This is the black hole at the center of our galaxy.

Photo of Sagittarius A*, the black hole at the center of the Milky Way. / Event Horizon Telescope Collaboration

Science | Astronomy

A team of more than 350 scientists achieves the first image of Sagittarius A*, a monster of 4 million solar masses that devours stars in the core of the Milky Way

“We are going to see something new, rare, extraordinary, in the center of the galaxy”, said the Spaniard Xabier Barçons, director of the European Southern Observatory (ESO). “It is very interesting to be here to show you the first image of this puzzle,” added Huib Jan van Langevelde, project manager for the Event Horizon Telescope (EHT). This is how the EHT scientists have just presented in Garching (Munich) the first photo of the black hole in the center of the Milky Way, our galaxy. Sagittarius A* is about 27,000 light-years from us, has 4 million solar masses, and acts as a gigantic sink into which surrounding stars rush. It's what you see on these lines. Or, rather, what they don't see in the center of the image, that black space inside a bright ring. The finding has been presented, at the same time, at press conferences in Washington, Santiago de Chile, Mexico City, Toio, Taipei and Madrid.

A black hole is born when, at the end of its life, a star explodes as a supernova, collapses and begins to suck all the matter from its surroundings. It is not empty space, but a region with a huge amount of matter compressed into a very small space. That makes its gravitational attraction billions of times greater than that of Earth and not even light can escape from it. There are two types: stellar ones –up to tens of solar masses– and supermassive ones from the center of galaxies, such as Sagittarius A*.

An antenna as big as the Earth

The EHT is a set of eight radio telescopes from America, Europe –the IRAM Pico Veleta Observatory, in Granada–, Africa and Antarctica that work in a network as if they were an antenna the size of the Earth. The collaboration, in which more than 350 scientists participate, was born in 2015 to obtain the first direct images of black holes, a type of star whose existence was proposed in 1784 by the English geologist and clergyman John Michell and predicted in 1915 by Albert Einstein.

What is a black hole?  How many types are there?  Which one is the biggest?

There are closer stellar black holes, but from the beginning the EHT focused its gaze on two distant but supermassive ones, Sagittarius A* and the one at the center of M87 –a very bright and nearby elliptical galaxy–, which, due to their large size, scientists considered more easy to hunt. They got it right. On April 10, 2019, the consortium presented to the world
the first photo of a black hole; in fact, of the ring of matter that is about to rush into its interior and disappear forever.

The image of M87, which is located in the center of the galaxy of the same name 55 million light years away, went around the world. There was a giant of 6,500 million solar masses and 9,000 million kilometers in diameter (the Solar System would enter it up to Neptune). "It is a historic day," the EHT scientists underlined in several simultaneous press conferences – in Brussels, Washington, Santiago de Chile, Shanghai and Taipei. And they announced what the next target would be, Sagittarius A*, much smaller, but also much closer.

The two black holes, the researchers have pointed out, look very similar despite the fact that the one in the Milky Way is much smaller. "We have two completely different types of galaxies and two black holes of very different masses, but near the edge of these black holes they look amazingly similar," said Sera Markoff, co-chair of the EHT Science Council and an astrophysicist at the University of Amsterdam.

anatomy of a

black hole

It is a celestial body that concentrates

a lot of mass in one volume

very small and therefore the field

gravitational pull that it creates around it

it's gigantic.

Sagittarius black hole radius A*

The 'monster' photographed is

a sphere of about 6 million

of kilometers of radius; it is

say, 15 times the distance from

the earth to the moon

Relativistic jet:

when the stars are sucked in

by black holes, jets of

particles and radiation

ejected at a speed

close to that of light.

Photon Sphere:

made up of emitted photons

by the hot plasma surrounding the

black hole, and they are curved

because of its immense gravity.

accretion disk:

superheated gas and dust

rush towards the black hole

immense speed,

producing radiation

electromagnetic (X-rays).

Singularity:

region of infinite density where

all energy and matter falls.

Event horizon:

the radius around the singularity

where light and matter cannot

escape the gravity of the hole.

It is the point of no return.

Anatomy of a black hole

It is a celestial body that concentrates a lot of mass in a very small volume and, therefore,

the gravitational field it creates around it

it's gigantic.

Sagittarius black hole radius A*

The 'monster' photographed is

a sphere of about 6 million

of kilometers of radius; it is

say, 15 times the distance from

the earth to the moon

Relativistic jet:

when stars are sucked into black holes, jets of particles and radiation are expelled at close to the speed of light.

Photon Sphere:

formed by the photons emitted by the hot plasma that surrounds the black hole, and that are curved by its immense gravity.

accretion disk:

superheated gas and dust precipitate

toward the black hole at immense speed, producing electromagnetic radiation

(X-rays).

Singularity:

region of infinite density where all falls

the energy and matter.

Event horizon:

the radius around the singularity where light and matter cannot escape

hole gravity. It's the point of not

return.

Anatomy of a black hole

It is a celestial body that concentrates a lot of mass in a very small volume.

small and, therefore, the gravitational field that it creates around it

it's gigantic.

Sagittarius black hole radius A*

The 'monster' photographed is

a sphere of about 6 million

of kilometers of radius; it is

say, 15 times the distance from

the earth to the moon

Relativistic jet:

when stars are sucked into black holes, jets of particles

and radiation are expelled at a speed close to the speed of light.

Photon Sphere:

formed by the photons emitted by the hot plasma that surrounds the black hole, and that are curved by its immense gravity.

accretion disk:

superheated gas and dust rush toward the black hole at a

immense speed, producing electromagnetic radiation (X-Rays).

Singularity:

region of infinite density where all energy and matter falls.

Event horizon:

the radius around the singularity where light and matter cannot

escape the gravity of the hole. It is the point of no return.

Stars at breakneck speeds

Astronomers Bruce Balick and Robert Brown discovered in 1974, using the National Radio Astronomy Observatory (NRAO) of the United States, a very compact and bright radio source in the center of the Milky Way. They did not know what it was and, to differentiate the strange object from others in the region, Brown named it Sagittarius A* in 1982. Over time, he gained weight for the hypothesis that the radio source was a supermassive black hole, and in 1994 its mass was calculated to be 3 million times that of the Sun.

Simulation of the orbits of stars very close to the supermassive black hole at the center of the Milky Way. /

ESO / L. Calçada / Spaceengine.org

In the 1990s, two independent teams of astronomers, led by the German Reinhard Genzel and the American Andrea Ghez, studied the orbits of stars near the galactic center –where there are 10 million stars in just 3 light years– and concluded that there were an invisible supermassive object, that could only be a black hole, that pulled them and made them move at breakneck speeds. That identification of Sagittarius A* as a black hole made them
winners of the 2020 Nobel Prize in Physics.

Today, astrophysicists believe that many, if not all, galaxies have a supermassive black hole like Sagittarius A* at their center. Although the first imaged, M87, is more than a thousand times more massive than the one at the center of our galaxy, it is not the largest known, far from it. That record is held by TON 618, with 66,000 million solar masses and a diameter of 390,000 million kilometers, 2,600 times the distance that separates the Earth from the Sun.

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