Photographing ghosts in space
- Shaun Smillie
Photographing a black hole in space, 55 million light years from Earth, seems an impossible task but scientists went to unprecedented lengths to achieve this.
Black holes are probably some of the most mysterious, scary and fascinating phenomena in space. These dark lords of the cosmos have the power to give birth to or destroy stars – or even prevent them from forming. Being invisible, they pose a challenge to find, and an even bigger challenge to photograph.
A multinational team of 300 scientists put their minds together to find their way around the problem: how do you photograph an object that is 55 million light years away, that you are not even able to see?
The team included Wits Professor Roger Deane, South African Radio Astronomy Observatory (SARAO)/National Research Foundation Chair in Radio Astronomy in the School of Physics and Wits/SARAO postdoctoral fellow, Dr Iniyan Natarajan – the only two scientists from Africa that were part of the collaboration.
Black holes are regions of spacetime where gravity is so strong that nothing can escape from them – even light. The gravity is so strong because large amounts of matter is squeezed into a tiny space, creating an extremely dense body. This can happen when a star is dying, or is found at the dense centre of a galaxy.
As the gravity is so strong, black holes emit no light. They have been mathematical mysteries since Albert Einstein published his theory of general relativity. According to this theory, a sufficiently compact mass can deform spacetime to form a black hole.
Black holes remained mere mathematical theories until the early 1970s. Then, several researchers independent of each other, identified the first black hole called Cygnus X-1, that is located about 6 070 light years from Earth.
Over the past three decades it has become clear that black holes are not some fantastical interesting, exotic thought, but they actually influence the growth of our galaxies dramatically.
“If a black hole gets all violent and angry, it can shut off all star formation in its galaxy. But it can also trigger some star formation,” says Deane.
Research has shown that there is a clear correlation between the mass of a central black hole and the speed at which stars and gas move around within the galaxy.
As black holes cannot be seen, scientists study the behaviour of objects – such as stars – around them to locate them. Each galaxy has a supermassive black hole at its centre. The one in our galaxy – the Milky Way – has a black hole called Sagittarius A*. It has a mass equal to about four million suns and could fit a few billion earths in its space.
In their attempt to capture the first ever image of a black hole, the group of scientists that collaborated to form the Event Horizon Telescope (EHT) focused their attention on a black hole, called M87*. This black hole sits in a galaxy about 55 million light years from Earth.
The team selected it because of its enormous mass and its relative proximity on cosmic scales. This combination gives it one of the largest black hole shadow sizes in the sky. M87* is 300 million times more massive than the first black hole discovered, Cygnus X-1. It is also 6.5 billion times the mass of our sun, and roughly the same size as our solar system.
Telescope as big as Earth
The sheer size of M87* demanded an imaging system that has never been seen before. The team virtually connected an array of eight telescopes, situated at very dry sites around the earth, to create the EHT.
The telescopes were able to synchronise their recorded data and exploit the rotation of the Earth. By stitching together the data streams caught by the telescopes, the EHT formed a virtual telescope that could be equated to the size of Earth.
“The sharpness of the images [that] a network of antennas can make is determined by two things, how far apart they are, and the wavelength of light they are tuned to. So while the Square Kilometre Array in the Karoo will make images of similar sharpness to the Hubble Space Telescope, the EHT sees details 1 000 times finer by placing antennas across the globe and capturing light waves barely one millimetre in size,” says Deane.
The synchronisation of the telescopes had to be so precise that atomic clocks were used. Each telescope produced about 350 terabytes of data per day, which was then fed into specialised supercomputers to synthesise the data streams. EHT members then used the data to reconstruct the image with purpose-built algorithms.
“The resulting EHT achieved mind-boggling resolution. It would be the equivalent of the ability to read a newspaper in Cairo, while sitting in Johannesburg,” says Deane.
In creating the actual image of M87*, the scientists decided to focus not on the black hole itself, but on capturing its shadow. They did this by imaging the halo-like ring of hot material that encircles M87* as light is bent around it or disappears into it.
“The problem is that the shadow feature or the deficit of light caused by M87* is so small against the sky it is basically like trying to photograph a doughnut on the moon.”
On 10 April 2019, the EHT collaboration released the first ever image of M87*. The image shows a large dark spot in the centre of a ring of light. It could be said that the image captured is 55 million years old – almost as old as the oldest dinosaurs – as that is the time that it would have taken the light to travel to Earth, for us to witness it.
In March this year, the EHT team went a step further, and announced that they had once again taken an image of M87*, this time catching the black hole as it appears in polarised light.
This image will help to understand how magnetic fields behave around black holes and help explain the mysterious jets associated with these galactic objects.
For Deane, studying black holes is just one of many projects that he is involved in that examines the bigger picture of the evolution of galaxies.
One day these mystery objects might even assist in humankind’s great quest to find life in the universe.
“The radiation from a black hole could break down any complex life, and that may give us a sense of where best to look,” says Deane.
- Shaun Smillie is a freelance writer.
- This article first appeared in Curiosity, a research magazine produced by Wits Communications and the Research Office.
- Read more in the 12th issue, themed: #Solutions. We explore #WitsForGood solutions to the structural, political and socioeconomic challenges that persist in South Africa, and we are encouraged by astounding ‘moonshot moments’ where Witsies are advancing science, health, engineering, technology and innovation.