We Just Saw Sagittarius A*! Our Milky Way's Black Hole
Something amazing happened on May 12, 2022: European Southern Observatory (ESO)
showed the world the fantastic image of Sagittarius A*, (Sagittarius A-star),
the black hole in the centre of our galaxy. Its bright and reddish colour has gone all
around the world, on every computer, laptop, smartphone and tv. That's because this image has
monstrous consequences on our understanding of the universe. We are the first humans getting to
see pictures of the black hole in our galaxy. It is like living in a big house, the house had
belonged to your family ever since, but there was a room that nobody could enter
because the keys were lost, and now you found the keys and you finally opened it.
To be honest, though, this is not the first time we took a picture of a black hole.
If you remember, something similar happened in 2019. But the earlier picture showed the black
hole in the centre of another distant galaxy, M87. So what's the difference between our black
hole and the M87's one? Follow me in this video to get to know the answer to this question and more.
1. What is Sagittarius A*? First of all, don't be scared:
nothing is gonna happen, and we will not be sucked into a black hole. Even if it belongs to our
galaxy, we are too distant for it to swallow us, and so are the great majority of stars,
planets and objects in our galaxy. The thing is, black holes are giant and scary, but they obey
the laws of physics, and nothing can go wrong as long as you are at a “safety distance”.
We, as a solar system, orbit at something like 8 kpc from it, which is the equivalent of more
than one point five billion astronomical units from this monster, so we are safe.
This set, here's what we know. Sagittarius A* sits at the centre
of our Milky Way galaxy, in the direction of the Sagittarius constellation. Astronomers
know it quite well because for decades they have been measuring blasts of radio waves coming from
a compact source in that region. There were speculations, and some of them claimed that
they were facing a black hole. But the real hint came later, in 1980, when two teams of astronomers
started tracking the motions of stars near this mysterious compact source,
this mysterious source breathing radio waves. This simulation is showing you what they saw.
As you can see, a special point exists in this region. In the neighbourhood of this point,
stars whirl around, speeding up. It's like they are accelerated by something.
What astronomers what seeing was this dark object speeding stars up
to a third of the speed of light. The only possible explanation that
astrophysicists knew of this was a result of general relativity by Albert Einstein:
black holes. That's how we started convincing ourselves that the mysterious object was indeed
a black hole. But we needed confirmation. One clear confirmation came with the study of
the orbit of S2, a star orbiting Sagittarius. The highly eccentric orbit of S2 allowed
astronomers to test for various effects predicted by general relativity and even extra-dimensional
effects. These effects reached a maximum at the closest approach, which occurred in mid-2018.
Given a recent estimate of 4.31 million M☉ for the mass of the Sgr A* black hole
and S2's close approach, this made S2's the fastest known ballistic orbit,
reaching speeds exceeding 5,000 km/s. In 2018, S2 made its closest approach to Sgr A*,
reaching 7650 km/s or almost 3% of the speed of light, while passing the black hole at a
distance of just 120 astronomical units. Can you believe it? 3 per cent of the speed of light!
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Anyway, the motion of the stars is important because it tells
us something about the mass of the black hole.
The study suggested that our black hole is 4 million times more massive than the Sun.
Reinhard Genzel and Andrea Ghez later shared the Nobel Prize in Physics for this discovery.
Another thing you need to know about the black hole is its size.
Every black hole's size is defined by its event horizon. The horizon is the distance from the
centre of the black hole within nothing can escape. For Sagittarius, scientists had previously
been able to calculate it to be 16 million miles or 26 million kilometres in diameter.
The Milky Way’s black hole is huge compared to the black holes left behind when massive stars die.
Another thing that we have to keep in mind is that M87 and Sagittarius
are not the only existing black holes. What astronomers actually think nowadays
is that there are supermassive black holes at the centre of nearly all galaxies.
Compared to most of these, Sagittarius A* is quite unremarkable in size.
Now let's go to the clue of this video because we want to talk about the image. What does it show?
In fact, you may ask: what do you exactly mean when you say we have “a picture of a black hole?”
“Aren't black holes completely dark?” And you would be right. Black holes themselves are
completely dark, since nothing, not even light, not even Usain Bolt, can escape their gravity.
But astronomers are smart, and after some studies, they realized that every black hole
must be surrounded by clouds of gas. Now, with modern instruments, we are able to measure this
gas to infer images of the black holes within. So what you really see in this image is not the
black hole, but the material surrounding its event horizon, most of it being gas and light.
The central dark region in the image is a shadow cast by the black hole onto the gas.
The bright ring is the gas itself glowing. The bright spots in the ring show areas of hotter
gas that may one day fall into the black hole. Some of the gas visible in the image is actually
behind Sagittarius A*. Light from that gas is being bent by the powerful gravity
of the black hole toward Earth. This effect, called gravitational lensing,
is a core prediction of general relativity and may be one of the most fascinating ones.
However, producing such an image is not easy at all. To understand how hard it was,
you just need to take into account that scientists worked on it for 3 years straight,
after the M87 black hole first came out. The thing is, supermassive black holes are extremely hard to
measure. They are far away and shrouded by the gas and dust that clogs the centre of galaxies.
They are also relatively small compared to the vastness of space.
And distances in the universe are huge, and the light that comes in
from an object has to travel a lot in order to reach a telescope, a camera, or our eyes.
From where Sagittarius A* sits, 26,000 light-years away at the centre of the Milky Way,
only 1 in 10 billion photons of visible light can reach Earth,
and most are absorbed by gas in the way. But radio waves, which are not visible,
pass through gas much more easily than visible light, so astronomers thought it
might be the case to study such distant objects in the so-called radio domain.
The result is that the orange colours in the image are representations of those radio waves.
In order to acquire information from the black hole in the form of radio waves,
the team needed a good receiver. In this case, astronomers used a number of eight
radio telescopes spread across the globe, and put the data together in such a smart way,
invoking the power of the interferometry technique.
The project is called Event horizon telescope, or EHT.
It is composed of a network of radio telescopes distributed over the globe
resulting in a single radio telescope with high sensitivity and high angular resolution.
Using the Very Long Baseline Interferometry (VLBI) technique, many independent radio antennas
thousands of kilometres apart can be used in a synchronous system to create a "virtual" telescope
having the distance between the most distant receiving stations as its effective aperture
One thing you need to know is that the observations generated so much data every night,
that the team couldn’t send it through the internet – they had to ship physical
hard drives to where they processed the data. Because black holes are so hard to see, there is
a lot of uncertainty in the data the telescopes collect. The only way to have an accurate image
is to use supercomputers to produce millions of different images, each one a mathematically
viable version of the black hole based on the data collected and the laws of physics.
They then blended all of these images together to produce the final, beautiful, accurate image.
The processing time was equivalent to running 2,000 laptops at full speed for a whole year!
As you can see, it's not enough to have an Einstein. We also need technicians and engineers
that let us perform such computations on our instruments! Technology and science go together.
The difference between the black hole at the centre of our galaxy and the one at the centre
of M87 is mostly a difference in size and mass. M87* is 2,000 times larger than Sagittarius A*
and 7 billion times the mass of the Sun. But because Sagittarius A* is 2,000 times closer
to Earth than M87*, the Event Horizon Telescope was able to observe both black holes at a similar
resolution – giving astronomers a chance to learn about the universe by comparing the two.
But M87 is “calmer”, in the sense that, being bigger, it rotates slower.
This is mainly the reason why we first had an image of M87 and it took three
more years to have a picture of Sagittarius A*. Compared to the one at the centre of M87,
our black hole is quite active and excited! The similarity between the two images is
striking because small stars and small galaxies look and behave very differently from large stars
or galaxies. While M87* is an unusually huge black hole, Sagittarius A* is likely pretty similar to
many of the hundreds of billions of black holes at the centre of other galaxies in the universe.
Sera Markoff, co-director of the scientific committee of EHT, said:
"We have two completely different galaxies and two black holes of very different masses,
yet, near their boundary, these black holes are incredibly similar.
This tells us that General Relativity governs these objects closely
and that any further differences we see must be due to differences in the material around them."
So what these pictures are really telling us is that Einstein has been proven right once again.
This is good news because it shows how the physics we are using is based on solid ground.
Of course, there are a lot of unanswered questions that we will need to answer, and maybe who knows:
one day Einstein could be proven wrong, or slightly wrong. Maybe we are missing something.
For the moment, let's just appreciate what we have: the honour to be making history.
"This video ends here! Thanks for watching everyone! What do you think about this video?
Are you in love, as we are, with the picture of our black hole? Let us know in the comments below,
be sure to subscribe, and I'll see you next time on the channel!"