The Sgr A* accretion flow is the hot, thin gas spiralling around the Milky Way’s central black hole, strikingly faint for something feeding a black hole.
At a glance
| type | nebula |
|---|---|
| distance from the Sun | 26,700 light-years (8.18 kpc) |
| its light left it | 26,700 years ago |
| size in the piece | about 0.2 light-years acrossmodel |
Read off the piece’s own catalogue, the same values it draws with. Model marks a property of the piece’s galaxy model rather than a measurement.
About Sgr A* accretion
A black hole is itself dark; what we see of Sgr A* is the gas around it, heated as it falls in. Here that gas is sparse and extremely hot, supplied mostly by the winds of massive stars near the centre, and only a small part of it ever reaches the black hole. It radiates very inefficiently, so the Galaxy’s nucleus is a quiet one, far dimmer than the active nuclei of other galaxies.
The flow is seen in radio, infrared and X-rays, and it flickers: flares in infrared and X-rays occur often, from hot spots very close to the black hole. The Event Horizon Telescope, linking radio dishes across the Earth, imaged the innermost glow as a bright ring around a dark shadow, the silhouette of the black hole against its own accretion flow.
The black hole’s mass is known from the orbits of stars that swing around it, a measurement recognised with a Nobel Prize.
What you see in the piece
It is a small warm glow at the very centre of the Galaxy, hidden from the Sun’s position in visible light by dust; it shows in the infrared and radio bands and when you fly to Sgr A*.
Measured or modelled
The piece draws a compact warm halo standing in for the accretion flow; its size and brightness are a representation, not a resolved image. How the flow is structured, and why it is so faint, are described by models that are still being refined.
Questions
Why is the Milky Way’s black hole so faint?
Very little gas reaches it, and the hot, thin gas that does flows in without radiating efficiently, so most of its energy is carried into the black hole or blown away rather than shining.
Has Sgr A* been photographed?
Yes. The Event Horizon Telescope imaged the radio glow of its accretion flow as a ring around a dark shadow, the black hole’s silhouette.