The Event Horizon Telescope: photographing black holesA black hole is a region in space in which gravity is so strong that nothing can escape, not even light. Some black holes form when stars reach the end of their lives and collapse under their own gravity. As a star collapses it shrinks, condenses, and gravity escalates. The gravity can become so great that the star turns into a black hole as its light fades from existence. The edge of the black hole is called the event horizon, the point of no return. Only very massive stars become black holes at the end of their life, our Sun for example is not one of them. Aside from black holes formed by stellar collapse, another classification is the supermassive black hole, believed to be found in the centre of galaxies. With masses of hundreds to billions of times the mass of our Sun, their formation is not fully understood. While black holes have been detected, none have actually been imaged. For over a decade, collaborators of the Event Horizon Telescope have been trying to take the first ever picture of a black hole, namely the one at the centre of our own galaxy. How do we know there’s a black hole at the centre of our galaxy? In 1931, a radio signal was discovered coming from the direction of the centre of the Milky Way. In 1974, the object responsible for this radiation, Sagittarius A* (Sgr A*) was discovered, believed to be a supermassive black hole. Of course, black holes don’t emit their own radiation. Due to their strong gravitational fields, any surrounding stars or clouds of gas are flung around in orbits at such speeds that they heat up and radiate. This is the radiation we observe and is detectable on Earth. More recent observations found that stars near the centre of the Milky Way had unusual, highly elliptical orbits around the object Sgr A*. What were they orbiting? Was it truly a black hole? By analysing the stars’ orbits over a number of years, the position and the mass of the object they were orbiting, SgrA*, could be determined. The location of SgrA* was found to be at centre of the Milky Way, with an inferred mass of around 3.6 million times that of our Sun. The only known object to fit this description is a black hole.
For over a decade, collaborators of the Event Horizon Telescope have been trying to take the first ever picture of a black hole, namely the one at the centre of our own galaxy.While SgrA* has been indirectly detected, its existence inferred from the behaviour of the stars surrounding it as well as the radiation detected from the superheated gas spiralling it, no one has ever truly seen it. How can we hope to see something that is, by definition, total darkness? If we could somehow take a picture of the matter that is glowing brightly around SgrA*, perhaps we could see a big black dot in the middle of it all. This was the vision of Dr. Sheperd Doeleman, the Director of the Event Horizon Telescope collaboration.Earth’s atmosphere blocks most of the high frequency radiation coming from space, luckily for us. However, low frequency radiation such as microwaves and radio waves are able to pass through the Earth’s atmosphere unimpeded, making them easier to observe on Earth. The light coming from the surroundings of SgrA* just happens to be a bright radio source, perfect for observation on Earth.
How can we hope to see something that is, by definition, total darkness?As radio waves have very long wavelengths, radio telescopes must be large in size. If a telescope is too large however, it may warp under its own weight. Instead, large arrays of smaller telescopes are often used to detect radio waves and the signals from each telescope are collated. In order to approximate an image of SgrA* however, astrophysicists calculated that we would need a radio telescope the size of the Earth. Obviously, this would be impossible. Astrophysicists are resourceful and were not deterred, however. Doeleman realised that by using telescopes capable of observing radio frequencies that were already in place, from the South Pole Telescope in Antarctica to The Greenland Telescope, and pointing them in the direction of the centre of our galaxy when conditions were good, we may not have a telescope the size of the Earth, but the next best thing.
So much data was collected that it couldn’t be sent over the internet and needed to be transported physically on various hard drives by air carrierThis network of telescopes together forms The Event Horizon Telescope (EHT). After many years of work, data was finally collected in April 2017 over the course of 10 days. Not only did the telescope image the location of SgrA* but also the supermassive black hole located in the M87 galaxy. So much data was collected that it couldn’t be sent over the internet and needed to be transported physically on various hard drives by air carrier. Data from the different telescopes must be matched in order to produce an overall image, much like putting a jigsaw puzzle together, and is currently still being processed at MIT and the Max Planck Institute for Radio Astronomy in Germany for analysis. According to recent updates, the EHT project is proving to be highly successful. Hopefully soon we will have our very first image of a black hole. That, or something much more mysterious.