April 10, 2019)M87* image, inferred 6.5-billion-solar-mass estimate, radio reconstruction and shadow versus horizon.
- Astronomers Reveal First Image of the Black Hole at the Heart of Our Galaxy ↗Event Horizon Telescope Collaboration (May 12, 2022)Sagittarius A* about four million solar masses, star-orbit context, surrounding gas and shadow.
- The curious case of GW190814: a stellar-mass black hole and a mystery compact object ↗LIGO Scientific Collaboration and Virgo Collaboration (2020)The approximately 2.6-solar-mass companion remains ambiguous; spin inference and signal-model qualifications.
- GW231123: black-hole masses inferred from a merger signal ↗LIGO-Virgo-KAGRA Collaboration (2025)Dated approximately 137/103-solar-mass component estimates and explicit model limits; descriptive title, no ranking claim.
- Two Models of Black Hole Spin ↗NASA (February 27, 2013)X-ray spectral approach and competing obscuring-gas interpretation; no present record or universal model claim.
- Dark Matter: primordial black-hole candidates ↗NASA Science (updated August 12, 2026)The primordial-black-hole subsection explicitly treats atom-sized possibilities as hypothetical; no claimed detection or dark-matter identification.
- Observational Constraints on Black Hole Spin ↗Christopher S. Reynolds, author review (arXiv submitted 2020)Full primary scholarly review: angular momentum, frame dragging, stable circular orbits and disk-model limits.
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