Sagittarius A* | The Milky Way’s dark centre

Andrea Ghez and Reinhard Genzel were awarded the Nobel Prize in 2020 for demonstrating that an extremely massive, invisible object existed at the center of the Milky Way that attracts masses and causes them to accelerate enormously. They estimate that the mass of about 4 million times the Sun was concentrated in this area. Now the Event Horizon Telescope, a collaboration of more than 300 researchers from 80 countries, has published an image of this region. Known as Sagittarius A* (SgrA* for short), this region is believed to host a supermassive black hole about 4 million times the solar mass. Other possibilities have been put forward by researchers in this context, but that of a supermassive black hole is the most likely.

The Event Horizon Telescope is not just a single telescope. It consists of a consortium of eight powerful telescope arrays around the world, which together form a giant eye, the size of the Earth and 3 million times sharper than the human eye. With this giant eye, the researchers looked at this point, which is about 27,000 light-years away from Earth. They observed SgrA* on multiple nights and collected data for hours, just as a camera would use long exposures. However, the technique was very different, as it uses a network of telescopes rather than a single one. This is called Very Long Baseline Interferometry.

Using this technique, the Event Horizon Telescope collaboration in 2019 had created an image of an area called M87*, the black hole at the center of the Messier 87 galaxy. Despite the fact that the two galaxies are so different and the masses of the black holes were different, the images are very similar.

The shadow of a black hole

Although we cannot see the black hole itself, the gas moving around the black hole emits light, which follows a curved path around the black hole, leaving behind a central dark area, which becomes the black hole’s “shadow”. called. This effect occurs due to the enormous gravity of the central region. This image is thus a confirmation of Einstein’s general theory of relativity. The ring-shaped image of SgrA*, very similar to that of M87*, took up 52 micro arcseconds in the field of view, which is the same span of our view as a donut on the moon!

Challenges in imaging

Despite the fact that M87* is much further away than SgrA*, the group was able to image the former earlier. This is because SgrA* is only one-thousandth the size of M87*; line of sight to SgrA* is obscured by much intervening matter; and finally, since SgrA* is much smaller than M87*, it takes only minutes for the gas swirling around it to complete orbit around SgrA*, rather than weeks to go around M87*. The latter gives a variability that makes it difficult to create an image. Clear imaging requires a long exposure of about 8-10 hours, where ideally the subject would not change much.

The telescopes that make up the array are Atacama Large Millimeter/sub-millimeter Array, Atacama Pathfinder Experiment, IRAM 30-meter Telescope, James Clerk Maxwell Telescope, Large Millimeter Telescope Alfonso Serrano, Submillimeter Array, UArizona Submillimeter Telescope and South Pole Telescope . Since 2017, when the observations about this started, the group has added the Greenland Telescope, Northern Millimeter Extended Array and UArizona 12-meter Telescope on Kitt Peak to the set.

While there is overwhelming evidence that SgrA* contains a very compact invisible object at its core, is the only possibility a black hole? This question has a non-trivial answer. According to Pankaj Joshi, founding director of the Cosmology Center and Distinguished Professor of Physics at Ahmedabad University, who is an expert in the field and not part of the collaboration: “In their papers, the researchers consider several alternatives, such as naked singularities and wormholes. Their report in one of the papers – article five of the series published in The Astrophysical Journal Letters – claims that JMN’s (Joshi-Malafarina-Narayan) bare singularity with photon sphere could be the best mimic of black holes. that the central object and its nature remain a matter of great mystery, this is because just as a black hole event horizon would create a shadow, so the naked singularity also creates a similar shadow and therefore it is impossible to distinguish between the two.”

The New Generation Event Horizon Telescope collaboration explores these deeper mysteries.


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