With the breathtaking photo, it has now been conclusively proven that our galaxy actually harbors a black hole.

Several years ago, astronomers made history when they captured the first image of a black hole. And now there’s another first. For the first time, the black hole at the center of our Milky Way has been captured on image. And with the photo it has been conclusively proven that our galaxy – as supposed – houses a black hole.

This is the first image of Sgr A*, the supermassive black hole at the center of our Milky Way Galaxy. It is the first direct visual evidence for the existence of this black hole. Image: EHT Collaboration

The image provides a long-awaited look at the massive object at the center of our Milky Way Galaxy. Because although we’ve known since about the mid-20th century that there is a ‘heavy object’ in the heart of our Milky Way, it wasn’t until 2002 before it was first cautiously suggested that it could be a black hole. It wasn’t until 2020 that astrophysicist Reinhard Genzel and astronomer Andrea Ghez provided the most compelling evidence that the center of the Milky Way is home to a supermassive black hole. It earned them the Nobel Prize in Physics.

Sagittarius A*

The black hole we are talking about is called Sagittarius A* (abbreviated Sgr A*). The object is about 25,800 light-years away from our parent star, toward the constellation Sagittarius. The black hole has a weight of four million times the mass of our sun.

The photo

Taking a picture of a black hole is not exactly easy. Black holes do not emit light. This allows researchers to photograph only their ‘shadow’ or ‘silhouette’ caused by the gravitational bending of light under extreme gravity. So, although we can’t see Sgr A* either because it’s completely dark, the glowing gas around it shows a characteristic signature: a dark central region (the so-called shadow), surrounded by a bright ring-shaped structure. This ring consists of light that has been deflected by the strong gravity of the black hole. “We were amazed that the ring size matched so well with Einstein’s general theory of relativity predictions,” said researcher Geoffrey Bower. “These unprecedented observations have greatly improved our understanding of what goes on at the center of our Milky Way Galaxy and provide new insights into how these giant black holes interact with their environment.”

Create images

To create the image, researchers used a network of eight existing radio observatories located around the world, which are linked together to form a single Earth-sized virtual telescope. These separate telescopes are located in remote places and at high altitudes. Their recorded signals are then combined and processed to obtain the final image.

This image shows the locations of some of the telescopes that make up the EHT, and gives an impression of the long baselines between the telescopes. Image: ESO/L. calcada

The image of Sgr A* follows on from the previous image of a black hole taken in 2019. In that image, the shadow of the black hole shines at the center of Messier 87 (M87* for short), a massive galaxy in the Virgo Cluster. The black hole is 55 million light-years from Earth and 6.5 billion times as massive as our sun. The photo was a world first. Because until then scientists suspected that black holes exist, this was only based on indirect observations and theories. With the photo of the black hole – or more precisely: the shadow of the event horizon – the definitive proof for the existence of black holes had been officially provided.

Equations

Scientists are delighted to finally have images of two black holes of very different sizes. This offers the opportunity to learn to understand their mutual differences and similarities. The two black holes are also remarkably similar, although the black hole in our Milky Way galaxy is more than a thousand times smaller and has less mass than M87*. “We have two completely different types of galaxies here and two black holes of very different masses,” said study researcher Sera Markoff. “But close to the edge of these black holes, they are surprisingly similar. This tells us that these objects up close are governed by general relativity, and that any differences we see further away must be due to differences in the material surrounding the black holes.”

Difficult

Although Sgr A* is much closer, it took much more effort to capture this black hole than it did with M87*. Why? “The gas near the black holes moves at the same speed — almost as fast as light — around Sgr A* and M87*,” explains researcher Chi-kwan Chan. “But where the gas takes days to weeks to orbit the black hole M87*, it takes a few minutes to orbit the much smaller black hole Sgr A*. This means that the brightness and pattern of the gas around Sgr A* changed rapidly during the observations – a bit like trying to get a sharp picture of a puppy chasing its own tail.” In addition, the central region of our galaxy is difficult to see from Earth. This is due to the presence of a lot of interstellar dust that obscures our view.

Mission accomplished

With the photo of Sgr A* another important goal has been achieved. Researchers aim to observe the immediate vicinity of a black hole directly. Such observations provide the best way to directly investigate the strong gravitational effects expected near a black hole and to trace the intricate dynamics of matter orbiting the black hole at nearly the speed of light. Researchers have already started to use the new data to test the theories and models that describe how gas behaves around supermassive black holes. This behavior is not yet fully understood, but it is believed to play a key role in the formation and evolution of galaxies.

Overall, the new image of the black hole at the heart of our galaxy provides valuable information about the workings of these giants, which are believed to be found in the centers of most galaxies. “We now have images of two supermassive black holes — one on the large side and one on the smaller side,” said researcher Keiichi Asada. “This means we can test how gravity behaves in these extreme environments better than ever before.”