The First Image of a Black Hole

by Hoi Kiu Wong

Credits: Event Horizon Telescope Collaboration

INTRODUCTION 

Last year, A team of international astronomers worked together and did something that no one previously thought was possible - capturing an image of the black hole. This image was a huge scientific breakthrough as it was well known that nothing can escape a black hole, not even light; the photo was made possible as it shows the glowing surrounding of the black hole - the silhouette of it. In this article, we will look at how this image was captured, the challenges that were faced in the process, and how scientists overcame them. 

WHAT IS A BLACK HOLE

Nasa defines a black hole as ‘a place in space where gravity pulls so much that even light can not get out’. It is formed from the death of a Red Supergiant (following a supernova, note that the Red Supergiant could also form a super-dense neutron star rather than a black hole) and because matter has been compressed in such a small space, this region in space becomes so dense, creating a gravity sink which is a deep curvature in space-time (See Fig. 1) [1]. Since no light can come out of any black hole, they are all invisible; therefore, the image of the black hole captured is actually a photo of its silhouette [2]. 

Fig. 1 Comparison of curvature in Space-time by the Sun, a white dwarf and a black hole 

Image Credit: Addison Wesley

Black holes vary in size and there are different types: stellar, intermediate, supermassive and miniature [1]. The Stellar black hole has a mass of approximately 20 times more than the mass of the sun. Whereas, the larger black holes - ‘supermassive’ black holes, have masses of around 1 million suns all together. Research has shown that every galaxy has a supermassive black hole in the middle. One example is Sagittarius A, the supermassive black hole in the center of our galaxy. It has a mass of approximately 4 millions suns and would fit in a large ball that could hold millions of Earths. Note that not all black holes are huge - there are some that are as small as the size of an atom called miniature blackholes [1] [2]. So far, these tiny black holes are theoretical, but they are believed to be ‘small vortices of darkness’ that have ‘swirled to life after the universe formed with the big bang, some 13.7 billion years ago’.

From watching Science Fiction, people have wondered about the possibility of a black hole destroying the Earth - imagining them as ‘cosmic vacuum cleaners’ [1]. The answer is no - there are no black holes close enough to the solar system for our planet to be ‘sucked in’. If a black hole with the same mass of the sun was present at the centre of the solar system, the Earth would not move into it because the black hole would have the same gravitational field strength as the sun; hence, the Earth would continue to orbit but there will be other consequences as a result of no more sunlight reaching the Earth - as life would not be able to be sustained [2]. 

HOW THEY DID IT 

Scientists had theorised that it would be possible to capture the silhouette of a black hole; however, the main challenge was the large distance between the black holes and the earth; taking an image of any black hole far away in space would undoubtedly be a difficult task. To overcome this, the Event Horizon Telescope (EHT) was made, and it uses a technique that allows imaging of far-away objects, known as Very long Baseline Interferometry (VLBI). Effectively, the EHT is a huge camera made up of telescopes at different observatories around the world - in Mexico, Hawaii, Arizona, Chile, Spain and Antarctica (See Fig. 2) [4]. VLBI allows the telescopes to synchronise and focus on the same object at the same time - in this case, the black hole. This technique of VLBI has also been used to track spacecraft and to capture images of distant cosmic radio sources such as quasars. To obtain a clear image, the EHT has a large diameter (aperture) which increases its ability to detect more light and the resolution gets higher as well. Since the black hole is very distant from us, lots of light needs to be gathered by these telescopes. The EHT’s aperture is around the distance between the two farthest-apart telescope stations: one at the South Pole and the other in Spain. All these telescopes focus on the black hole and obtain data simultaneously. If you compare EHT with the Hubble Space Telescope, its degree of precision is much higher; capable of resolving objects about 4000 times better [3]. 

Fig. 2 The Telescopes around the world that make up the EHT

Image taken from: https://www.nationalgeographic.com/science/2019/04/first-picture-black-hole-revealed-m87-event-horizon-telescope-astrophysics/

The EHT Team decided to capture images of two black holes: Sagittarius A* and M87*. Sagittarius A* was selected because it is located at the centre of our galaxy, the Milky Way, and is approximately 26,000 light years away. It would be the ‘easiest’ black hole to capture as it appears largest from Earth; however, the only challenge is that there are lots of stars and other celestial objects in the way that creates ‘pollution’. To make a clear image, a large amount of data needs to be processed and removed - which takes a long time. M87* was the other black hole chosen and it is located at the center of the elliptical galaxy Messier 87, at the center of the Virgo cluster [4]. It was considered to be ‘better’ than the Sagittarius A* because it is an active black hole. Moreover, it is massive compared to the Sagittarius A*, as it contains 6.5 billion solar masses (compared to 4 million solar masses for Sagittarius A*). The only challenge was that M87* was further away from earth. [3] 

In April 2017, the telescopes worked together and of the ten days chosen for observation, four days were clear at all eight sites. These telescopes observed M87* in short radio wavelengths and collected approximately five petabytes of data. As this amount of data exceeds what the current internet speeds can handle, the recorded media were physically transported in computer disks to a central location where computer scientists and astronomers collaborated together in trying to process the image. After two years, the scientists successfully assembled the image of the silhouette of the black hole M87* together [3] [4]. 

CONCLUSION 

This image of the silhouette of the black hole is important as it gives us a deeper insight into physics and allows us to test theories like Einstein’s theory of general relativity. We still do not know a lot about black holes; hence with new technology, combined with relativity, we can discover more about the structure and behaviour of black holes, as well as the mechanism by which some black holes emit enormous jets to particles travelling at the speed of light. One of the major results of the EHT black hole imaging project is a more precise calculation of the black hole’s mass - as we can now measure the radius of M87*’s event horizon (also known as its Schwarzschild radius) and use computer programming to give an estimation of the Black Hole’s mass (See Fig. 3)  [3]. 

Fig. 3 Artist’s impression of a spinning supermassive black hole surrounded by an accretion disc 

Image credit: ESO 

Image taken from https://www.jpl.nasa.gov/edu/news/2019/4/19/how-scientists-captured-the-first-image-of-a-black-hole/ [3] 

BIBLIOGRAPHY 

[1] Wei-Haas, Maya. “Black Holes, Explained.” What Is a Black Hole?, 17 Sept. 2019, www.nationalgeographic.com/science/space/universe/black-holes/. 

[2] Dunbar, Brian. “What Is a Black Hole?” NASA, NASA, 21 May 2015, www.nasa.gov/audience/forstudents/k-4/stories/nasa-knows/what-is-a-black-hole-k4.html. 

[3] “How Scientists Captured the First Image of a Black Hole - Teachable Moments.” NASA, NASA, 19 Apr. 2019, www.jpl.nasa.gov/edu/news/2019/4/19/how-scientists-captured-the-first-image-of-a-black-hole/. 

[4] Tafreshi, Photograph by Babak. “First-Ever Picture of a Black Hole Unveiled.” National Geographic, 10 Apr. 2019, www.nationalgeographic.com/science/2019/04/first-picture-black-hole-revealed-m87-event-horizon-telescope-astrophysics/.