Astrophysics Knowledge

The Sgr A* Black Hole Image: 3 Reasons Why it is Extra Special

If you love science and astronomy, you surely did not miss the above picture. This is the first-ever image of the supermassive black hole at the centre of our Milky Way galaxy! Actually, it’s the shadow and the surrounding region, but we will come to that in a bit. It was published on May 12, 2022, by a global collaboration of more than 300 astronomers from 80 institutes using the Event Horizon Telescope (EHT).

This is not the first time the EHT team published such an image. In April 2019, the same collaboration released the first-ever image of the black hole in the galaxy M87 using the EHT. This marked the beginning of a new era in the study of black holes. You may be wondering, well, you already took a picture before, then what is so special about this second image? What is the point of spending more than two years for a picture of a few blurry orange blobs? As an astronomer, my research is not exactly about black holes but the galaxies they reside in. But I am as excited and curious about the work as you are. So, I went through the technical details of the Sgr A* imaging for you and would like to share why I think this is absolutely worth the effort and more.

1. Central black hole of our home galaxy! 

First of all, this is the supermassive black hole of our own Milky Way galaxy! It is more known as the Sagittarius A* (or Sgr A*, pronounced “sadge-ay-star”). The part ‘Sagittarius’ of the name comes from the direction of the Milky Way’s centre from us, which is towards the Sagittarius constellation. For a long time, general relativity predicted that we expect a very condensed and massive object (aka, a black hole) at the centre of our galaxy and most of the massive galaxies.

In the Milky Way, we can see the nearby stars around the galactic centre. These stars move around the centre very fast because they are closer (similar to how the planets closer to the Sun move faster than the farther ones in our Solar System). Astronomers have already measured the motion of these stars and estimated the mass of this central object (about 4 million times more massive than the Sun). Given the small area where such a high amount of mass resides, it was highly likely that this is indeed a supermassive black hole. With the direct imaging by the EHT team, now we have another compelling evidence that black holes exist and that our galaxy also has one. 

Do we actually see the black hole?

A point to note is that we cannot see the black hole itself because no light can escape from it by definition. We can see the central dark region or the shadow where the black hole is. The surrounding bright ring-like structure is the light coming from the accreting gas around the black hole – bent by the powerful gravity of the enormous mass in it.

2. The technical wonders to make it happen

Another extraordinary aspect of the image is the technical marvel the team needed to accomplish such unprecedented zoom-in capacity and noise reduction. Sgr A* is about 27,000 light-years away from Earth, and the diameter of the orange ring around the black hole is roughly equal to the orbit of Mercury around the Sun. Imagine being able to see a regular sized doughnut on the ground of Moon from the Earth, that is how much the magnifying power had to be! Below is a cool video published by the European Southern Observatory (ESO) showing the zoom-in from the sky as we see on Earth to Sgr A*.

Let us compare the Sgr A* image with the previous black hole image of the M87 galaxy (M87*), a monster even for a black hole. With 7 billion times the mass of the Sun, M87* is about 2000 times more massive (and bigger) than Sgr A*. But it is also about 2000 times farther. That is why the EHT setup could be used to observe these two black holes with very different mass and sizes. They are also residing in two very different types of galaxies. Yet, the edges of both of these black holes look very similar. This tells us that the same physics governs the regions close to a black hole, explained by General Relativity. 

One puzzle to solve after another

Surprisingly, it was more difficult to image SgrA* compared to M87*, even though Sgr A* is much closer to us. This is because of the rapidly rotating gas around the black hole. The gas moves around both of these black holes at a similar speed, which is close to the speed of light. As M87* is a giant, it takes the gas longer to rotate around the black hole, from days to weeks. In comparison, the gas around Sgr A* takes only a few minutes to move around it. To make the image, the astronomers needed to gather light for over a few hours, like taking a long-exposure shot (or night shots) in a camera.

This resulted in a chaotic image with nothing clearly visible. EHT scientist Chi-Kwan Chan very appropriately compared the process as “a bit like trying to take a clear picture of a puppy chasing its tail.” To account for this, the team chunked their long exposure data into smaller bits of few minutes each. They made snapshots from these chunks to capture individual states of gas movement. Finally, they took an average of these shots to improve the data quality.  

3. Entire Earth as one giant telescope (and aiming for even bigger ones)!

Finally, there is the ingenuity of using the entire Earth as one telescope. The idea is not new, the same principle of using multiple telescopes around the world was also used for the M87* imaging. It is still very impressive. Why do we need something like this? To observe a small target, we need a telescope with higher resolving power. It refers to the smallest object a telescope can separate from its surroundings. The smaller the target, the higher this power needs to be in any wavelength. And to increase it, we need a larger telescope. To observe something as small as the shadow of central black hole in galaxies through radio wavelengths, the telescope size had to be of the scale of the Earth. It was made possible by connecting multiple powerful radio telescopes across the Earth by a method called interferometry.

Looking ahead . . .

For future efforts, the collaboration is working on joining more telescopes in the system. This will make the image clearer and help to obtain a better understanding of the black holes. Another major upgrade for the upcoming years is the scope to include space telescopes in the system. This will enable the system to capture images of a wider range of black hole mass and size in different galaxies. With these very exciting line-ups to look forward to, we are surely in for a series of surprises in the following decades from the collaboration and its extensions! If you are thinking about a career in astronomy, maybe you can also be one of the forerunners of this field in future!

References for more reading:

Curious to learn more about the Sgr A* image? Here are some of the best articles on the topic that I found:

  1. Announcement and summary of the Sgr A* image reveal from the EHT collaboration.
  2. Details (simplified) about the imaging techniques and challenges from an EHT collaboration member.
  3. A well-explained overview of what is Sgr A*, what went into making the image, and what information can we get from the image, from an astronomy professor.
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