Black Holes
Sgr A*
Supermassive black hole at center of Milky Way — 4 million solar masses, imaged in 2022
Sagittarius A* (Sgr A*) is the supermassive black hole at the center of the Milky Way galaxy, located 26,000 light-years from Earth in the direction of Sagittarius. Mass: ~4.3 million M_sun. Distance: 8.0 kiloparsecs. Confirmed via decades of observation of stars orbiting it (Genzel and Ghez, Nobel Prize 2020). Imaged by Event Horizon Telescope in May 2022 — second supermassive black hole imaged after M87*. Provides direct test of general relativity.
- Mass~4.3 million M_sun
- Distance~8 kpc (~26,000 light-years)
- Schwarzschild radius~12 million km (~0.08 AU)
- Stars orbitingS2, S62, S0-2 — short-period orbits
- First imagedMay 2022 by Event Horizon Telescope
- Discovery1974 (Brown & Lo); confirmed via S-stars (Genzel/Ghez)
Interactive visualization
Press play, or step through manually. The visualization is yours to drive — try it before reading on.
Watch the 60-second explainer
A condensed visual walkthrough — narrated, captioned, under a minute.
Why Sgr A* matters
- Galactic center. Anchors Milky Way structure.
- GR test. Most extreme tests of Einstein's theory.
- S-stars. Direct mass measurement via orbits.
- Dynamics. Reveals galactic rotation curves.
- SMBH formation. Origin of supermassive BHs.
- EHT. Imaging frontier — global VLBI network.
- Astrophysical extremes. Lab for plasma physics, accretion.
Common misconceptions
- Sgr A* threatens Earth. Far away — no danger.
- Galaxy rotates around Sun. Both rotate around galactic center near Sgr A*.
- Sgr A* is largest BH known. Smaller than M87* and many others.
- Sgr A* always glowed brightly. Low luminosity now; periods of activity.
- Stars orbit slowly. S2 reaches 7,650 km/s at perihelion.
- Sgr A* is a star. A: It's a black hole — radio source name.
Frequently asked questions
How do we know it's a black hole?
(1) Stars orbit it at incredible speeds — implying enormous mass within tiny region. S2 reaches 7,650 km/s at perihelion, orbital period 16 years. (2) Mass concentration: 4.3 million M_sun in volume comparable to the Solar System. Only BH explanation works for such density. (3) X-ray, radio emission consistent with accretion disk.
What's the S-star cluster?
Group of stars in extreme orbits around Sgr A*. S2 (also called S0-2): orbital period 16 yr, perihelion ~120 AU (Solar System scale). S62: closer, faster. Observation of these stars over decades by Genzel and Ghez teams (independently) provided definitive proof of supermassive BH. Awarded Nobel Prize 2020.
Is Sgr A* active?
Currently quiet. Low accretion rate. Emits ~10⁻⁹ × Eddington luminosity (much less than M87*). Periodic flares. Past activity: evidence for major outbursts thousands of years ago (Fermi bubbles). Future activity: when more material accretes, could become brighter.
How was it imaged?
Event Horizon Telescope (EHT) — same VLBI network that imaged M87*. Observed radio at 1.3 mm. Synchronized data. Image reveals "shadow" of black hole — dark region surrounded by bright accretion ring. Released May 2022 — 3 years after M87* image. Smaller and dimmer than M87*; harder to image.
Is Sgr A* dangerous?
No. Earth is 26,000 light-years away. Gravitational pull from Sgr A* on Sun is balanced by galactic mass distribution. Sun orbits it at 230 km/s, period ~250 Myr. Even if Sgr A* became active (quasar), distance would protect Earth. Nothing nearby that would be affected.
What's the Milky Way doing around it?
Galaxy rotates around Sgr A*. Sun orbits at ~230 km/s, completing one orbit in ~225-250 Myr. Sgr A* anchors galactic core. Stars and gas in central region show Keplerian orbits — direct mass measurement. Galactic structure builds outward from this central engine.
Was Sgr A* always quiet?
No — fluctuates over time. Fermi bubbles (giant gamma-ray structures above/below galactic plane) suggest ~Myr-scale outbursts. Past episodes of quasar-like luminosity. Sgr A* now in quiet phase. Could brighten if a star or gas cloud falls into accretion zone — like G2 cloud near miss in 2014.