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How to Take a “Picture” of a Black Hole

April 29, 2019

After many years of artist renderings, the first ever image of a black hole was released to the world in April. It looks just about how you would expect: a ring-like structure with a dark central region.

The image was created using the Event Horizon Telescope, an array of eight ground-based radio telescopes across the globe — more on that later.

The black hole itself has a mass 6.5 billion times that of the Sun. It is 5 million light-years away at the center of the massive galaxy Messier 87, which is in the nearby Virgo galaxy cluster, visible in the Northern Hemisphere from March to July.

How was the image made?

We see other celestial objects like stars and meteors — and can take pictures of them — because the light that they emit travels all the way down to our telescopes, eyeballs and cameras.

Black holes don’t emit light. The gravity in a black hole is so strong that not even light can escape. They are truly black.

To overcome that problem, astronomers and astrophysicists use radio telescopes. Rather than collecting visible light, radio telescopes collect radio light waves, bring them into focus and make them available for analysis. Each pixel in the image of the black hole represents some particular wavelength of a radio wave.

But that wasn’t the biggest challenge

The biggest challenge of producing an image of a black hole wasn’t that it’s black, but that from earth it is absolutely tiny. To explain how tiny we must introduce the term angular size (or angular measurement), which in astronomy and optics describes the apparent size of an object from any fixed point. The units of angular measurement are degrees.

Imagine breaking up the visual world around your eyeballs into 360 degrees. A tiny object, or a large object very far away, may only take up a few of those 360 degrees.

The moon usually has an angular size of about 0.5 degrees. Your thumb held out in front of you at arm’s length also has an angular size of about 0.5 degrees. That’s why you can cover the moon with your thumb. Your thumb is much smaller than the moon, but their angular size is the same when your thumb is the right distance from you.

Degrees are broken down into arcs. An arcminute is 1/60th of a degree. An arcsecond is 1/3600th of a degree. A microarcsecond is an arcsecond broken down into a million more pieces.

The black hole has an angular size of around 40 microarcseconds, meaning the moon — and your thumb at arm’s length — is 45 million times larger than the apparent size of the black hole.

How did they capture anything at the microarcsecond level?

To overcome the laws of diffraction, which make accurately recording anything of that size near impossible, you would need a radio telescope the size of the Earth.

Enter very-long-baseline interferometry (VLBI), a technique that synchronizes multiple radio telescopes positioned across the globe and exploits the rotation of the planet to mimic the power of one Earth-size telescope.

You extrapolate on the data you collect from those radio telescopes — positioned in Arizona, Hawai’i, Mexico, Chile, Spain, and the South Pole — and use an algorithm to fill in the missing pieces.

That’s how the Event Horizon Telescope can achieve unprecedented resolution down to 25 millionths of an arcsecond at its observing wavelength of 1.3 mm.

More on the technique used to create the first image of a black hole can be found here and here.

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