Four important things that this picture tells us

Scientists working on the Event Horizon telescope have taken the very first images of a black hole – commonly referred to as M87 * (the * in the title represents a black hole).

M87 * is a super-massive black hole in the center of the M87 galaxy. This galaxy is also known as Virgo A and is part of the constellation Virgo.

M87 is a member of the group of the Virgin and can be seen at the bottom left of this group picture. Image: European Southern Observatory

Here are four things we learned about this black hole.

1) Even if the light can not escape, you can still see the shadow of the black hole

Black holes are defined by the fact that they can trap light. So, how do you see an object that traps light in the vast black expanses of space?

OK, it's still very difficult. But we think that the biggest black holes are hidden in the center of the galaxies, which are not so empty.

There are a lot of things to lose under the weight of the powerful gravitational pull of the black hole.

We found evidence showing both stars and clouds of hot plasma orbiting these super massive black holes. The closer the plasma is to the black hole, the faster it will spin around it (just as Mercury rotates around the Sun in 88 days, but Neptune takes 165 years).

It seems that the plasma in the image is moving at a significant fraction of the speed of light. So in this plasma we have electrons that spin very rapidly in a circle around the black hole.

This is a giant version of the Australian synchrotron, which uses the gravity of a black hole instead of magnets to keep the electrons moving at relativistic speeds.

Like the Australian synchrotron, these electrons emit light (synchrotron radiation) which then traveled from M87 * to Earth to create this beautiful image.

However, some of this light is intercepted by the black hole in the middle. The black hole leaves a shadow on what would otherwise be a brilliant disc.

2) General relativity works on a very wide range of masses

A prediction of general relativity is that the size of a black hole is linearly proportional to its mass. If the mass of a black hole doubles, the diameter of the event horizon (the region where the light can not escape) doubles as well.

LIGO gravitational wave detections have confirmed the size of black holes with masses of tens of solar masses. The Horizon telescope's image of events has now measured the size of a black hole of more than six billion solar masses. It's a mass range of 100,000,000 (or one hundred million).

The general relativity relationship between mass and size has been maintained throughout this range, limiting some alternative theories for gravity.

3) How big is the SUPERMASSIVE black hole?

We already had an idea of ​​the magnitude of M87 * by measuring the speed of gas and stars near the center of the galaxy. However, the estimates disagreed by about a factor of two.

The dynamics of the gas suggest masses close to 3.5 billion solar masses while the stellar measurements preferred an upper mass close to 6.6 billion solar masses.

Part of the Milky Way, the constellation Sagittarius. Image: Advanced Camera for NASA / ESA Hubble Space Telescope Surveys

The size of the black hole shadow in the new image is clearly in favor of the higher mass estimate. M87 * weighs 6.5 billion solar masses, plus or minus 10%. This extreme mass also explains why our first black hole image is in M87 *.

The bigger a black hole, the bigger and easier to see. We expected the two largest black holes (in apparent size) to be M87 * and the black hole of the Milky Way, Sagittarius A *.

Sagittarius A * is clearly much closer to us (just 26,000 light years away from 53 million light-years), but it is also much smaller, with four million instead of six billion solar masses.

The net result is that M87 * is almost the same apparent size as Sagittarius A *.

4) There are giant black holes in the center of most galaxies

We already had several reasons to believe that most galaxies harbor a giant black hole near their center. However, as the saying goes, see, it's believe.

The black hole imaging at the center of the M87 galaxy confirms the presence of supermassive black holes in the center of the galaxies.

But how did we manage to imagine a black hole at 53 million light-years?

The Event Horizon telescope created an image of the black hole using a technique called interferometry to combine data from eight telescopes around the world, including my favorite, the South Pole Telescope in Antarctica.

Image analysis even benefited from the rotation of the Earth between observations (effectively moving the relative positions of telescopes and the black hole).

Together, these instruments provided an effective telescope almost the size of the Earth, with a diameter of nearly 13 million meters. And it will probably take a while to build such a big mirror.

Image: Collaboration with Event Horizon Telescope

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