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Black holes · the galactic centre

Sagittarius A*: the black hole at the centre of the Milky Way

Sagittarius A*, a black shadow ringed by bent light, drawn as the Event Horizon Telescope saw it, in Exo-View

At the centre of our galaxy, 26,670 light-years from the Sun in the constellation Sagittarius, sits a black hole of about four million Suns. Nobody has seen it directly; we know it from the stars that race round it, and since 2022 from a picture of its shadow. In Exo-View you can fly there from the Sun.

Fly to Sagittarius A*See the Milky Way
Distance
26,670 light-years
Mass
About 4.15 million Suns
Horizon
About 12 million km in radius, a size that would fit inside Mercury's orbit
Kind
Supermassive black hole, quiet: it gives off only a few hundred Suns' worth of light
Nearest star
S2, 16 years round, 120 AU from the hole at its nearest, in May 2018
Pictured
By the Event Horizon Telescope, announced in May 2022

How we know it is there

The centre of the galaxy is hidden from us by dust, but infrared telescopes see through it. For almost thirty years two teams, led by Reinhard Genzel and Andrea Ghez, tracked the stars at the very centre, and found them whirling round an invisible point at thousands of km a second. The star S2 goes round it once in 16 years on a long oval; in 2018 it swept past at about 7,650 km/s, 2.5% of the speed of light, and its light was stretched by the hole's gravity just as Einstein's theory says it should. Only a black hole of four million Suns, squeezed into a space smaller than S2's orbit, fits. The work won the 2020 Nobel Prize in Physics.

The picture of its shadow

In May 2022 the Event Horizon Telescope, a set of radio dishes across the Earth working as one, released an image of Sagittarius A*: a dark shadow ringed by a bright circle of hot gas, which is light bent round the hole by its gravity. The ring is about 52 millionths of an arcsecond across, like seeing a doughnut on the Moon from the Earth. The hole is quiet, feeding on little, which is why it is so faint for its mass.

In Exo-View

The key below flies from the Sun to the galactic centre and draws the hole as that picture shows it, a shadow edged by its ring with the sky bent round it. S2's orbit is drawn closer in than it is, on its true shape and timing, and the plate says so. The star is next at its nearest in 2034.

Fly to Sagittarius A*See the Milky Way

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