5 questions to help you quickly understand the first photo of the black hole at the center of the Milky Way

Last night, the first photo of the black hole at the center of the Milky Way was released.

The first photo of the black hole at the center of the Milky Way! (Provided by EHT Collaboration)

At 21:07 on May 12, at a press conference held simultaneously around the world including Shanghai, astronomers showed people the first photo of the supermassive black hole at the center of our Milky Way galaxy!

This photo was "taken" by an international research team organized by the Event Horizon Telescope (EHT) collaboration using a network of radio telescopes distributed around the world.

Seeing this photo of the black hole at the center of the Milky Way, we are all very excited, because it has been three years since the first black hole photo was released on April 10, 2019. Now, this precious photo has raised more questions for us to explore and discover.

Question 1: The black hole at the center of the Milky Way is closer to the Earth, so why wasn’t the first black hole photographed?

Ever since seeing the first photo of a black hole (the photo of the black hole at the center of M87) in 2019, people have been obsessed with the photo of the black hole at the center of their own galaxy and have always kept it in their hearts.

After the photo was taken in 2017, two years later in 2019, we got a photo of the black hole of M87, which is 55 million light years away from us. This is the only time we can clearly see the appearance of a black hole. Compared with the black hole in the Milky Way, the M87 black hole has a great advantage. Its rotation axis is only 17 degrees, and it is almost along its rotation axis. So there is almost no obstruction, so it is relatively easy for us to see the photo of the M87 black hole.

Photo of the black hole at the center of M87 (provided by the EHT collaboration)

The Milky Way's supermassive black hole is located at the center of the Milky Way. It is the supermassive black hole of our own galaxy. Some people will definitely think that since it is right next to us, isn't it easier to take a picture?

In fact, as the poem says, "I don't know the true face of Mount Lu because I am inside this mountain." Although this black hole is located at the center of the Milky Way, we ourselves are inside the Milky Way, making it even more difficult to photograph.

We have gradually come to know our galaxy with the help of radio and infrared bands other than optical ones, as well as other galaxies. Although the black hole of our own Milky Way (called Sgr. A*) is close, data processing is more difficult and time-consuming due to obstruction, so "taking pictures" requires more time.

However, the wait also makes the release of this photo even more exciting, because this is a photo of the black hole at the center of our own Milky Way! This is also another major breakthrough after the EHT collaboration released the first black hole photo of mankind in 2019, capturing the central black hole (M87*) in the more distant galaxy M87.

Question 2: How was this photo of the black hole at the center of the Milky Way taken? Compared with the photo of the black hole at the center of the M87 galaxy, what new methods were used?

As we all know, M87 is almost in the direction of the rotation axis, while we are on the Milky Way disk, so compared with M87, the black hole at the center of the Milky Way will be blocked a lot when imaging. For example, when observing the Milky Way in the optical band, we will see a lot of dust and other gases blocking it. At this time, we must use the infrared or radio bands with longer wavelengths. The mature ones are the millimeter wave and submillimeter wave bands, that is, the event horizon telescope. It is worth mentioning that it uses different submillimeter and millimeter wave telescopes around the world to form an array with a diameter of tens of thousands of kilometers.

This photo is very similar to the photo of M87 taken in 2019. Both were taken using 8 different millimeter-wave telescopes around the world, or event horizon telescopes for short.

This huge telescope combination is: ALMA (Atacma Large Milimiieter/Submeter Array) in Chile, SPT (South Pole Telescope) in Antarctica, SMA (Submilleter Array) in Hawaii, USA, LMT (Large Millimeter Array) in Mexico, JCMT (James Clerk Maxwell Telescope) in Hawaii, USA, IRAM (IRAM 30-m telescope) in Spain, APEX (Atacama Pathfinder EXperiment) in Chile, and SMT (Submillimeter Telescope) in Arizona, USA.

It is worth mentioning that the JCMT telescope in Hawaii, USA, is a telescope operated by China, and many Chinese scientists should have conducted observations here. Unfortunately, the maximum diameter that can be achieved by infrared observations is hundreds of meters. For example, the VLT/gravity of the European Southern Observatory can observe a diameter of 130 meters, but the aperture is still far from kilometers. I hope that we can use the infrared band to see photos of black holes in the future.

The picture shows the Milky Way in various wavelengths, with the energy increasing from top to bottom, from millimeter waves to gamma rays.

We know that the black hole in the Milky Way is only about 4 million times the mass of the sun (according to the 2020 Nobel Prize results), while the black hole in M87 reaches 6.5 billion times the mass of the sun. The former is 1,650 times smaller than the latter.

In terms of size, the black hole at the center of the Milky Way is obviously slightly smaller, but the black hole at the center of the Milky Way is more difficult to photograph. This is because the mass of the black hole at the center of the Milky Way is much smaller than that of M87 and the distance is much closer, so the possibility of changes in the surrounding matter is much greater.

Compared to the observation of the black hole in M87, the changes that originally took several days now occur in a few minutes, so the observation is more difficult. For example, for this photo, scientists have developed new and complex tools to consider the gas of Sgr A*.

Question 3: Compared with the photo of the black hole at the center of the M87 galaxy, what are the differences and what new information is there?

Because individual observations are very difficult, the photo of the black hole at the center of the Milky Way (Sgr A*) that we saw this time was the result of the research team spending a lot of time extracting different photos and then averaging them.

This is also the first time that a picture of the black hole hidden at the center of our galaxy has finally been presented.

We can recall the time of the last photo: we started taking pictures in 2017, and in 2019 we got a picture of the black hole at the center of M87.

However, it was not until five years later that scientists used supercomputers to synthesize and analyze data and strictly compared the black hole simulation database with the observation results, allowing us to see the photo of the black hole at the center of the Milky Way for the first time. Thanks to the wisdom and hard work of scientists, we can see an unprecedented picture!

Question 4: The black hole at the center of the Milky Way accounts for less than 0.0005% of the Milky Way. Why can it bind hundreds of billions of stars?

If we look at the structure of the Milky Way, it can be divided into three parts: the galactic nucleus (including the black hole), the galactic disk, and the galactic halo; in terms of mass, the mass of the big black hole at the center of the Milky Way is less than 0.0005% of the mass of the Milky Way; and from the perspective of the core of the Milky Way, the Milky Way black hole is only a part of the galactic nucleus.

So what force holds the Milky Way's hundreds of billions of stars within a limited radius? How is all visible matter held together?

In fact, this question was raised at the beginning of the last century.

Astrophysicist Fritz Zwicky measured the stars in the Coma Cluster and discovered the existence of dark matter. Because Zwicky's personality was not very popular, although this concept was correct, it was not taken seriously.

It was not until 1970 that the young Verin Rubin and her mentor Kent Ford studied the rotation speed of stars in the Andromeda Galaxy. Using high-precision spectral measurement technology, they were able to detect the relationship between the speed of stars in the outer regions far from the galactic core and the distance around the galaxy. According to Newton's law, if the mass of a galaxy is mainly concentrated in the visible stars in the galactic core, the speed of stars in the outer regions of the galaxy will decrease with distance.

However, observations show that the speed of stars in the outer reaches of galaxies is constant over a fairly large range, which means that there may be a large amount of invisible matter in galaxies that is not only distributed in the core of galaxies, and its mass is much greater than the sum of the masses of luminous stars.

We now know that the mass of invisible matter (dark matter) is about 10 times heavier than the visible mass, and this is true for almost all galaxies.

This is the answer to the previous question. Although the black hole at the center of our Milky Way has such a small mass, with the help of dark matter, it can bind hundreds of billions of stars!

Question 5: What is the significance of taking this photo for research?

Before the press conference, many people may have expected to see a black hole similar to the one in the movie "Interstellar" when they heard about the photo of the black hole at the center of the Milky Way, but the result was not the case.

This is because what we see is the part very close to the black hole. If it were relatively far away, we would see a scene similar to that in the movie "Interstellar".

Regardless, this photo is more intimate than the previous one of M87 because it's a picture of our own galaxy's black hole, and it's much more difficult to photograph.

Let us once again thank all the scientific workers and the Chinese scientists who participated in this research for allowing us to take a glimpse of what the black hole at the center of the Milky Way looks like. Astronomical exploration will never stop!

Produced by: Science Popularization China

Author: Gou Lijun, National Astronomical Observatory, Chinese Academy of Sciences

Producer: China Science Expo

This article is produced by the Science Popularization China Frontier Technology Project. Please indicate the source as Science Popularization China when reprinting.

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