Scientists in Europe have observed probably the closest material revolving around a black hole that is believed to exist at the centre of the Milky Way.

Bengaluru: For the first time ever, astronomers have obtained a close-up of the environment around a black hole.

The Very Large Telescope (VLT) interferometer of the European Southern Observatory (ESO) has seen what is probably the closest material revolving around the ‘supermassive black hole (SMBH)’ at the centre of our galaxy, with the help of the GRAVITY instrument.

This material is the closest anything can get to the black hole that makes our entire galaxy go around and orbits at 30 per cent of the speed of light. Anything closer will find itself spinning out of control into a vortex, finally entering into a black hole from which even light cannot escape.

The research was conducted by scientists from Max Planck Institute for Extraterrestrial Physics (MPE) and the Max Planck Institute for Astronomy, Germany; the Observatoire de Paris, the Université Grenoble Alpes, CNRS, and the University of Cologne, France; the Portuguese CENTRA — Centro de Astrosica e Gravitação; and the ESO, which is headquartered in Germany.

The study was published in the journal Astronomy and Astrophysics.

Supermassive black hole in the Milky Way

Just under a century ago, astronomers discovered that a radio signal was being regularly emitted by a source at the centre of our home galaxy, the Milky Way, in the direction of the constellation Sagittarius.

The source was named Sagittarius A* (Sgr A*), the asterisk denoting a pun: The phenomenon was exciting and all excited states of atoms are denoted so.

Sgr A* is a ‘supermassive black hole’, the largest type of black holes to exist. There is one in the centre of every galaxy we know, each of them hundreds or thousands or even billions times the mass of our Sun. Sgr A* is estimated to have a mass 4 billion times the Sun’s.

Sgr A* has never been directly observed. We haven’t seen or captured photographic evidence of a black hole. However, we are amply familiar with the phenomenon itself and several of its relativistic effects, many of which were predicted by Albert Einstein.

We can also see its gravitational effects: The way it bends light, when it sucks an asteroid flying by, or when we can directly see that pretty much our entire galaxy and all the stars in it, with their planets, revolve around it.

Hot spots and cosmic luck

Located in Chile, the VLT is actually a combination of four telescopes in the Atacama Desert that look out into the universe in the visible and infrared spectrum.

Using a technique called interferometry, their individual observations can be combined and strengthened to see things that would otherwise be difficult to spot through just one telescope.

The MPE astronomers working on the VLT were observing a star called S2 that was flying near Sgr A* when they noticed flashes of light near the black hole.

The three bright flares or “hot spots” they saw were coincidentally positioned in such a way that their plane of revolution was directly perpendicular to ours. The astronomers were able to actually track the flares as they orbited Sgr A*, once every 45 minutes.

Given their location and the estimated size of Sgr A*, the astronomers were able to deduce that the hot spots were travelling at 30 per cent of the speed of light.

The researchers also released a video of simulations of orbital motions of gas swirling around at about 30% of the speed of light on a circular orbit around the supermassive black hole Sagittarius A*.

“It’s mind-boggling to actually witness material orbiting a massive black hole at 30 per cent of the speed of light,” said Oliver Pfuhl, a scientist at the MPE. “GRAVITY’s tremendous sensitivity has allowed us to observe the accretion processes in real time in unprecedented detail.”

The hot spots were orbiting in the region called the ‘accretion disk’ of a black hole, where material spins around it, some of it eventually falling into the event horizon or the outer edge of the black hole. Specifically, these hot spots were at the closest safest distance to the event horizon, the innermost stable orbit.

Why it matters

The findings lend even more evidence to the existence of Sgr A*, proving Einstein right yet again.

“Understanding accretion around Sgr A* will help us understand how galaxies evolve and how their central black holes work,” said Abhijeet Borkar, a post-doctoral researcher in radio astronomy at the Astronomical Institute of Czech Academy of Sciences who has formerly worked at MPE.

“By now, it’s more or less clear that there is no alternative object in place of Sgr A*,” he added. “We now hope to find answers to questions like why our accretion disk is so thin, how gases lose their stability and fall into the event horizon, etc.”

As our tech catches up with our theoretical science, we expect to learn more about these astrophysical phenomena operating at unimaginable scales.

“The Event Horizon Telescope is making observations of the event horizon scale physics around Sgr A*,” added Borkar. “If successful, the first results on the physics around a black hole and its properties might be published by February 2019.”

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