A donut in the sky

On 12 May 2022, a global network of astronomers from the Event Horizon Telescope Collaboration revealed the first image of the supermassive black hole at the heart of our Milky Way galaxy. This black hole, called Sagittarius A* or Sgr A*, lies 27 000 light years away, and its shadow is only 52 micro-arcseconds wide on the sky. That’s about the size of a donut on the surface of the moon as seen from Earth! 

This tiny speck is not visible even to the largest individual telescope. The Event Horizon Telescope combines data from multiple radio telescopes located all across the globe, including ALMA and APEX, co-owned by ESO. This effectively creates one giant, “Earth-sized'' virtual telescope, using a technique known as very-long-baseline interferometry. Scientists then use a series of complex algorithms to sift through the data and reconstruct the image we see today.

Black holes cannot be directly imaged as they are completely dark, but astronomers can measure the radio waves emitted by the superheated glowing gas and dust surrounding them, creating the donut-like shape we see in the image of Sgr A*.

Links:

• Recording of the ESO press conference on the Sgr A* EHT image
• Press release on the Sgr A* EHT image
• ESO Press Conference replay, visuals, blog posts and other background information on Sgr A*