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Sagittarius A*
Invisible itself — traced by the stars it moves.
- classification
- Supermassive black hole
- system
- Milky Way galactic centre
Sagittarius A* is the compact radio source at the centre of the Milky Way, identified as a supermassive black hole. It is the closest object of its kind, which makes it the best-studied.
- Distance from Earth
- about 26,000 light years
- Mass
- about 4.3 million solar masses
- Event-horizon radius
- roughly 12 million km (Schwarzschild radius)
- Location
- constellation Sagittarius, galactic centre
Only values with an established scientific consensus are listed. Where a quantity is uncertain or model-dependent, it is discussed in the sections below rather than stated as data.
A vast mass sits in a tiny volume
Stars orbit an unseen object at high speed on tight, closed orbits, requiring millions of solar masses within a region smaller than our solar system.
It has been imaged in silhouette
A ring of emission surrounding a central dark region was resolved in 2022, matching predictions for a black hole shadow.
It accretes faintly
Compared with active galactic nuclei, Sagittarius A* is remarkably quiet, consuming very little material.
Long-term stellar orbit tracking
Decades of infrared imaging traced individual stars through complete orbits, giving a precise central mass.
Radio interferometry
A global array of radio telescopes acted as one Earth-sized instrument to resolve the emission ring.
Multi-wavelength monitoring
X-ray and infrared flares track how material behaves close to the horizon.
- Why is its accretion rate so low compared with similar black holes?
- How do the flares originate — magnetic reconnection, orbiting hot spots, or something else?Competing hypotheses remain.
- What happens to information that crosses the horizon?An unresolved theoretical problem.
Items above are unresolved. Hypotheses are labelled as such and are not presented as confirmed findings.
Event Horizon Telescope Collaboration // 2022
First Sagittarius A* Event Horizon Telescope Results. I (ApJL 930, L12) ↗
2022 Sgr A* shadow image.