College College Space and Planetary Sciences Recent Breakthroughs TSS

How to Photograph a Black Hole

Black holes are difficult to observe. However, the recent image of Messier 87 shows that it is possible to see what is around a black hole at very large distances.

Black holes are one of the most mysterious and least understood phenomena in our universe. Imaging black holes directly is impossible since they vacuum up everything including light into their singularity. However, on April 10, 2019, scientists were able to get the best image of a black hole to date, specifically Messier 87 (M87).

The first issue with creating a photo of a black hole is that they are nearly impossible to spot in the first place. This is because on average black holes are very tiny compared to other celestial objects like planets. There are also no black holes nearby, so we are out of luck finding an average sized black hole to image. However, there are supermassive blackholes like Sagittarius A*, which is at the center of the Milky Way and over 4 million times as massive as our Sun. These kinds of black holes are perfect candidates for observation as they suck up matter more frequently than smaller black holes. (1)

Another problem that arises when trying to photograph a black hole is the lack of emitted light. Black holes are so massive that nothing can escape their gravitation pull. This strong gravitational region of space forms when a dying star cannot go against the force of gravity and collapses into a black hole. If we are fortunate enough to observe a black hole with stars nearby or even in a binary system, then we can see an accretion disk that rotates around the black hole. This disk contains matter from the nearby stars that being consumed by the black hole. However, an accretion disk only forms if a black hole was consuming something, so black holes in emptier regions of space are nearly undetectable unless you look for gravitational lensing. (2) Gravitational lensing is an outcome of mass bending light as predicted in Einstein’s theory of general relativity. Essentially, the light from galaxies and stars behind the black hole warp around it. You can see an example of this phenomenon in the picture below. (3)

Diagram of gravitational lensing. Image from NASA.
Image of gravitational lensing around a black hole. Image from Seeker.

So how come we were able to observe M87? This black hole is over 55 million light years away, so one telescope could not do the job alone. Messier 87 happens to be a supermassive black hole within a galaxy of the same name. In fact, it is much larger than Sagittarius A*; M87 has the mass of over 6.5 billion Suns. Additionally, M87 is fairly active as it has a bright accretion disk, which allows us to see the size of the event horizon, the region of the black hole that we cannot observe. Because of these properties, scientists determined that M87 would be a good candidate for observation. The Event Horizon Telescope project was a collaboration of many different observatories around the world that combined all their data to form the famous images of M87. The combination of all the telescopes in this project created a virtual telescope the size of the Earth, which was necessary to get to the resolution of the current image. (1)

M87 over 6 days. Image from Event Horizon Telescope collaboration.

Sources

1) https://www.space.com/event-horizon-telescope.html

2) https://science.nasa.gov/astrophysics/focus-areas/black-holes

3) http://www.cfhtlens.org/public/what-gravitational-lensing

 

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