On April 10, 2019, with great fanfare, an international team of astronomers unveiled the very first photograph of a black hole. Let’s revisit that historic exploit.
A black hole is a star whose constituent material is so compressed that light itself can’t escape from it if it gets too close. It deforms spacetime and superheats any matter located close by. Astronomers predicted the existence of these objects with Einstein’s general theory of relativity over 100 years ago.
An Earth-sized telescope
Unfortunately, it’s extremely difficult to observe a black hole directly. To do so requires a telescope with very high resolution – and for that reason more than 200 astronomers from 20 different countries combined their efforts in the Event Horizon Telescope (EHT) project. That project brings together eight radio telescopes spread out over the entire surface of the planet. Once they’re linked together, what results is a virtual telescope the scale of the Earth. The resolution of the EHT is 20 microarcseconds. With that much resolution, someone living in Montréal could read the date on a 25-cent coin in Vancouver.
The astronomers also used atomic clocks to record the precise instant of their observations, with the goal of ensuring near-perfect synchronization.
After years of tests and calibrations, the teams of astronomers had four days of observations in April 2017 when all optimal conditions were in place.
The collected data, amounting to nearly five petaoctets (five million gigaoctets), were stored on hard drives and flown to two supercomputers located in German and the U.S. Then they were synchronized to within a millionth of a millionth of a second! That took two years of hard work.
A fascinating image
The picture, which can be seen above, is utterly gripping. Gas, as it falls towards the black hole, begins to swirl rapidly around it and forms a ring that emits light. When the light photons cross a region called the event horizon, they’re inexorably captured by the black hole and create a dark area at the center of the ring. Some photons nevertheless succeed in escaping, allowing us to see a great ring.
The picture reveals the existence of a black hole at the centre of the massive galaxy M87, located 55 millions light years from the Earth. The diameter of the black hole is in the neighborhood of 40 billion kilometers, for a mass equivalent to 6.5 billion times that of the Sun.
The matter in the ring is spinning clockwise. The brighter zone in the lower part of the ring corresponds to matter getting closer to us, while the dark area higher up consists of matter moving away.
The picture taken by the EHT admirably accords with astronomers’ theoretical models. Which confirms the validity of the results obtained.
In the future, astronomers hope to have a better understanding of how matter behaves close to a black hole. They also want to understand the role of the magnetic field in the processes of matter accretion and how a galaxy like M87 emits immense jets of matter.
But above all, the astronomer community is impatient to see the second EHT picture, scheduled for late in the year, the one of the black hole that sits at the center of our Milky Way. That one being smaller than the one in M87, matter turns much faster in the ring. Meaning that a different and probably more complex picture can be expected. A story to keep an eye on…