Earth-Sized Telescope Bags Magnetic Fields from the Milky Way's Black Hole Core

The Event Horizon Telescope starts earning its keep.

Michael Byrne

Michael Byrne


For the first time, astrophysicists have directly detected magnetic fields very near the event horizon of the Milky Way's immense central black hole.

In a paper published Thursday in Science, a large international team of scientists describe observations made with the Event Horizon Telescope (EHT)—what's really a global net of telescopes adding up to a single giant Earth-sized instrument—of the Galactic Center supermassive black hole, Sagittarius A*. Until only recently, a view like this would have been deemed impossible given the cosmic-scale forces at work near the black hole's event horizon, the point of no return for material and even light itself as it falls into the gravitational crunch of 4.5 million solar masses.

So, yes, the Milky Way does cruise around a central supermassive black hole. Its scale is almost impossible to imagine; if our own Sun had the same mass, it would be something like 500 times the size of the Solar System (according to the back of my envelope). The thing is so dense, however, that in reality it's only about 100 times the diameter of the Sun (for 4.5 million times the mass).

We know about the Sag A* black hole because of its gravitational effects. For many years, astronomers have been charting the paths of 30 or so individual stars in the suspected black hole's neighborhood, while also using statistical techniques to sort through the movements of many thousands more. The existence of the Sag A* black hole isn't much in dispute.

Image: M. Weiss/CfA

Where magnetic fields come in is with the black hole's spinning entourage of assorted junk, e.g. its accretion disc. This disc is magnetized but unstable, so as it whirls around, small variations get amplified and blasted out into space. This is called synchrotron radiation and is the result of charged particles being accelerated. Around a black hole, the result can be high-energy jets of gravitationally driven radiation erupting out into space. You can see synchrotron radiation as a blue haze in the images above.

The EHT is still a work in progress. During the study, the meta-telescope consisted of four different telescopes spread across the globe: the Submillimeter Array and the James Clerk Maxwell telescopes in Hawaii; the Submillimeter Telescope in Arizona; and the Combined Array for Research in Millimeter-wave Astronomy (CARMA) in California. The basic idea is that they all collect radio wave data and correlate it using near-perfect atomic clocks. With such synchronization, it becomes possible to have a radio telescope spanning the entire planet.

Now, the EHT array consists of nine different telescopes, with a total of 12 planned for the future. Once complete, it will be able to image an orange on the Moon, as noted by Physics World.

"There are now enough telescopes in the array, in principle, to make images in the next couple of years," study co-author Avery Broderick offers in a Perimeter Institute statement. "This might be the point, like the turn of the last century, when all of a sudden the puzzle pieces click into place."