The Darkest Black

Image of the black hole at the centre of M87 Image from: Akiyama, K., Alberdi, A., Alef, W., Asada, K., Azulay, R., Baczko, A.-K., … Ziurys, L. (2019). First M87 Event Horizon Telescope Results. II. Array and Instrumentation. The Astrophysical Journal, 875(1), L2. http://doi.org/10.3847/2041-8213/ab0c96

On April 10th The Event Horizon Telescope Collaborative released an image so exciting that I, like my parents with the Moon landing, will remember where I was and what I was doing when I saw it – which happened to be on my partner’s stationary bike in our garage watching the YouTube broadcast on my smartphone.

The image that popped up on the screen before me was a ring of orange hues, weighted and thicker towards the bottom left. A dark, gaping, empty, expanse of black sat inside the ring. I was looking at a black hole and the shadow its event horizon. The orange hues were ionized gasses dizzyingly swirling around it at speeds a fraction of the speed of light; sending out their blazingly hot swansong before crossing a frontier into an area of space so unknown we can only conjecture at what is behind the veil of the event horizon.

The light from that gas travelled incredible distances of time and space before reaching not our eyes, but a group of radio telescopes spanning the globe, interconnected through an ambitious and creative collaborate effort. The end result of which is nothing short of breathtaking.

Being so enthralled in the image, I missed a good portion of what the researchers announced about their findings so far. To get an idea, I turned to the five articles that were published in The Astrophysical Journal Letters and thumbed through them. Between the formulas, diagrams and interpretations, I quickly saw the incredible amount of collaboration and work that went into capturing and processing the images taken between April 4 and 11, 2017. Numerous radio telescopes across the Earth all had to simultaneously have good weather, the petabytes of data that had to be transferred, standardized, aligned and consolidated. New algorithms were created, faster data processing were invented and countless hours spent to produce an image of a dark region in space, the shadow of the black hole, at the centre of M87 that spans 19 to 38 microarcseconds!

If you are like me, you want to know how much that is in light years not arcseconds and you’re not worried about the margins of error. Let us have a little fun and work that out for ourselves. We’ll need a few things: the small angle formula, the distance to M87 and a calculator.

The small angle formula (SAF) is: arcseconds = 206,265(diameter of object/ distance to object)

The distance to M87 is about 53.5 million Light Years

Let us take the upper end of the measurement because who really wants a small shadow? 38 microarcseconds become … 3.8 x 10^-5 arcseconds.

We want the diameter of the shadow, that means we rewrite the SAF to become diameter of object = (distance to object x arcseconds)/206,265 then plug in the numbers.

diameter = (53,500,000 x 0.000,038)/206,265

We get around 0.01 light years which we can convert into km by multiplying by 9.5 x 10^12 … and voila! 9.5 x 10^10 km or 95 billion km! Not bad for a shadow.

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