Astronomers Get as Close as They Can to Seeing the Black Hole at the Heart of the Milky Way

Since the 1970s, astronomers have theorized that at the center of our galaxy,  about 26,000 light-years from Earth, there exists a supermassive black hole (SMBH) known as Sagittarius A*. Measuring an estimated 44 million km (27.3 million mi) in diameter and weighing in at roughly 4 million Solar masses, this black hole is believed to have had a profound influence on the formation and evolution of our galaxy.

And yet, scientists have never been able to see it directly and its existence has only been inferred from the effect it has on the stars and material surrounding it. However, new observations conducted by the GRAVITY collaboration** has managed to yield the most detailed observations to date of the matter surrounding Sagittarius A*, which is the strongest evidence yet that a black hole exists at the center of the Milky Way.

The study which describes their findings – “Detection of orbital motions near the last stable circular orbit of the massive black hole SgrA*“, which recently appeared in the journal Astronomy and Astrophysics – was led by Reinhard Genzel of the Max Planck Institute for Extraterrestrial Physics (MPE) and included the various scientists that make up the GRAVITY collaboration.

The four Unit Telescopes that make up the ESO’s Very Large Telescope, at the Paranal Observatory. Credit: ESO/H.H.Heyer

The GRAVITY collaboration (which is made up of scientists from multiple European research institutes and universities) is so-named because of their association with the  GRAVITY instrument, which is part of the ESO’s Very Large Telescope Interferometer (VLTI). This instrument combines the light from the VLT’s four Unit Telescopes to create a virtual telescope that measures 130 m (426.5 ft) in diameter.

For the past two years, this team has been using this instrument to observe the Galactic center and Sgr A* to observe the effects it has on the surrounding environment. The purpose of these observations has been to test the predictions made by Einstein’s Theory of General Relativity and learn more about SMBHs by studying the closest available candidate.

Another purpose was to search for the orbital motions of flares of infrared radiation (aka. ‘hot spots’) in Sag A* accretion disc (the belt of gas orbiting the black hole). The flares happen when this gas, which is accelerated to relativistic speeds, is pulled as close as possible the black hole’s event horizon – what is known as the innermost stable circular orbit (ISCO) – without being consumed.

Using the GRAVITY instrument on the VLTI, the team observed flares coming from belt that was accelerated to 30% the speed of light in a circular orbit around Sag A*. Not only was this the first time material has been observed orbiting close to a black hole’s point of no return, it was the most detailed observations yet of material orbiting this close to a black hole.

As Oliver Pfuhl, a scientist at the Max Planck Institute for Extraterrestrial Physics and a co-author on the paper, said in a recent ESO press release:

It’s mind-boggling to actually witness material orbiting a massive black hole at 30% of the speed of light. GRAVITY’s tremendous sensitivity has allowed us to observe the accretion processes in real time in unprecedented detail.

The observations they conducted also confirmed the theory that Sag A* is indeed a supermassive black hole – otherwise known as the “massive black hole paradigm”. As Genzel explained, this accomplishment is something scientists have been looking forward to for decades. “This always was one of our dream projects but we did not dare to hope that it would become possible so soon,” he said.

Interestingly, this is not the first time that the GRAVITY collaboration has used the VLTI to observe the center of our galaxy. Earlier this year, the team used GRAVITY and the Spectrograph for INtegral Field Observations in the Near Infrared (SINFONI) instrument to measure the movements of a star as it conducted a close fly-by with Sag A*.

As the star (S2) passed close to the extreme gravitational field of Sagittarius A*, the team measured the star’s position and velocity and compared these to previous measurements. After comparing them to various theories of gravity, they were able to confirm that the star’s behavior was consistent with predictions made by Einstein’s Theory of General Relativity.

This was a major accomplishment, as it was the first time that General Relativity had been confirmed in such an extreme environment. As Pfuhl explained:

We were closely monitoring S2, and of course we always keep an eye on Sagittarius A*. During our observations, we were lucky enough to notice three bright flares from around the black hole — it was a lucky coincidence!

In the end, these groundbreaking observations were made possible thanks to a combination of international collaboration and state-of-the-art instruments. In the future, more advanced instruments – and improved methods of data-sharing – are sure to unlock even more mysteries of the Universe and help scientists understand how it came to be.

And be sure to check out this ESOcast that talks about this recent discovery, courtesy of the ESO:

**The GRAVITY collaboration is made up of members from the Max Planck Institute for Extraterrestrial Physics, the LESIA Paris Observatory, the Centre Nationale de Researches Scientifique (CNRS), the Max Planck Institute for Astronomy, the Centro de Astrofísica e Gravitação (CENTRA), the European Southern Observatory (ESO), and multiple European universities.

Further Reading: ESO, Astronomy and Astrophysics

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